research article on the bishop-phelps-bollobás property...
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Research ArticleOn the Bishop-Phelps-Bollobaacutes Property for Numerical Radius
Sun Kwang Kim1 Han Ju Lee2 and Miguel Martiacuten3
1 Department of Mathematics Kyonggi University Suwon 443-760 Republic of Korea2Department of Mathematics Education Dongguk University-Seoul Seoul 100-715 Republic of Korea3 Departamento de Analisis Matematico Facultad de Ciencias Universidad de Granada E-18071 Granada Spain
Correspondence should be addressed to Han Ju Lee hanjuleedonggukedu
Received 30 December 2013 Accepted 14 February 2014 Published 6 April 2014
Academic Editor Manuel Maestre
Copyright copy 2014 Sun Kwang Kim et alThis is an open access article distributed under theCreativeCommonsAttribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited
We study the Bishop-Phelps-Bollobas property for numerical radius (in short BPBp-nu) and find sufficient conditions for Banachspaces to ensure the BPBp-nu Among other results we show that 119871
1(120583)-spaces have this property for every measure 120583 On the
other hand we show that every infinite-dimensional separable Banach space can be renormed to fail the BPBp-nu In particularthis shows that the Radon-Nikodym property (even reflexivity) is not enough to get BPBp-nu
1 Introduction
Let 119883 be a (real or complex) Banach space and 119883lowast its dualspace The unit sphere of 119883 will be denoted by 119878
119883 We write
L(119883) for the space of all bounded linear operators on119883 For119879 isinL(119883) its numerical radius is defined by
V (119879) = sup 1003816100381610038161003816119909lowast1198791199091003816100381610038161003816 (119909 119909
lowast) isin Π (119883) (1)
where Π(119883) = (119909 119909lowast) isin 119878119883times 119878
119883lowast 119909
lowast(119909) = 1 It is clear
that V is a seminorm on L(119883) We refer the reader to themonographs [1 2] for background An operator 119879 isin L(119883)attains its numerical radius if there exists (119909
0 119909
lowast
0) isin Π(119883)
such that V(119879) = |119909lowast0119879119909
0|
In this paper we will discuss the density of numericalradius attaining operators actually on a stronger propertycalled Bishop-Phelps-Bollobas property for numerical radiusLet us present first a short account on the known resultsabout numerical radius attaining operators Motivated by thestudy of normattaining operators initiated by J Lindenstraussin the 1960s Sims [3] asked in 1972 whether the numericalradius attaining operators are dense in the space of allbounded linear operators on a Banach space Berg and Sims[4] gave a positive answer for uniformly convex spaces andCardassi showed that the answer is positive for ℓ
1 119888
0 119862(119870)
(where 119870 is a metrizable compact) 1198711(120583) and uniformly
smooth spaces [5ndash7] Acosta showed that the numericalradius attaining operators are dense in 119862(119870) for every
compact Hausdorff space119870 [8] Acosta and Paya showed thatnumerical radius attaining operators are dense inL(119883) if 119883has the Radon-Nikodym property [9] On the other handPaya [10] showed in 1992 that there is a Banach space119883 suchthat the numerical radius attaining operators are not densein L(119883) which gave a negative answer to Simsrsquo questionSome authors also paid attention to the study of densenessof numerical radius attaining nonlinear mappings [11ndash14]
Motivated by the work [15] of Acosta et al on theBishop-Phelps-Bollobas property for operators Guirao andKozhushkina [16] introduced very recently the notion ofBishop-Phelps-Bollobas property for numerical radius
Definition 1 (see [16]) A Banach space 119883 is said to havethe Bishop-Phelps-Bollobas property for numerical radius (inshort BPBp-nu) if for every 0 lt 120576 lt 1 there exists 120578(120576) gt 0such that whenever 119879 isin L(119883) and (119909 119909lowast) isin Π(119883) satisfyV(119879) = 1 and |119909lowast119879119909| gt 1 minus 120578(120576) there exit 119878 isin L(119883) and(119910 119910
lowast) isin Π(119883) such that
V (119878) = 1003816100381610038161003816119910lowast1198781199101003816100381610038161003816 = 1 119879 minus 119878 lt 120576
1003817100381710038171003817119909 minus 1199101003817100381710038171003817 lt 120576
1003817100381710038171003817119909lowastminus 119910
lowast1003817100381710038171003817 lt 120576
(2)
Notice that if a Banach space119883 has the BPBp-nu then thenumerical radius attaining operators are dense inL(119883) Oneof the main results of this paper is to show that the converseresult is no longer true (Section 5)
Hindawi Publishing CorporationAbstract and Applied AnalysisVolume 2014 Article ID 479208 15 pageshttpdxdoiorg1011552014479208
2 Abstract and Applied Analysis
It is shown in [16] that the real or complex spaces 1198880and
ℓ1have the BPBp-nu This result has been extended to the
real space 1198711(R) by Falco [17] Aviles et al [18] give sufficient
conditions on a compact space 119870 for the real space 119862(119870) tohave the BPBp-nu which in particular include all metrizablecompact spaces
The content of this paper is the following First weintroduce in Section 2 a modulus of the BPBp-nu analogousto the one introduced in [19] for the Bishop-Phelps-Bollobasproperty for the operator norm and we will use it as a toolin the rest of the paper As easy applications we prove thatfinite-dimensional spaces always have the BPBp-nu and thata reflexive space has the BPBp-nu if and only if its dual doesNext Section 3 is devoted to prove that Banach spaces whichare both uniformly convex and uniformly smooth satisfy aweaker version of the BPBp-nu and to discuss such weakerversion In particular it is shown that 119871
119901(120583) spaces have the
BPBp-nu for every measure 120583 when 1 lt 119901 lt infin 119901 = 2We show in Section 4 that given any measure 120583 the real orcomplex space 119871
1(120583) has the BPBp-nu Finally we prove in
Section 5 that every separable infinite-dimensional Banachspace can be equivalently renormed to fail the BPBp-nu(actually to fail the weaker version) In particular this showsthat reflexivity (or even superreflexivity) is not enough for theBPBp-nu while the Radon-Nikodym property was knownto be sufficient for the density of numerical radius attainingoperators
Let us introduce some notations for later use The 119899-dimensional space with the ℓ
1norm is denoted by ℓ(119899)
1
Given a family 119883119896infin
119896=1of Banach spaces [⨁infin
119896=1119883
119896]1198880
(resp[⨁
infin
119896=1119883
119896]ℓ1
) is the Banach space consisting of all sequences(119909
119896)infin
119896=1such that each 119909
119896is in 119883
119896and lim
119896rarrinfin119909
119896 = 0
(resp suminfin
119896=1119909
119896 lt infin) equipped with the norm (119909
119896)infin
119896=1 =
sup119896119909
119896 (resp (119909
119896)infin
119896=1 = sum
infin
119896=1119909
119896)
2 Modulus of the Bishop-Phelps-Bollobaacutes forNumerical Radius
Analogously to what is done in [19] for the BPBp for theoperator norm we introduce here a modulus to quan-tify the Bishop-Phelps-Bollobas property for numericalradius
Notation 1 Let119883 be a Banach space Consider the set
Πnu (119883) = (119909 119909lowast 119879) (119909 119909
lowast) isin Π (119883)
119879 isinL (119883) V (119879) = 1 = 1003816100381610038161003816119909lowast1198791199091003816100381610038161003816
(3)
which is closed in 119878119883times 119878
119883lowast times L(119883) with respect to the
following metric
dist ((119909 119909lowast 119879) (119910 119910lowast 119878))
= max 1003817100381710038171003817119909 minus 1199101003817100381710038171003817 1003817100381710038171003817119909
lowastminus 119910
lowast1003817100381710038171003817 119879 minus 119878
(4)
The modulus of the Bishop-Phelps-Bollobas property fornumerical radius is the function defined by
120578nu (119883) (120576)
= inf 1 minus 1003816100381610038161003816119909lowast1198791199091003816100381610038161003816 (119909 119909
lowast) isin Π (119883) 119879 isinL (119883)
V (119879) = 1 dist ((119909 119909lowast 119879) Πnu (119883)) ⩾ 120576
(5)
for every 120576 isin (0 1) Equivalently 120578nu(119883)(120576) is the supremumof those scalars 120578 gt 0 such that whenever 119879 isin L(119883) and(119909 119909
lowast) isin Π(119883) satisfy V(119879) = 1 and |119909lowast119879119909| gt 1 minus 120578 there
exist 119878 isinL(119883) and (119910 119910lowast) isin Π(119883) such that
V (119878) = 1003816100381610038161003816119910lowast1198781199101003816100381610038161003816 = 1 119879 minus 119878 lt 120576
1003817100381710038171003817119909 minus 1199101003817100381710038171003817 lt 120576
1003817100381710038171003817119909lowastminus 119910
lowast1003817100381710038171003817 lt 120576
(6)
It is immediate that a Banach space119883 has the BPBp-nu ifand only if 120578nu(120576) gt 0 for every 0 lt 120576 lt 1 By construction ifa function 120576 997891rarr 120578(120576) is valid in the definition of the BPBp-nuthen 120578nu(120576) ⩾ 120578(120576)
An immediate consequence of the compactness of theunit ball of a finite-dimensional space is the following resultIt was previously known to A Guirao (private communica-tion)
Proposition 2 Let 119883 be a finite-dimensional Banach spaceThen119883 has the Bishop-Phelps-Bollobas property for numericalradius
Proof Let 119870 = 119878 isin L(119883) V(119878) = 0 Then 119870 is anorm-closed subspace of L(119883) Hence L(119883)119870 is a finite-dimensional space with two norms
V ([119879]) = inf V (119879 minus 119878) 119878 isin 119870 = V (119879)
[119879] = inf 119879 minus 119878 119878 isin 119870 (7)
where [119879] is the class of 119879 in the quotient space L(119883)119870Hence there is a constant 0 lt 119888 ⩽ 1 such that
119888 [119879] ⩽ V (119879) ⩽ [119879] (8)
Suppose that119883 does not have the BPBp-nuThen there is0 lt 120576 lt 1 such that 120578nu(119883)(120576) = 0That is there are sequences(119909
119899 119909
lowast
119899) isin Π(119883) and (119879
119899) isinL(119883) with V(119879
119899) = 1 such that
dist ((119909119899 119909
lowast
119899 119879
119899) Πnu (119883)) ⩾ 120576 (119899 isin N)
lim119899
1003816100381610038161003816119909lowast
119899119879119899119909119899
1003816100381610038161003816 = 1
(9)
By compactness wemay assume that lim119899[119879
119899]minus[119879
0] = 0 for
some1198790isinL(119883) and V(119879
0) = 1 Hence there exists a sequence
119878119899119899in 119870 such that lim
119899119879
119899minus (119879
0+ 119878
119899) = 0 Observe that
V(1198790+ 119878
119899) = V(119879
0) = 1 for every 119899 isin N
By compactness again we may assume that (119909119899 119909
lowast
119899)
converges to (1199090 119909
lowast
0) isin 119883 times 119883
lowast This implies that (1199090 119909
lowast
0) isin
Π(119883) and |119909lowast0(119879
0+ 119878
119899)119909
0| = V(119879
0+ 119878
119899) = 1 that is
(1199090 119909
lowast
0 119879
0+ 119878
119899) isin Πnu(119883) for all 119899 This is a contradiction
with the fact that0 = lim
119899dist ((119909
119899 119909
lowast
119899 119879
119899) (119909
0 119909
lowast
0 119879
0+ 119878
119899))
⩾ lim119899
dist ((119909119899 119909
lowast
119899 119879
119899) Πnu (119883)) ⩾ 120576
(10)
Abstract and Applied Analysis 3
We may also give the following easy result concerningduality
Proposition 3 Let119883 be a reflexive space Then
120578nu (119883) (120576) = 120578nu (119883lowast) (120576) (11)
for every 120576 isin (0 1) In particular 119883 has the BPBp-nu if andonly if 119883lowast has the BPBp-nu
Wewill use that V(119879lowast) = V(119879) for all 119879 isinL(119883) where 119879lowastdenotes the adjoint operator of 119879 This result can be found in[1] but it is obvious if119883 is reflexive
Proof By reflexivity it is enough to show that 120578nu(119883)(120576) ⩽120578nu(119883
lowast)(120576) Let 120576 isin (0 1) be fixed If 120578nu(119883)(120576) = 0 there is
nothing to prove Otherwise consider 0 lt 120578 lt 120578nu(119883)(120576)Suppose that 119879
1isinL(119883lowast
) and (119909lowast1 119909
1) isin Π(119883
lowast) satisfy
V (1198791) = 1
100381610038161003816100381611990911198791119909lowast
1
1003816100381610038161003816 gt V (1198791) minus 120578 (12)
By considering 119879lowast1isin L(119883) we may find 119878
1isin L(119883) and
(1199101 119910
lowast
1) isin Π(119883) such that
1003816100381610038161003816119910lowast
111987811199101
1003816100381610038161003816 = V (1198781) = 1
10038171003817100381710038171199101 minus 11990911003817100381710038171003817 lt 120576
1003817100381710038171003817119910lowast
1minus 119909
lowast
1
1003817100381710038171003817 lt 1205761003817100381710038171003817119879
lowast
1minus 119878
1
1003817100381710038171003817 lt 120576
(13)
Then 119878lowast1isinL(119883lowast
) and (119910lowast1 119910
1) isin Π(119883
lowast) satisfy
1003816100381610038161003816⟨1199101 119878lowast
1119910lowast
1⟩1003816100381610038161003816 = V (119878
1) = 1
1003817100381710038171003817119910lowast
1minus 119909
lowast
1
1003817100381710038171003817 lt 120576
10038171003817100381710038171199101 minus 11990911003817100381710038171003817 lt 120576
10038171003817100381710038171198791 minus 119878lowast
1
1003817100381710038171003817 lt 120576
(14)
This implies that 120578nu(119883lowast)(120576) ⩾ 120578 We finish by just taking
supremum on 120578
We do not know whether the result above is valid in thenonreflexive case
3 Spaces Which Are Both Uniformly Convexand Uniformly Smooth
For a Banach space which is both uniformly convex anduniformly smooth we get a property which is weaker thanBPBp-nu This result was known to A Guirao (privatecommunication)
Proposition 4 Let 119883 be a uniformly convex and uniformlysmooth Banach space Then given 120576 gt 0 there exists 120578(120576) gt 0such that whenever 119879
0isinL(119883) with V(119879
0) = 1 and (119909
0 119909
lowast
0) isin
Π(119883) satisfy |119909lowast011987901199090| gt 1 minus 120578(120576) there exist 119878 isin L(119883) and
(119910 119910lowast) isin Π(119883) such that
V (119878) = 1003816100381610038161003816119910lowast1198781199101003816100381610038161003816
1003817100381710038171003817119909 minus 1199101003817100381710038171003817 lt 120576
1003817100381710038171003817119909lowastminus 119910
lowast1003817100381710038171003817 lt 1205761003817100381710038171003817119878 minus 1198790
1003817100381710038171003817 lt 120576
(15)
Proof Notice that the uniform smoothness of119883 is equivalentto the uniform convexity of 119883lowast Let 120575
119883(120576) and 120575
119883lowast(120576) be the
moduli of convexity119883 and119883lowast respectively Given 0 lt 120576 lt 1consider
120578 (120576) =120576
4min 120575
119883(120576
4) 120575
119883lowast (120576
4) gt 0 (16)
Consider 1198790isin L(119883) with V(119879
0) = 1 and (119909
0 119909
lowast
0) isin Π(119883)
satisfying |119909lowast011987901199090| gt 1 minus 120578(120576) Define 119879
1isinL(119883) by
1198791119909 = 119879
0119909 + 120582
1(120576
4) 119909
lowast
0(119909) 119909
0(17)
for all 119909 isin 119883 where 1205821is the scalar satisfying |120582
1| = 1 and
|119909lowast
011987901199090+ 120582
1(1205764)| = |119909
lowast
011987901199090| + 1205764 Now choose 119909
1isin 119878
119883
and 119909lowast1isin 119878
119883lowast such that |119909lowast
1(119909
1)| = 1 119909lowast
1(119909
0) = |119909
lowast
1(119909
0)| and
1003816100381610038161003816119909lowast
111987911199091
1003816100381610038161003816 ⩾ V (1198791) minus 120578(
1205762
42) (18)
Now we define a sequence (119909119899 119909
lowast
119899 119879
119899) in 119878
119883times 119878
119883lowast times L(119883)
inductively Indeed suppose that we have a defined sequence(119909
119895 119909
lowast
119895 119879
119895) for 0 ⩽ 119895 ⩽ 119899 and let
119879119899+1119909 = 119879
119899119909 + 120582
119899+1
120576119899+1
4119899+1119909lowast
119899(119909) 119909
119899 (19)
Then choose 119909119899+1
isin 119878119883
and 119909lowast119899+1
isin 119878119883lowast such that
|119909lowast
119899+1(119909
119899+1)| = 1 and |119909lowast
119899+1(119909
119899)| = 119909
lowast
119899+1(119909
119899)
1003816100381610038161003816119909lowast
119899+1119879119899+1119909119899+1
1003816100381610038161003816 ⩾ V (119879119899+1) minus 120578(
120576119899+2
4119899+2) (20)
Notice that for all 119899 ⩾ 0 we have
1003817100381710038171003817119879119899+1 minus 1198791198991003817100381710038171003817 ⩽120576119899+1
4119899+1
1003816100381610038161003816V (119879119899+1) minus V (119879119899)1003816100381610038161003816 ⩽120576119899+1
4119899+1
(21)
This implies that (119879119899) is a Cauchy sequence and assume that
it converges to 119878 isinL(119883) Then we have
lim119899119879119899= 119878
10038171003817100381710038171198790 minus 1198781003817100381710038171003817 lt 120576
lim119899
1003816100381610038161003816119909lowast
119899119879119899119909119899
1003816100381610038161003816 = lim119899V (119879
119899) = V (119878)
(22)
We will show that both sequences (119909119899) and (119909lowast
119899) are Cauchy
From the definition we have
V (119879119899+1) minus 120578(
120576119899+2
4119899+2)
⩽1003816100381610038161003816119909
lowast
119899+1119879119899+1119909119899+1
1003816100381610038161003816
4 Abstract and Applied Analysis
⩽
100381610038161003816100381610038161003816100381610038161003816
119909lowast
119899+1119879119899119909119899+1+ 120582
119899+1
120576119899+1
4119899+1119909lowast
119899(119909
119899+1) 119909
lowast
119899+1(119909
119899)
100381610038161003816100381610038161003816100381610038161003816
⩽ V (119879119899) +120576119899+1
4119899+1119909lowast
119899+1(119909
119899)
V (119879119899+1)
⩾1003816100381610038161003816119909
lowast
119899119879119899+1119909119899
1003816100381610038161003816 =
100381610038161003816100381610038161003816100381610038161003816
119909lowast
119899119879119899119909119899+ 120582
119899+1
120576119899+1
4119899+1
100381610038161003816100381610038161003816100381610038161003816
=1003816100381610038161003816119909
lowast
119899119879119899119909119899
1003816100381610038161003816 +120576119899+1
4119899+1⩾ V (119879
119899) minus 120578(
120576119899+1
4119899+1) +120576119899+1
4119899+1
(23)
In summary we have
V (119879119899) +120576119899+1
4119899+1119909lowast
119899+1(119909
119899)
⩾ V (119879119899) minus 120578(
120576119899+1
4119899+1) +120576119899+1
4119899+1minus 120578(
120576119899+2
4119899+2)
(24)
Hence
119909lowast
119899+1(119909
119899) ⩾ 1 minus 2
4119899+1
120576119899+1120578(120576119899+1
4119899+1)
= 1 minus1
2min120575
119883(120576119899+1
4119899+2) 120575
119883lowast (120576119899+1
4119899+2)
1003817100381710038171003817100381710038171003817
119909119899+ 119909
119899+1
2
1003817100381710038171003817100381710038171003817⩾ 119909
lowast
119899+1(119909119899+ 119909
119899+1
2) ⩾ 1 minus 120575
119883(120576119899+1
4119899+2)
100381710038171003817100381710038171003817100381710038171003817
119909lowast
119899+ 119909
lowast
119899+1
2
100381710038171003817100381710038171003817100381710038171003817
⩾119909lowast
119899+ 119909
lowast
119899+1
2(119909
119899) ⩾ 1 minus 120575
119883lowast (120576119899+1
4119899+2)
(25)
This means that
1003817100381710038171003817119909119899 minus 119909119899+11003817100381710038171003817 ⩽ 120576
119899+14
119899+2
1003817100381710038171003817119909lowast
119899minus 119909
lowast
119899+1
1003817100381710038171003817 ⩽ 120576119899+14
119899+2
(26)
for all 119899 So (119909119899) and (119909lowast
119899) are Cauchy Let 119909
infin= lim
119899119909119899and
119909lowast
infin= lim
119899119909lowast
119899 Then 119909
0minus 119909
infin lt 1205764 and 119909lowast
0minus 119909
lowast
infin lt 1205764
Hence |119909lowastinfin(119909
infin)| = lim
119899|119909
lowast
119899(119909
119899)| = 1 and
V (119878) = lim119899V (119879
119899) = lim
119899
1003816100381610038161003816119909lowast
119899119879119899119909119899
1003816100381610038161003816 =1003816100381610038161003816119909
lowast
infin119878119909
infin
1003816100381610038161003816 (27)
Let 120572 = 119909lowastinfin(119909
infin) 119910lowast = 119909lowast
infin and 119910 = 119909
infin Then we have
119910lowast(119910) = 1 V(119878) = |119910lowast119878119910| and 119910 minus 119909
0 lt 120576 Notice that
|120572 minus 1| =1003816100381610038161003816119909
lowast
infin(119909
infin) minus 119909
lowast
0(119909
0)1003816100381610038161003816
⩽1003816100381610038161003816(119909
lowast
infinminus 119909
lowast
0) (119909
infin)1003816100381610038161003816 +1003816100381610038161003816119909
lowast
0(119909
infin) minus 119909
lowast
0(119909
0)1003816100381610038161003816 lt120576
2
(28)
Therefore
1003817100381710038171003817119910lowastminus 119909
lowast1003817100381710038171003817 ⩽1003817100381710038171003817119910
lowastminus 119910
lowast1003817100381710038171003817 +1003817100381710038171003817119910
lowastminus 119909
lowast1003817100381710038171003817 lt120576
2+120576
4lt 120576 (29)
This completes the proof
Let us discuss a little bit about the equivalence betweenthe property in the result above and the BPBp-nu Forconvenience let us introduce the following definition
Definition 5 A Banach space 119883 has the weak Bishop-Phelps-Bollobas property for the numerical radius (in short weak-BPBp-nu) if given 120576 gt 0 there exists 120578(120576) gt 0 such thatwhenever 119879
0isin L(119883) with V(119879
0) = 1 and (119909
0 119909
lowast
0) isin Π(119883)
satisfy |119909lowast011987901199090| gt 1 minus 120578(120576) there exist 119878 isin L(119883) and
(119910 119910lowast) isin Π(119883) such that
V (119878) = 1003816100381610038161003816119910lowast1198781199101003816100381610038161003816
1003817100381710038171003817119909 minus 1199101003817100381710038171003817 lt 120576
1003817100381710038171003817119909lowastminus 119910
lowast1003817100381710038171003817 lt 120576 119878 minus 119879 lt 120576
(30)
Notice that the only difference between this concept andthe BPBp-nu is the normalization of the operator 119878 by thenumerical radius Of course if the numerical radius andthe operator norm are equivalent these two properties arethe same This equivalence is measured by the so-callednumerical index of the Banach space as follows For a Banachspace119883 the numerical index of119883 is defined by
119899 (119883) = inf V (119879) 119879 isinL (119883) 119879 = 1 (31)
It is clear that 0 ⩽ 119899(119883) ⩽ 1 and 119899(119883)119879 ⩽ V(119879) ⩽ 119879for all 119879 isin L(119883) The value 119899(119883) = 1 means that V equalsthe usual operator norm This is the case of 119883 = 119871
1(120583) and
119883 = 119862(119870) among many others On the other hand 119899(119883) gt 0if and only if the numerical radius is equivalent to the normof L(119883) We refer the reader to [20] for more informationand background
The following result is immediate We include a proof forthe sake of completeness
Proposition 6 Let119883 be a Banach space with 119899(119883) gt 0 Then119883 has the BPBp-nu if and only if119883 has the weak-BPBp-nu
Proof Thenecessity is clear For the converse assume that wehave 120578(120576) gt 0 satisfying the conditions of the weak-BPBp-nufor all 0 lt 120576 lt 1 If 119879 isin L(119883) with V(119879) = 1 and (119909
0 119909
lowast
0) isin
Π(119883) satisfy |119909lowast0119879119909
0| gt 1 minus 120578(120576) for 0 lt 120576 lt 1 then there exist
119878 isinL(119883) and (119910 119910lowast) isin Π(119883) such that
V (119878) = 1003816100381610038161003816119910lowast1198781199101003816100381610038161003816 119878 minus 119879 lt 120576
1003817100381710038171003817119909 minus 1199101003817100381710038171003817 lt 120576
1003817100381710038171003817119909lowastminus 119910
lowast1003817100381710038171003817 lt 120576
(32)
As V(119878) gt 0 by the above let 1198781= (1V(119878))119878 Then we have
1 = V (1198781) =1003816100381610038161003816119910
lowast11987811199101003816100381610038161003816
1003817100381710038171003817119909 minus 1199101003817100381710038171003817 lt 120576
1003817100381710038171003817119909lowastminus 119910
lowast1003817100381710038171003817 lt 120576
(33)
Abstract and Applied Analysis 5
Finally we have
10038171003817100381710038171198781 minus 1198791003817100381710038171003817 ⩽
10038171003817100381710038171003817100381710038171003817
1
V (119878)119878 minus 119878
10038171003817100381710038171003817100381710038171003817
+ 119878 minus 119879
=119878
V (119878)|V (119878) minus 1| + 119878 minus 119879
⩽1
119899 (119883)|V (119878) minus V (119879)| + 119878 minus 119879
⩽ (1
119899 (119883)+ 1) 119878 minus 119879 lt
119899 (119883) + 1
119899 (119883)120576
(34)
An obvious change of parameters finishes the proof
We do not know whether the hypothesis of 119899(119883) gt 0 canbe omitted in the above result
Putting together Propositions 4 and 6 we get the follow-ing
Corollary 7 Let 119883 be a uniformly convex and uniformlysmooth Banach space with 119899(119883) gt 0 Then 119883 has the BPBp-nu
Let us comment that every complex Banach space 119883satisfies 119899(119883) ⩾ 1119890 so the above corollary automaticallyapplies in the complex case In the real case this is no longertrue as the numerical index of a Hilbert space of dimensiongreater than or equal to two is 0 On the other hand it isproved in [21] that real 119871
119901(120583) spaces have nonzero numerical
index for every measure 120583when 119901 = 2Therefore we have thefollowing examples
Example 8 (a) Complex Banach spaces which are uniformlysmooth and uniformly convex satisfy the BPBp-nu
(b) In particular for every measure 120583 the complex spaces119871119901(120583) have the BPBp-nu for 1 lt 119901 lt infin(c) For every measure 120583 the real spaces 119871
119901(120583) have the
BPBp-nu for 1 lt 119901 lt infin 119901 = 2
Note Added in Revision Very recently H J Lee M Martınand J Merı have proved that Proposition 6 can be extendedto some Banach spaces with numerical index zero as forinstance real Hilbert spaces Hence they have shown thatHilbert spaces have the BPBp-nu These results will appearelsewhere
4 L1
Spaces
In this section we will show that 1198711(120583) has the BPBp-nu for
every measure 120583 In the proof we are dealing with complexintegrable functions since the real case is followed easilyby applying the same proof Our main result here is thefollowing
Theorem 9 Let 120583 be a measure Then 1198711(120583) has the Bishop-
Phelps-Bollobas property for numerical radius More preciselygiven 120576 gt 0 there exists 120578(120576) gt 0 (which does not depend on120583) such that whenever 119879
0isin L(119871
1(120583)) with V(119879
0) = 1 and
(1198910 119892
0) isin Π(119871
1(120583)) satisfy |⟨119879
01198910 119892
0⟩| gt 1 minus 120578(120576) then there
exist 119879 isinL(1198711(120583)) (119891
1 119892
1) isin Π(119871
1(120583)) such that
1003816100381610038161003816⟨1198791198911 1198921⟩1003816100381610038161003816 = V (119879) = 1 10038171003817100381710038171198910 minus 1198911
1003817100381710038171003817 lt 120576
10038171003817100381710038171198920 minus 11989211003817100381710038171003817 lt 120576
1003817100381710038171003817119879 minus 11987901003817100381710038171003817 lt 120576
(35)
As a first step we have to start dealing with finite regularpositive Borel measures for which a representation theoremfor operators exists
Proposition 10 Let 119898 be a finite regular positive Borelmeasure on a compact Hausdorff spaceΩ Then 119871
1(119898) has the
Bishop-Phelps-Bollobas property for numerical radius Moreprecisely given 120576 gt 0 there is 120578(120576) gt 0 (which is independent ofthemeasure119898) such that if a norm-one element119879 inL(119871
1(119898))
and (1198910 119892
0) isin Π(119871
1(119898)) satisfy |⟨119879119891
0 119892
0⟩| gt 1 minus 120578(120576) then
there exist an operator 119878 isin L(1198711(119898)) (119891
1 119892
1) isin Π(119871
1(119898))
such that1003816100381610038161003816⟨1198781198911 1198921⟩
1003816100381610038161003816 = 119878 = 110038171003817100381710038171198910 minus 1198911
1003817100381710038171003817 ⩽ 120576
10038171003817100381710038171198920 minus 11989211003817100381710038171003817 ⩽ 120576 119879 minus 119878 ⩽ 120576
(36)
To prove this proposition we need some backgroundon representation of operators on Lebesgue spaces on finiteregular positive Borel measures and several preliminarylemmas
Let 119898 be a finite regular positive Borel measure on acompact Hausdorff space Ω If 120583 is a complex-valued Borelmeasure on the product space Ω times Ω then define theirmarginal measures 120583119894 on Ω (119894 = 1 2) as follows 1205831(119860) =120583(119860 times Ω) and 1205832(119861) = 120583(Ω times 119861) where 119860 and 119861 are Borelmeasurable subsets of Ω
Let 119872(119898) be the complex Banach lattice of measuresconsisting of all complex-valued Borel measures 120583 on theproduct spaceΩ timesΩ such that |120583|119894 are absolutely continuouswith respect to119898 for 119894 = 1 2 endowed with the norm
1003817100381710038171003817100381710038171003817100381710038171003817
11988910038161003816100381610038161205831003816100381610038161003816
1
119889119898
1003817100381710038171003817100381710038171003817100381710038171003817infin
(37)
Each 120583 isin 119872(119898) defines a bounded linear operator 119879120583from
1198711(119898) to itself by
⟨119879120583(119891) 119892⟩ = int
ΩtimesΩ
119891 (119909) 119892 (119910) 119889120583 (119909 119910) (38)
where 119891 isin 1198711(119898) and 119892 isin 119871
infin(119898) Iwanik [22] showed that
the mapping 120583 997891rarr 119879120583is a lattice isometric isomorphism from
119872(119898) onto L(1198711(119898)) Even though he showed this for the
real case it can be easily generalized to the complex caseFor details see [22 Theorem 1] and [23 IV Theorem 15(ii)Corollary 2]
We will also use that given an arbitrary measure 120583 every119879 isin L(119871
1(120583)) satisfies V(119879) = 119879 [24] (that is the space
1198711(120583) has numerical index 1)
Lemma 11 (see [15 Lemma 33]) Let 119888119899 be a sequence of
complex numbers with |119888119899| ⩽ 1 for every 119899 and let 120578 gt 0 such
6 Abstract and Applied Analysis
that for a convex series sum120572119899 Resuminfin
119899=1120572119899119888119899gt 1 minus 120578 Then for
every 0 lt 119903 lt 1 the set 119860 = 119894 isin N Re 119888119894gt 119903 satisfies the
estimate
sum
119894isin119860
120572119894⩾ 1 minus
120578
1 minus 119903 (39)
From now on 119898 will be a finite regular positive Borelmeasure on the compact Hausdorff space Ω
Lemma 12 Suppose that there exist a nonnegative simplefunction 119891 isin 119878
1198711(119898)
and a function 119892 isin 119878119871infin(119898)
such that
Re ⟨119891 119892⟩ gt 1 minus 1205763
16 (40)
Then there exist a nonnegative simple function 1198911isin 119878
1198711(119898)
anda function 119892
1isin 119878
119871infin(119898)
such that
1198921(119909) = 120594supp(119891
1)(119909) + 119892 (119909) 120594
Ωsupp(1198911)(119909)
⟨1198911 119892
1⟩ = 1
1003817100381710038171003817119891 minus 119891110038171003817100381710038171lt 120576
1003817100381710038171003817119892 minus 11989211003817100381710038171003817infinlt radic120576 supp (119891
1) sub supp (119891)
(41)
Proof Let119891 = sum119898
119895=1(120573
119895119898(119861
119895))120594
119861119895
for some (120573119895) such that
120573119895⩾ 0 for all 119895 andsum119898
119895=1120573119895= 1 and119861
119895rsquos aremutually disjoint
By the assumption we have
Re ⟨119891 119892⟩ =119899
sum
119895=1
120573119895
1
119898 (119861119895)
int119861119895
Re119892 (119909) 119889119898 (119909) gt 1 minus 1205763
16
(42)
and letting
119869 = 119895 1 ⩽ 119895 ⩽ 1198991
119898 (119861119895)
int119861119895
Re119892 (119909) 119889119898 (119909) gt 1 minus 1205762
4
(43)
we have by Lemma 11
sum
119895isin119869
120573119895gt 1 minus
120576
4 (44)
For each 119895 isin 119869 we have
1 minus1205762
4lt
1
119898 (119861119895)
int119861119895
Re119892 (119909) 119889119898 (119909)
=1
119898 (119861119895)
int119861119895capRe119892⩽1minus120576
Re119892 (119909) 119889119898 (119909)
+ int119861119895capRe119892gt1minus120576
Re119892 (119909) 119889119898 (119909)
⩽1
119898 (119861119895)
((1 minus 120576)119898 (119861119895cap Re119892 ⩽ 1 minus 120576)
+ 119898 (119861119895cap Re119892 gt 1 minus 120576))
= 1 minus 120576
119898 (119861119895cap Re119892 ⩽ 1 minus 120576)
119898 (119861119895)
(45)
This implies that
119898(119861119895cap Re119892 ⩽ 1 minus 120576)
119898 (119861119895)
lt120576
4 (46)
Define 119861119895= 119861
119895cap Re119892 gt 1 minus 120576 for all 119895 isin 119869
1198911= (1sum
119895isin119869
120573119895)sum
119895isin119869
120573119895(120594
119861119895
119898 (119861119895)) (47)
and 1198921(119909) = 1 on supp(119891
1) and 119892
1(119909) = 119892(119909) elsewhereThen
it is clear that supp(1198911) sub supp(119891) 119892 minus 119892
1infinlt radic120576 and
⟨1198911 119892
1⟩ = 1 Finally we will show that 119891 minus 119891
1 lt 120576 Notice
first that10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
⩽
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
= 2sum
119895isin119869
120573119895
119898(119861119895 119861
119895)
119898 (119861119895)
lt120576
2
(48)
Hence1003817100381710038171003817119891 minus 1198911
1003817100381710038171003817
⩽
10038171003817100381710038171003817100381710038171003817100381710038171003817
1
sum119895isin119869120573119895
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus 119891
10038171003817100381710038171003817100381710038171003817100381710038171003817
⩽
1 minus sum119895isin119869120573119895
sum119895isin119869120573119895
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+120576
4
= (1 minussum
119895isin119869
120573119895) +
120576
2+120576
4
⩽120576
4+120576
2+120576
4= 120576
(49)
Lemma 13 (see [25 Lemma 33]) Suppose that 119879120583is a norm-
one element in L(1198711(119898)) for some 120583 isin 119872(119898) and there is
a nonnegative simple function 1198910such that 119891
0is a norm-one
element of 1198711(119898) and 119879
1205831198910 ⩾ 1 minus 120576
32
6 for some 0 lt 120576 lt 1
Abstract and Applied Analysis 7
Then there exist a norm-one bounded linear operator 119879] forsome ] isin 119872(119898119898) and a nonnegative simple function 119891
1
in 1198781198711(119898)
such that 119879120583minus 119879] ⩽ 120576 1198911 minus 1198910 ⩽ 3120576 and
(119889|]|1119889119898)(119909) = 1 for all 119909 isin supp(1198911)
Lemma 14 Suppose that 119879] isin L(1198711(119898)) is a norm-one
operator 119891 = sum119899
119894=1120573119894(120594
119861119894
119898(119861119894)) where 119898(119861
119895) gt 0 for all
1 ⩽ 119895 ⩽ 119899 and 119861119895119899
119895=1are mutually disjoint Borel subsets ofΩ
is a norm-one nonnegative simple function and 119892 is an elementof 119878
119871infin(119898)
such that
Re ⟨119892 119879]119891⟩ ⩾ 1 minus1205766
27(50)
for some 0 lt 120576 lt 1 and
119889|]|1
119889119898(119909) = 1 119892 (119909) = 1 (51)
for all x in the support of 119891Then there exist a nonnegative simple function 119891 isin 119878
1198711(119898)
a function119892 isin 119878
119871infin(119898)
and an operator119879] inL(1198711(119898) 1198711(119898))such that
⟨119892 119879]119891⟩ =1003817100381710038171003817119879]1003817100381710038171003817 = 1
1003817100381710038171003817119879] minus 119879]1003817100381710038171003817 ⩽ 2120576
10038171003817100381710038171003817119891 minus 119891
10038171003817100381710038171003817⩽ 3120576
1003817100381710038171003817119892 minus 1198921003817100381710038171003817 ⩽radic120576 ⟨119891 119892⟩ = 1
(52)
Proof Since
Re ⟨119892 119879]119891⟩ ⩾ 1 minus1205766
27 (53)
we have
1 minus1205766
27lt Re ⟨119892 119879]119891⟩
= intΩtimesΩ
119891 (119909)Re119892 (119910) 119889120583 (119909 119910)
=
119899
sum
119895=1
120573119895intΩtimesΩ
120594119861119895
(119909)
119898 (119861119895)
Re119892 (119910) 119889] (119909 119910)
(54)
Let
119869 = 119895 intΩtimesΩ
(
120594119861119895
(119909)
119898 (119861119895)
)Re119892 (119910) 119889] (119909 119910) gt 1 minus 1205763
26
(55)
Then from Lemma 11 we have sum119895isin119869120573119895gt 1 minus 120576
32 Let 119891
1=
sum119895isin119869120573119895(120594
119861119895
119898(119861119895)) where 120573
119895= 120573
119895(sum
119895isin119869120573119895) for all 119895 isin 119869
Then
10038171003817100381710038171198911 minus 1198911003817100381710038171003817 ⩽
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
(120573119895minus 120573
119895)
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+ sum
119895isin119869
120573119895⩽ 120576
3⩽ 120576 (56)
Note that there is a Borel measurable function ℎ on Ω times Ωsuch that 119889](119909 119910) = ℎ(119909 119910) 119889|]|(119909 119910) and |ℎ(119909 119910)| = 1 forall (119909 119910) isin Ω times Ω Let
119862 = (119909 119910) 1003816100381610038161003816119892 (119910) ℎ (119909 119910) minus 1
1003816100381610038161003816 ltradic120576
232 (57)
Define two measures ]119891and ]
119888as follows
]119891(119860) = ] (119860 119862) ]
119888(119860) = ] (119860 cap 119862) (58)
for every Borel subset 119860 ofΩ times Ω It is clear that
119889] = 119889]119891+ 119889]
119888 119889
10038161003816100381610038161003816]119891
10038161003816100381610038161003816= ℎ 119889]
119891
1198891003816100381610038161003816]1198881003816100381610038161003816 = ℎ 119889]119888 119889 |]| = 119889
10038161003816100381610038161003816]119891
10038161003816100381610038161003816+ 1198891003816100381610038161003816]1198881003816100381610038161003816
(59)
Since (119889|]|11198891198981)(119909) = 1 for all 119909 isin ⋃119899
119895=1119861119895 we have
1 =119889|]|1
1198891198981
(119909) =
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909) +1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909) (60)
for all 119909 isin 119861 = ⋃119899
119895=1119861119895 and we deduce that |]|1(119861
119895) = 119898
1(119861
119895)
for all 1 ⩽ 119895 ⩽ 119899We claim that |]
119891|1(119861
119895)119898
1(119861
119895) ⩽ 120576
22
2 for all 119895 isin 119869Indeed if |119892(119910)ℎ(119909 119910) minus 1| ⩾ radic120576232 then
Re (119892 (119910) ℎ (119909 119910)) ⩽ 1 minus 12057624 (61)
So we have
1 minus1205763
26
⩽1
1198981(119861
119895)
ReintΩtimesΩ
120594119861119895(119909)119892 (119910) 119889] (119909 119910)
=1
1198981(119861
119895)
intΩtimesΩ
120594119861119895(119909)
Re (119892 (119910) ℎ (119909 119910)) 119889 |]| (119909 119910)
=1
1198981(119861
119895)
intΩtimesΩ
120594119861119895(119909)
Re (119892 (119910) ℎ (119909 119910)) 119889 10038161003816100381610038161003816]11989110038161003816100381610038161003816(119909 119910)
+1
1198981(119861
119895)
intΩtimesΩ
120594119861119895(119909)
Re (119892 (119910) ℎ (119909 119910)) 119889 1003816100381610038161003816]1198881003816100381610038161003816 (119909 119910)
⩽1
1198981(119861
119895)
((1 minus120576
24)10038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
(119861119895) +1003816100381610038161003816]1198881003816100381610038161003816
1
(119861119895))
= 1 minus120576
24
10038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
(119861119895)
1198981(119861
119895)
(62)
This proves our claimWe also claim that for each 119895 isin 119869 there exists a Borel
subset 119861119895of 119861
119895such that
(1 minus120576
2)119898
1(119861
119895) ⩽ 119898
1(119861
119895) ⩽ 119898
1(119861
119895)
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909) ⩽120576
2
(63)
8 Abstract and Applied Analysis
for all 119909 isin 119861119895 Indeed set 119861
119895= 119861
119895cap 119909 isin Ω (119889|]
119891|1119889119898
1)
(119909) ⩽ 1205762 Then
int119861119895119861119895
120576
2119889119898
1(119909) ⩽ int
119861119895
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909) 1198891198981(119909)
=10038161003816100381610038161003816]1119891
10038161003816100381610038161003816(119861
119895) ⩽1205762
22119898
1(119861
119895)
(64)
This shows that 1198981(119861
119895 119861
119895) ⩽ (1205762)119898
1(119861
119895) This proves our
second claimNow we define 119892 by 119892(119910) = 119892(119910)|119892(119910)| if |119892(119910)| ⩾ 1 minus
radic120576232 and 119892(119910) = 119892(119910) if |119892(119910)| lt 1 minus radic120576232 and we
write 119891 = sum119895isin119869120573119895(120594
119861119895
1198981(119861
119895)) It is clear that 119892 isin 119878
119871infin(119898)
119892 minus 119892 lt radic120576 and 119892(119910) = 1 for all 119909 isin supp119891
Finally we define the measure
119889] (119909 119910)
= sum
119895isin119869
120594119861119895
(119909) 119892 (119910)ℎ (119909 119910)119889]119888(119909 119910)(
1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+ 1205941198691119861(119909) 119889] (119909 119910)
(65)
where 119861 = ⋃119895isin119869119861119895 It is easy to see that (119889|]|1119889119898
1)(119909) = 1
on 119861 and (119889|]|11198891198981)(119909) ⩽ 1 elsewhere Note that
119889 (] minus ]) (119909 119910)
= sum
119895isin119869
120594119861119895
(119909) [
[
119892 (119910)ℎ (119909 119910)(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1]
]
119889]119888(119909 119910)
minus sum
119895isin119869
120594119861119895
(119909) 119889]119891(119909 119910)
(66)
If (119909 119910) isin 119862 then |119892(119910)| ⩾ 1 minus radic120576232 ⩾ 1 minus 1232 and
100381610038161003816100381610038161003816119892 (119910)ℎ (119909 119910) minus 1
100381610038161003816100381610038161003816
=
100381610038161003816100381610038161003816100381610038161003816
119892 (119910)
1003816100381610038161003816119892 (119910)1003816100381610038161003816
ℎ (119909 119910) minus 1
100381610038161003816100381610038161003816100381610038161003816
⩽
1003816100381610038161003816119892 (119910) ℎ (119909 119910) minus 11003816100381610038161003816
1003816100381610038161003816119892 (119910)1003816100381610038161003816
+
10038161003816100381610038161 minus1003816100381610038161003816119892 (119910)
1003816100381610038161003816
10038161003816100381610038161003816100381610038161003816119892 (119910)
1003816100381610038161003816
⩽ 2
1003816100381610038161003816119892 (119910) ℎ (119909 119910) minus 11003816100381610038161003816
1003816100381610038161003816119892 (119910)1003816100381610038161003816
⩽ 2radic120576
232
232
232 minus 1⩽ 2radic120576
(67)
Hence for all (119909 119910) isin 119862 we have
10038161003816100381610038161003816100381610038161003816100381610038161003816
119892 (119910)ℎ (119909 119910)(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
⩽100381610038161003816100381610038161003816119892 (119910)ℎ (119909 119910) minus 1
100381610038161003816100381610038161003816(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+
10038161003816100381610038161003816100381610038161003816100381610038161003816
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
⩽ 2radic120576(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+
10038161003816100381610038161003816100381610038161003816100381610038161003816
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
(68)
So we have for all 119909 isin 1198691
119889|] minus ]|1
1198891198981
(119909)
⩽ sum
119895isin119869
120594119861119895
(119909) [
[
2radic120576(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+
10038161003816100381610038161003816100381610038161003816100381610038161003816
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
]
]
1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909)
+ sum
119895isin119869
120594119861119895
(119909)
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909)
⩽ sum
119895isin119869
120594119861119895
(119909)(2radic120576 + (1 minus1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909)))
+sum
119895isin119869
120594119861119895
(119909)(
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909))
⩽ 2radic120576 + 120576 lt 3radic120576
(69)
This gives that 119879] minus 119879] lt 3radic120576 Note also that for all 119895 isin 119869
⟨119879]
120594119861119895
1198981(119861
119895)
119892⟩
= intΩtimesΩ
120594119861119895
(119909)
1198981(119861
119895)
119892 (119910) 119889] (119909 119910)
= intΩtimesΩ
120594119861119895
(119909)
1198981(119861
119895)
ℎ (119909 119910)(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
119889]119888(119909 119910)
Abstract and Applied Analysis 9
= intΩ
120594119861119895
(119909)
1198981(119861
119895)
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
1198891003816100381610038161003816]1198881003816100381610038161003816
1
(119909)
= intΩ
120594119861119895
(119909)
1198981(119861
119895)
1198891198981(119909) = 1
(70)
Hence we get ⟨119879]119891 119892⟩ = 1 which implies that 119879]119891 =119879] = 1 Finally10038171003817100381710038171003817119891 minus 119891
10038171003817100381710038171003817
⩽10038171003817100381710038171003817119891 minus 119891
1
10038171003817100381710038171003817+10038171003817100381710038171198911 minus 119891
1003817100381710038171003817
=
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
1198981(119861
119895)
minus sum
119895isin119869
120573119895
120594119861119895
1198981(119861
119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+ 120576
⩽ sum
119895isin119869
120573119895(
10038171003817100381710038171003817100381710038171003817100381710038171003817
120594119861119895
1198981(119861
119895)
minus
120594119861119895
1198981(119861
119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+
10038171003817100381710038171003817100381710038171003817100381710038171003817
120594119861119895
1198981(119861
119895)
minus
120594119861119895
1198981(119861
119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
) + 120576
= 2sum
119895isin119869
120573119895
1198981(119861
119895 119861
119895)
1198981(119861
119895)
+ 120576
⩽ 2sum
119895isin119869
120573119895
(1205762)1198981(119861
119895)
1198981(119861
119895)
+ 120576 ⩽120576
1 minus 1205762+ 120576 lt 3120576
(71)
We are now ready to present the proof of the main resultin the case of finite regular positive Borel measures
Proof of Proposition 10 Let 1205751= 120575
3
2(5sdot2
4) 120575
2= 120575
12
3(3
2sdot2
14)
and 1205753= (12057610)
2 for some 0 lt 120576 lt 1 Suppose that 119879 isinL(119871
1(119898)) with 119879 = 1 and that there is an 119891
0isin 119878
1198711(119898)
and1198920isin 119878
119871infin(119898)
such that ⟨1198910 119892
0⟩ = 1 and |⟨119879119891
0 119892
0⟩| gt 1minus120575
3
12
6Then there is an isometric isomorphism Ψ from 119871
1(119898) onto
itself such thatΨ(1198910) = |119891
0| and there is a scalar number 120572 in
119878R such that |⟨1198791198910 119892
0⟩| = ⟨120572119879119891
0 119892
0⟩ Then letting 119891
1= Ψ119891
0
1198921= (Ψ
minus1)lowast1198920 and 119879
1= 120572Ψ119879Ψ
minus1 we have
⟨1198781198911 119892
1⟩ = ⟨120572Ψ119879
0Ψ
minus1Ψ119891
0 (Ψ
minus1)lowast
1198920⟩
= ⟨1205721198791198910 119892
0⟩ gt 1 minus
1205753
1
26
⟨1198911 119892
1⟩ = ⟨Ψ119891
0 (Ψ
minus1)lowast
1198920⟩ = 1
(72)
Since 11987911198911 gt 1 minus 120575(120575
3
12
6) by Lemma 13 there exists a
norm-one bounded operator 119879] and a nonnegative simplefunction119891
2isin 119878
1198711(119898)
such that 1198791minus119879] ⩽ 1205751 1198912minus1198911 ⩽ 31205751
and (119889|]|11198891198981)(119909) = 1 for all 119909 isin supp(119891
2) Then
⟨119879]1198912 1198921⟩
= ⟨11987911198911 119892
1⟩ minus ⟨119879
11198911minus 119879
11198912 119892
1⟩ minus ⟨119879
11198912minus 119879]1198912 1198921⟩
⩾ ⟨11987911198911 119892
1⟩ minus10038171003817100381710038171198911 minus 1198912
1003817100381710038171003817 minus10038171003817100381710038171198791 minus 119879]
1003817100381710038171003817
⩾ 1 minus1205753
1
26minus 3120575
1minus 120575
1⩾ 1 minus 5120575
1= 1 minus
1205753
2
16
(73)
Notice also that
⟨1198912 119892
1⟩ = ⟨119891
1 119892
1⟩ minus ⟨119891
1minus 119891
2 119892
1⟩ ⩾ 1 minus
10038171003817100381710038171198911 minus 11989121003817100381710038171003817
⩾ 1 minus 31205751⩾ 1 minus 5120575
1= 1 minus
1205753
2
16
(74)
By Lemma 12 there are a nonnegative simple function 1198913isin
1198781198711(119898)
and a function 1198923isin 119878
119871infin(119898)
such that
1198923(119909) = 120594supp119891
3
(119909) + 1198922(119909) 120594
Ωsupp1198913
(119909)
10038171003817100381710038171198912 minus 11989131003817100381710038171003817 ⩽ 1205752
10038171003817100381710038171198923 minus 11989211003817100381710038171003817 ⩽radic120575
2 ⟨119891
3 119892
3⟩ = 1
(75)
So we have
⟨119879]1198913 1198923⟩ = ⟨119879]1198912 1198921⟩ minus ⟨119879]1198912 minus 119879]1198913 1198921⟩
minus ⟨119879]1198913 1198921 minus 1198923⟩
⩾ 1 minus1205753
2
16minus 2radic120575
2⩾ 1 minus 3radic120575
2= 1 minus
1205756
3
27
(76)
By Lemma 14 there exist 1198914isin 119878
1198711(119898)
and 1198924isin 119878
119871infin(119898)
and anoperator 119879
4such that
⟨1198924 119879
41198914⟩ = 1 =
100381710038171003817100381711987941003817100381710038171003817
10038171003817100381710038171198794 minus 119879]1003817100381710038171003817 ⩽ 21205753
10038171003817100381710038171198914 minus 11989131003817100381710038171003817 ⩽ 31205753
10038171003817100381710038171198924 minus 11989231003817100381710038171003817 ⩽radic120575
3 ⟨119891
4 119892
4⟩ = 1
(77)
So we have
10038171003817100381710038171198794 minus 11987911003817100381710038171003817 ⩽10038171003817100381710038171198794 minus 119879]
1003817100381710038171003817 +1003817100381710038171003817119879] minus 1198791
1003817100381710038171003817
⩽ 1205751+ 2120575
3⩽ 3120575
3
10038171003817100381710038171198911 minus 11989141003817100381710038171003817 ⩽10038171003817100381710038171198911 minus 1198912
1003817100381710038171003817 +10038171003817100381710038171198912 minus 1198913
1003817100381710038171003817 +10038171003817100381710038171198913 minus 1198914
1003817100381710038171003817
⩽ 31205751+ 120575
2+ 3120575
3⩽ 10120575
3
10038171003817100381710038171198921 minus 11989241003817100381710038171003817 ⩽10038171003817100381710038171198921 minus 1198923
1003817100381710038171003817 + 1198923 minus 1198924
⩽ 1205752+ radic120575
3⩽ 2radic120575
3
(78)
10 Abstract and Applied Analysis
Let 119878 = 120572Ψminus11198794Ψ 119891 = Ψminus1
1198914 and 119892 = Ψlowast
1198924 then we have
119879 minus 119878 =10038171003817100381710038171003817119879 minus 120572Ψ
minus11198794Ψ10038171003817100381710038171003817=10038171003817100381710038171003817120572Ψ119879Ψ
minus1minus 119879
4
10038171003817100381710038171003817
=10038171003817100381710038171198791 minus 1198794
1003817100381710038171003817 ⩽ 31205753
100381710038171003817100381710038171198910minus 11989110038171003817100381710038171003817=100381710038171003817100381710038171198910minus Ψ
minus11198914
10038171003817100381710038171003817=10038171003817100381710038171198911 minus 1198914
1003817100381710038171003817 ⩽ 101205753
100381710038171003817100381710038171198920minus 11989110038171003817100381710038171003817=10038171003817100381710038171198920 minus Ψ
lowast1198924
1003817100381710038171003817 =100381710038171003817100381710038171003817(Ψ
minus1)lowast
1198920minus 119892
4
100381710038171003817100381710038171003817
=10038171003817100381710038171198921 minus 1198924
1003817100381710038171003817 ⩽ 2radic120575
3
⟨119891 119892⟩ = ⟨Ψminus11198914 Ψ
lowast1198924⟩ = ⟨119891
4 119892
4⟩ = 1
10038161003816100381610038161003816⟨119878119891 119892⟩
10038161003816100381610038161003816=10038161003816100381610038161003816⟨120572Ψ
minus11198794ΨΨ
minus11198914 Ψ
lowast1198924⟩10038161003816100381610038161003816= |120572| = 1
(79)
This completes the proof
Finally we may give the proof of the main result in fullgenerality
Proof of Theorem 9 Notice that the Kakutani representationtheorem (see [26] for a reference) says that for every 120590-finitemeasure ] the space 119871
1(]) is isometrically isomorphic to
1198711(119898) for some positive Borel regular measure on a compact
Hausdorff space Then by Proposition 10 there is a universalfunction 120576 997891rarr 120578(120576) gt 0 which gives the BPBp-nu for 119871
1(]) for
every 120590-finite measure ]Fix 120576 gt 0 Suppose that 119879
0isin L(119871
1(120583)) with V(119879
0) = 1
and (1198910 119891
lowast
0) isin Π(119871
1(120583)) satisfy
1003816100381610038161003816⟨119891lowast
0 119879
01198910⟩1003816100381610038161003816 gt 1 minus 120578 (120576) (80)
Choose a sequence 119891119899 in 119871
1(120583) such that sup
119899119879
0119891119899 = 1
and let 119866 be the closed linear span of
119879119899
0119891119898 119899 119898 isin N cup 0 (81)
As 119866 is separable there is a dense subset 119892119899 119899 isin N of 119866
and let 119864 = ⋃infin
119899=1supp119892
119899 where supp119892
119899is the support of 119892
119899
Then the measure 120583|119864is 120590-finite Let
119884 = 119891 isin 1198711(120583) supp (119891) sub 119864 (82)
be a closed subspace of 1198711(120583) It is clear that 119871
1(120583) = 119884oplus
1119885
and 119884 is isometrically isomorphic to 1198711(120583|
119864) So 119884 has the
BPBp-nu with 120578(120576)Now write 119878
0= 119879
0|119884 119884 rarr 119884 consider 119910
0= 119891
0isin
119878119884 119910lowast
0= 119891
lowast
0|119884isin 119878
119884lowast and observe that 119910lowast
0(119910
0) = 1 and
|119910lowast
0(119878
01199100)| = |119891
lowast
0(119879
01198910)| gt 1 minus 120578(120576) Hence there exist 119878 isin
L(119884) and (1199100 119910
lowast
0) isin Π(119884) such that
1003816100381610038161003816119910lowast
0(119878119910
0)1003816100381610038161003816 = 1 = V (119878) 1003817100381710038171003817119878 minus 1198780
1003817100381710038171003817 lt 120576
10038171003817100381710038171199100 minus 11991001003817100381710038171003817 lt 120576
1003817100381710038171003817119910lowast
0minus 119910
lowast
0
1003817100381710038171003817 lt 120576
(83)
Finally consider the operator 119879 isinL(1198711(120583)) given by
119879 (119910 119911)
= (119878119910 0) + 1198790(0 119911) ((119910 119911) isin 119871
1(120583) equiv 119884oplus
1119885)
(84)
We have 119879 = 1 (and so V(119879) = 1) Indeed1003817100381710038171003817119879 (119910 119911)
1003817100381710038171003817 =1003817100381710038171003817(119878119910 0)
1003817100381710038171003817 +10038171003817100381710038171198790 (0 119911)
1003817100381710038171003817 ⩽10038171003817100381710038171199101003817100381710038171003817 + 119911 =
1003817100381710038171003817(119910 119911)1003817100381710038171003817
(85)
for all (119910 119911) isin 1198711(120583) and 119879(119910
0 0) = (119878119910
0 0) = 119878119910
0 = 1
Let 119909 = (1199100 0) and 119909lowast = (119910lowast
0 119891
0|119885) Then (119909 119909lowast) isin Π(119871
1(120583))
Moreover we have1003816100381610038161003816119909
lowast1198791199091003816100381610038161003816 =1003816100381610038161003816119910
lowast
0119878119910
0
1003816100381610038161003816 = 1 = V (119879) 1003817100381710038171003817119909 minus 1198910
1003817100381710038171003817 =1003817100381710038171003817119910 minus 1199100
1003817100381710038171003817 lt 120576
1003817100381710038171003817119909lowast
0minus 119891
lowast
0
1003817100381710038171003817
= max 1003817100381710038171003817119910 minus 119891lowast
0|119884
1003817100381710038171003817 1003817100381710038171003817119891
lowast
0|119885minus 119891
lowast
0|119885
1003817100381710038171003817
=1003817100381710038171003817119910
lowastminus 119910
lowast
0
1003817100381710038171003817 lt 120576
1003817100381710038171003817119879 minus 11987901003817100381710038171003817 = sup119910+119911⩽1
1003817100381710038171003817119879 (119910 119911) minus 1198790 (119910 119911)1003817100381710038171003817
= sup119910⩽1
1003817100381710038171003817119878119910 minus 11987801199101003817100381710038171003817 =1003817100381710038171003817119878 minus 1198780
1003817100381710038171003817 lt 120576
(86)
This completes the proof
5 Examples of Spaces Failing theBishop-Phelps-Bollobaacutes Property forNumerical Radius
Our goal here is to prove that the density of numerical radiusattaining operators does not imply the BPBp-nu Actuallywe will show that among separable spaces there is noisomorphic property implying the BPBp-nu other than finite-dimensionality
We need to relate the BPBp-nu to the Bishop-Phelps-Bollobas property for operators which as mentioned in theintroduction was introduced in [15] A pair (119883 119884) of Banachspaces has the Bishop-Phelps-Bollobas property for operators(in short BPBp) if given 120576 gt 0 there exists 120578(120576) gt 0 suchthat given 119879 isin L(119883 119884) with 119879 = 1 and 119909 isin 119878
119883such that
119879119909 gt 1 minus 120578(120576) then there exist 119911 isin 119878119883and 119878 isin L(119883 119884)
satisfying
119878 = 119878119911 = 1 119909 minus 119911 lt 120576
119879 minus 119878 lt 120576
(87)
We refer the reader to [15 19 25] and references therein formore information and background Among the interestingresults on the BPBp we emphasize that a pair (119883 119884) when119883is finite-dimensional does not necessarily have the BPBp Forinstance if119884 is a strictly convex space which is not uniformlyconvex then the pair (ℓ(2)
1 119884) fails to have the BPBp (this is
contained in [15] see [19 Section 3])
Abstract and Applied Analysis 11
The next result relates the BPBp-nu to the BPBp foroperators in a particular case We will deduce our examplefrom it
Theorem 15 If 1198711(120583)oplus
1119883 has the BPBp-119899119906 then the pair
(1198711(120583) 119883) has the BPBp for operators
Before proving this proposition we will use it to get themain examples of this section The first example shows thatthe density of numerical radius attaining operators does notimply the BPBp-nu
Example 16 There is a reflexive space (and so numerical radiusattaining operators on it are dense) which fails to have theBPBp-nu Indeed let 119884 be a reflexive separable space whichis not superreflexive and we may suppose that 119884 is strictlyconvex Observe that 119884 cannot be uniformly convex since itis not superreflexive Now 119883 = ℓ(2)
1oplus1119884 is reflexive but the
pair (ℓ(2)1 119884) fails the BPBp since 119884 is strictly convex but not
uniformly convex [19 Corollary 33] Therefore Theorem 15gives us that119883 does not have the BPBp-nu
The example above can be extended to get the result thatevery infinite-dimensional separable Banach space can berenormed to fail the BPBp-nu This follows from the factthat every infinite-dimensional separable Banach space canbe renormed to be strictly convex but not uniformly convex(this result can be proved ldquoby handrdquo an alternative categoricalargument for it can be found in [27] and references therein)With a little more of effort we may get the main result of thesection
Theorem 17 Every infinite-dimensional separable Banachspace can be renormed to fail the weak-BPBp-nu (and so inparticular to fail the BPBp-nu)
We need the following result which is surely well knownAs we have not found a reference we include a nice and easyproof kindly given to us by Vladimir Kadets We recall thatgiven a Banach space 119884 the set of all equivalent norms on119884 can be viewed as a metric space using the Banach-Mazurdistance
Lemma 18 Let119884 be an infinite-dimensional separable Banachspace Then the set of equivalent norms on 119884 which are strictlyconvex and are not (locally) uniformly convex is dense in the setof all equivalent norms on119884 (with respect to the Banach-Mazurdistance)
Proof Fix 119890 isin 119878119884and 119890lowast
1isin 119878
119884lowast such that 119890lowast
1(119890) = 1 For a fixed
120576 isin (0 12) denote
119902 (119910) = max (1 minus 120576) 10038171003817100381710038171199101003817100381710038171003817 1003816100381610038161003816119890lowast
1(119910)1003816100381610038161003816 (119910 isin 119884) (88)
Evidently (1 minus 120576)119910 ⩽ 119902(119910) ⩽ 119910 for every 119910 isin 119884 Fix asequence 119890lowast
119896 119896 ⩾ 2 of norm-one functionals separating
the points of 119884 and denote
119901 (119910) = radic
infin
sum
119896=1
1
2119896
1003816100381610038161003816119890lowast
119896(119910)1003816100381610038161003816
2
(119910 isin 119884) (89)
Then 119901 is a strictly convex norm on 119884 119901(119890) ⩾ 1radic2 and119901(119910) ⩽ 119910 for all 119910 isin 119883 Finally write
100381710038171003817100381711991010038171003817100381710038171= (1 minus 120576) 119902 (119910) + 120576
119901 (119910)
119901 (119890)(119910 isin 119884) (90)
Then sdot 1is a strictly convex norm on 119884 and
(1 minus 120576)2 10038171003817100381710038171199101003817100381710038171003817 ⩽100381710038171003817100381711991010038171003817100381710038171⩽ (1 + 120576)
10038171003817100381710038171199101003817100381710038171003817 (119910 isin 119884) (91)
Wewill finish the proof by showing that sdot 1is not uniformly
convex (actually it is not locally uniformly convex) Indeedfor each 119899 isin N we select 119910
119899isin ⋂
119899
119896=1ker 119890lowast
119896with 119910
119899 = 1 and
consider 119890119899= 119890 + (1205764)119910
119899 Then 119902(119890) = 1 119902(119890
119899) = 1 and
119902(119890+119890119899) = 2 At the same time 119901(119910
119899) rarr 0 so 119901(119890
119899) rarr 119901(119890)
and 119901(119890 + 119890119899) rarr 2119901(119890) Consequently
1198901 = 1100381710038171003817100381711989011989910038171003817100381710038171997888rarr 1
1003817100381710038171003817119890 + 11989011989910038171003817100381710038171997888rarr 2 (92)
but 119890 minus 1198901198991= (1205764)119910
1198991⩾ (1 minus 120576)
2(1205764) which means the
absence of local uniform convexity at 119890
Proof of Theorem 17 Let 119883 be an infinite-dimensional sepa-rable Banach space Take a closed subspace 119884 of 119883 of codi-mension two By [28 Proposition 2] the map carrying everyequivalent norm on 119884 to its numerical index is continuousand so the set of values of the numerical index of 119884 upto reforming is a nontrivial interval [28 Theorem 9] ThenLemma 18 allows us to find an equivalent norm | sdot | on 119884 insuch a way that (119884 | sdot |) is strictly convex and is not uniformlyconvex and 119899(119884 | sdot |) gt 0 Now the space 119883 = ℓ(2)
1oplus1(119884 | sdot |)
is an equivalent renorming of 119883 which does not have theBPBp-119899119906 (indeed otherwise the pair (ℓ(2)
1 (119884 | sdot |)) would
have the BPBp for the operator norm and so (119884 | sdot |) wouldbe uniformly convex by [19 Corollary 33] a contradiction)Moreover as
119899 (119883) = min 119899 (ℓ(2)1) 119899 (119884 |sdot|) gt 0 (93)
(see [20 Proposition 2] for instance) 119883 also fails the weak-BPBp-nu by Proposition 6
To finish the section with the promised proof ofTheorem 15 we first see the following stability result
Lemma 19 Let119883 = [⨁infin
119896=1119883
119896]1198880
or [⨁infin
119896=1119883
119896]ℓ1
If119883 has theBishop-Phelps-Bollobas property for numerical radius with afunction 120578 then each Banach space 119883
119894has the Bishop-Phelps-
Bollobas property for numerical radius with 120578nu(119883119894) ⩾ 120578 That
is inf119894120578nu(119883119894
)(120576) ⩾ 120578nu(119883)(120576) for all 0 lt 120576 lt 1
Proof Let 119875119894 119883 rarr 119883
119894and 1198751015840
119894 119883
lowastrarr 119883
lowast
119894be the natural
projections and let 119876119894 119883
119894rarr 119883 and 1198761015840
119894 119883
lowast
119894rarr 119883
lowast be thenatural embeddings
Assume that an operator 119879119894 119883
119894rarr 119883
119894and a pair
(119909119894 119909
lowast
119894) isin Π(119883
119894) satisfy that
V (119879119894) = 1
1003816100381610038161003816119909lowast
119894119879119894119909119894
1003816100381610038161003816 gt 1 minus 120578 (120576) (94)
We define an operator 119879 119883 rarr 119883 and (119909 119909lowast) isin Π(119883) by
119879 = 119876119894∘ 119879
119894∘ 119875
119894 (119909 119909
lowast) = (119876
119894119909119894 119876
1015840
119894119909lowast
119894) (95)
12 Abstract and Applied Analysis
then clearly we see that1003816100381610038161003816119909
lowast1198791199091003816100381610038161003816 =1003816100381610038161003816119909
lowast
119894119879119894119909119894
1003816100381610038161003816 gt 1 minus 120578 (120576) (96)
From the assumption there exist 119878 119883 rarr 119883 and a pair(119910 119910
lowast) isin Π(119883) such that
1003816100381610038161003816119910lowast1198781199101003816100381610038161003816 = 1 = V (119878) 119878 minus 119879 lt 120576
1003817100381710038171003817119910lowastminus 119909
lowast1003817100381710038171003817 lt 1205761003817100381710038171003817119910 minus 119909
1003817100381710038171003817 lt 120576
(97)
Since this clearly shows that1003817100381710038171003817119875119894 ∘ 119878 ∘ 119876119894
minus 119879119894
1003817100381710038171003817 lt 120576100381710038171003817100381710038171198751015840
119894119910lowastminus 119909
lowast
119894
10038171003817100381710038171003817lt 120576
1003817100381710038171003817119875119894119910 minus 1199091198941003817100381710038171003817 lt 120576
(98)
we only need to show that |1198751015840119894119910lowast(119875
119894∘ 119878 ∘ 119876
119894)119875
119894119910| = 1
We first show the case of 1198880sum Since
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817=10038171003817100381710038171003817119875119895119910 minus 119875
11989511990910038171003817100381710038171003817⩽1003817100381710038171003817119910 minus 119909
1003817100381710038171003817 lt 120576 (99)
for every 119895 = 119894 we have
1 = 119910lowast(119910) = sum
119895isinN
1198751015840
119895119910lowast(119875
119895119910) ⩽ sum
119895isinN
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817
⩽100381710038171003817100381710038171198751015840
119894119910lowast10038171003817100381710038171003817+ 120576 sum
119895isinN119895 = 119894
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817⩽1003817100381710038171003817119910
lowast1003817100381710038171003817 = 1
(100)
This shows that 1198751015840119894119910lowast = 1 and 1198751015840
119895119910lowast= 0 for every 119895 = 119894
So 119910lowast = 1198761015840
1198941198751015840
119894119910lowast and 1198751015840
119894119910lowast(119875
119894119910) = 1 This and the fact that
119910 minus 119876119894119875119894119910 lt 120576 imply that
(119876119894119875119894119910 + (
1
120576) (119910 minus 119876
119894119875119894119910) 119876
1015840
1198941198751015840
119894119910lowast) isin Π (119883) (101)
So we get that (1198761015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910 + (1120576)(119910 minus 119876
119894119875119894119910))V(119878) = 1
Hence we have
1 =1003816100381610038161003816119910
lowast1198781199101003816100381610038161003816 =10038161003816100381610038161003816(119876
1015840
1198941198751015840
119894119910lowast) 11987811991010038161003816100381610038161003816
=
1003816100381610038161003816100381610038161003816(1 minus 120576) (119876
1015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910)
+120576 (1198761015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910 +1
120576(119910 minus 119876
119894119875119894119910))
1003816100381610038161003816100381610038161003816⩽ 1
(102)
and so we get |1198751015840119894119910lowast(119875
119894∘119878 ∘119876
119894)119875
119894119910| = |(119876
1015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910)| = 1
We next show the case of ℓ1sum The proof is almost
the same as that of the 1198880case However for the sake of
completeness we provide it hereSince 1198751015840
119895119910lowast = 119875
1015840
119895119910lowastminus 119875
1015840
119895119909lowast ⩽ 119910
lowastminus 119909
lowast lt 120576 for every
119895 = 119894 we have
1 = 119910lowast(119910) = sum
119895isinN
1198751015840
119895119910lowast(119875
119895119910) ⩽ sum
119895isinN
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817
⩽10038171003817100381710038171198751198941199101003817100381710038171003817 + 120576 sum
119895isinN119895 = 119894
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817⩽10038171003817100381710038171199101003817100381710038171003817 = 1
(103)
which shows 119875119894119910 = 1 and 119875
119895119910 = 0 for every 119895 = 119894 Since this
implies (119876119894119875119894119910119876
1015840
1198941198751015840
119894119910lowast+(1120576)(119910
lowastminus119876
1015840
1198941198751015840
119894119910lowast)) isin Π(119883) we get
that |(1198761015840
1198941198751015840
119894119910lowast+ (1120576)(119910
lowastminus 119876
1015840
1198941198751015840
119894119910lowast))119878(119876
119894119875119894119910)| ⩽ V(119878) = 1
Hence we have
1 =1003816100381610038161003816119910
lowast1198781199101003816100381610038161003816 =1003816100381610038161003816119910
lowast119878 (119876
119894119875119894119910)1003816100381610038161003816
=
1003816100381610038161003816100381610038161003816(1 minus 120576) (119876
1015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910)
+120576 (1198761015840
1198941198751015840
119894119910lowast+1
120576(119910
lowastminus 119876
1015840
1198941198751015840
119894119910lowast)) 119878 (119876
119894119875119894119910)
1003816100381610038161003816100381610038161003816⩽ 1
(104)
and so |1198751015840119894119910lowast(119875
119894∘ 119878 ∘ 119876
119894)119875
119894119910| = |(119876
1015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910)| = 1
Proof of Theorem 15 Note that 120578nu(1198711(120583)oplus1119883)(120576) rarr 0 as120576 rarr 0 Fix 0 lt 120576
0lt 1 and choose 0 lt 120576 lt 1 such that
6120576 + 120578nu(1198711(120583)oplus1119883)(120576) lt 1205760 Let 120578(1205760) = 120578nu(1198711(120583)oplus1119883)(120576)Suppose that 119879
0isin L(119871
1(120583) 119883) with 119879
0 = 1 and 119891
0isin
1198781198711(120583)
satisfy
1003817100381710038171003817119879011989101003817100381710038171003817 gt 1 minus 120578 (1205760) (105)
For any measurable subset 119861 let
1198711(120583|
119861) = 119891|
119861 119891 isin 119871
1(120583) (106)
with the norm 119891|119861 = 119891120594
1198611 Then it is easy to see
that 1198711(120583|
119861) is isometrically isomorphic to a complemented
subspace of 1198711(120583) Let 119875
119861 119871
1(120583) rarr 119871
1(120583|
119861) be the
restriction defined by 119875119861(119891) = 119891|
119861for all 119891 isin 119871
1(120583) and
let 119869119861 119871
1(120583|
119861) rarr 119871
1(120583) be the extension defined by
119869119861(119891)(120596) = 119891(120596) if 120596 isin 119861 and 119869
119861(119891)(120596) = 0 otherwise
It is clear that 119875119861119869119861= Id
1198711(120583|119861)and 119869
119861119875119861(119891) = 119891120594
119861for all
119891 isin 1198711(120583) Notice also that 119871
1(120583) is isometrically isomorphic
to 1198711(120583|
119861)oplus
11198711(120583|
119861119888)
Let 119860 = supp1198910and 119892
0= 119875
1198601198910 Then
1003817100381710038171003817119879011986911986011989201003817100381710038171003817 =100381710038171003817100381711987901198910
1003817100381710038171003817 gt 1 minus 120578 (1205760) gt 0 (107)
and define the operator 119879119860 119871
1(120583|
119860) rarr 119883 by 119879
119860119891 =
1198790119869119860119891119879
0119869119860 for every 119891 isin 119871
1(120583|
119860) Then
100381710038171003817100381711987911986011989201003817100381710038171003817 ⩾100381710038171003817100381711987901198910
1003817100381710038171003817 gt 1 minus 120578 (1205760) (108)
Since 120583|119860is 120590-finite 119871
1(120583|
119860)lowast= 119871
infin(120583|
119860) Let 119892lowast
0isin 119878
119871infin(120583|119860)
be a function such that ⟨119892lowast0 119892
0⟩ = 1 and choose 119909lowast
0isin 119878
119883lowast
such that 119909lowast0(119879
1198601198920) = 119879
1198601198920 Define the operator 119878
0isin
L(1198711(120583|
119860)oplus
1119883) by
1198780(119891 119909) = (0 119879
119860119891) ((119891 119909) isin 119871
1(120583|
119860) oplus
1119883) (109)
and observe that 1198780 = V(119878
0) = 1 Indeed
100381710038171003817100381711987801003817100381710038171003817 ⩽ 1 =
10038171003817100381710038171198791198601003817100381710038171003817
= sup 1003816100381610038161003816119909lowast1198791198601198911003816100381610038161003816 119909
lowastisin 119878
119883lowast 119891 isin 119878
1198711(120583|119860)
= sup 1003816100381610038161003816(119891lowast 119909
lowast) 119878
0(119891 119909)
1003816100381610038161003816
((119891lowast 119909
lowast) (119891 119909)) isin Π (119871
1(120583|
119860) oplus
1119883)
= V (1198780) ⩽100381710038171003817100381711987801003817100381710038171003817
(110)
Abstract and Applied Analysis 13
It is immediate that
(119892lowast
0 119909
lowast
0) 119878
0(119892
0 0) = 119909
lowast
0(119879
1198601198920) =10038171003817100381710038171198791198601198920
1003817100381710038171003817 gt 1 minus 120578 (1205760)
(111)
By Lemma 19 1198711(120583|
119860)oplus
1119883 has the BPBp-nu with the func-
tion 120578 Therefore there exist 1198781isin L(119871
1(120583|
119860)oplus
1119883) (119892
1 119909
1) isin
1198781198711(120583|119860)oplus1119883 and (119892lowast
1 119909
lowast
1) isin 119878
119871infin(120583|119860)oplusinfin119883lowast such that
1003817100381710038171003817(1198921 1199091) minus (1198920 0)1003817100381710038171003817 lt 120576
1003817100381710038171003817(119892lowast
1 119909
lowast
1) minus (119891
lowast
0 119909
lowast
0)1003817100381710038171003817 lt 120576
10038171003817100381710038171198781 minus 11987801003817100381710038171003817 lt 120576 ⟨(119892
lowast
1 119909
lowast
1) (119892
1 119909
1)⟩ = 1
1003816100381610038161003816(119892lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)1003816100381610038161003816 = V (119878
1) = 1
(112)
Claim 1 We claim that 1199091= 0
Otherwise
1 = Re ⟨(119892lowast1 119909
lowast
1) (119892
1 119909
1)⟩
=100381710038171003817100381711989211003817100381710038171003817Re⟨(119892
lowast
1 119909
lowast
1) (119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
⟩
+100381710038171003817100381711990911003817100381710038171003817Re⟨(119892
lowast
1 119909
lowast
1) (0 119909
1)
100381710038171003817100381711990911003817100381710038171003817
⟩ ⩽ 1
(113)
We deduce that
((119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
(119892lowast
1 119909
lowast
1)) (
(0 1199091)
100381710038171003817100381711990911003817100381710038171003817
(119892lowast
1 119909
lowast
1))
isin Π (1198711(120583|
119860) oplus
1119883)
(114)
Since 1198781((0 119909
1)119909
1) = (119878
1minus 119878
0)((0 119909
1)119909
1) lt 120576 we
get that
1 =1003816100381610038161003816⟨(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)⟩1003816100381610038161003816
=
100381610038161003816100381610038161003816100381610038161003816
100381710038171003817100381711989211003817100381710038171003817⟨(119892
lowast
1 119909
lowast
1) 119878
1((119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
)⟩
+100381710038171003817100381711990911003817100381710038171003817⟨(119892
lowast
1 119909
lowast
1) 119878
1((0 119909
1)
100381710038171003817100381711990911003817100381710038171003817
)⟩
100381610038161003816100381610038161003816100381610038161003816
⩽100381710038171003817100381711989211003817100381710038171003817 V (1198781) + 120576
100381710038171003817100381711990911003817100381710038171003817 lt100381710038171003817100381711989211003817100381710038171003817 +100381710038171003817100381711990911003817100381710038171003817 = 1
(115)
a contradiction This proves the claimWe define the operator 119878
2 119871
1(120583|
119860)oplus
1119883 rarr 119871
1(120583|
119860)oplus
1119883
by 1198782(119891 119909) = 119878
1(119891 0) for every 119891 isin 119871
1(120583|
119860) and for every
119909 isin 119883 Then we have
V (1198782) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816 = 1
10038171003817100381710038171198781 minus 11987821003817100381710038171003817 ⩽ 120576 (116)
Indeed from Claim 1 we have
V (1198781) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)1003816100381610038161003816 =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816 ⩽ V (119878
2)
(117)
On the other hand we see that1003816100381610038161003816(119891
lowast 119909
lowast) 119878
2(119891 119909)
1003816100381610038161003816
=1003816100381610038161003816(119891
lowast 119909
lowast) 119878
1(119891 0)
1003816100381610038161003816 ⩽10038171003817100381710038171198911003817100381710038171003817 V (1198781) ⩽ V (119878
1)
(118)
for every ((119891lowast 119909lowast) (119891 119909)) isin Π(1198711(120583|
119860)oplus
1119883) Therefore
V(1198782) ⩽ V(119878
1) Also
10038171003817100381710038171198781 minus 11987821003817100381710038171003817 ⩽ sup
119909isin119878119883
10038171003817100381710038171198781 (0 119909)1003817100381710038171003817
= sup119909isin119878119883
10038171003817100381710038171198781 (0 119909) minus 1198780 (0 119909)1003817100381710038171003817 ⩽ 120576
(119)
Claim 2 There exists 1198783 119871
1(120583|
119860)oplus
1119883 rarr 119871
1(120583|
119860)oplus
1119883 such
that 1198783(119892
1 0) = 119878
3 = 1 119878
3(0 119909) = 0 119878
3(119891 119909) isin 0oplus
1119883
for every (119891 119909) isin 1198711(120583|
119860)oplus
1119883 and 119878
3minus 119878
2 lt 4120576
Indeed write 1198781= (119863
1 119863
2) where 119863
1 119871
1(120583|
119860)oplus
1119883 rarr
1198711(120583|
119860) and119863
2 119871
1(120583|
119860)oplus
1119883 rarr 119883 We have that
sup 1003816100381610038161003816119892lowast119863
1(119892
1 0) + 119909
lowast119863
2(119892
1 0)1003816100381610038161003816
119909lowastisin 119878
119883lowast ⟨119892
lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860)
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816 +10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
⟨119892lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860)
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816
⟨119892lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860) +10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
⩽ V (1198782) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816119892
lowast
1119863
1(119892
1 0) + 119909
lowast
1119863
2(119892
1 0)1003816100381610038161003816
(120)
This implies that
1003816100381610038161003816119909lowast
1119863
2(119892
1 0)1003816100381610038161003816 =10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
1003816100381610038161003816119892lowast
1119863
1(119892
1 0)1003816100381610038161003816
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816 ⟨119892
lowast 119892
1⟩ = 1 119892
lowastisin 119871
infin(120583|
119860)
(121)
Therefore |119892lowast1| equals 1 on the support of 119863
1(119892
1 0) As
|⟨119892lowast
1 119892
1⟩| = 1 we also have that |119892lowast
1| equals 1 on the support
of 1198921 Changing the values of 119892lowast
1by the ones of 119891lowast
0on 119860
(supp(1198631(119892
1 0)) cup supp(g
1)) we may and do suppose that
|119892lowast
1| = 1 on the whole 119860We also have 119863
2(119892
1 0) gt 0 Indeed
10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198920 0)1003817100381710038171003817
⩽10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198921 0)
1003817100381710038171003817 +10038171003817100381710038171198780 (1198921 0) minus 1198780 (1198920 0)
1003817100381710038171003817
lt 2120576 + 120576 = 3120576
(122)
So we have
10038171003817100381710038171198632(119892
1 0) minus 119879
1198601198920
1003817100381710038171003817
⩽10038171003817100381710038171198631(119892
1 0)1003817100381710038171003817 +10038171003817100381710038171198632(119892
1 0) minus 119879
1198601198920
1003817100381710038171003817
14 Abstract and Applied Analysis
=1003817100381710038171003817(1198631
(1198921 0) 119863
2(119892
1 0)) minus (0 119879
1198601198920)1003817100381710038171003817
=10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198920 0)
1003817100381710038171003817 lt 3120576
(123)
and 1198632(119892
1 0) gt 119879
1198601198920 minus 3120576 ⩾ 1 minus 120578(120576
0) minus 3120576 gt 0
Finally define the operator 1198783by
1198783(119891 119909) = (0119863
2(119891 0) + 119892
lowast
1(119863
1(119891 0))
1198632(119892
1 0)
119909lowast
1119863
2(119892
1 0))
(124)
for (119891 119909) isin 1198711(120583|
119860)oplus
1119883 It is clear that
100381710038171003817100381711987831003817100381710038171003817 ⩽ sup
119891isin1198781198711(120583|119860)
(10038171003817100381710038171198632(119891 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119891 0)
1003816100381610038161003816) (125)
Notice also that
10038171003817100381710038171198631(119891 0)
1003817100381710038171003817
⩽10038171003817100381710038171198631(119891 0)
1003817100381710038171003817 +10038171003817100381710038171198632(119891 0) minus 119879
1198601198911003817100381710038171003817
=1003817100381710038171003817(1198631
(119891 0) 1198632(119891 0)) minus (0 119879
119860119891)1003817100381710038171003817
=10038171003817100381710038171198782 (119891 119909) minus 1198780 (119891 119909)
1003817100381710038171003817
(126)
for all (119891 119909) isin 1198711(120583|
119860)oplus
1119883 Hence we have
10038171003817100381710038171198783 minus 11987821003817100381710038171003817
= 2 sup119891isin1198781198711(120583|119860)
10038171003817100381710038171198631(119891 0)
1003817100381710038171003817 ⩽ 210038171003817100381710038171198782 minus 1198780
1003817100381710038171003817 lt 4120576(127)
On the other hand let 119866 1198711(120583|
119860) rarr 119871
1(120583|
119860) be defined by
119866(119891) = 119892lowast
1119891 for every 119891 isin 119871
1(120583|
119860) Then we have
V (1198782)
= sup 1003816100381610038161003816119911lowast11987821199111003816100381610038161003816 (119911 119911
lowast) isin Π (119871
1(120583|
119860) oplus
1119883)
⩾ sup10038161003816100381610038161003816100381610038161003816
119909lowast119863
2(119866(
1
120583 (119862)120594119862) 0)
+119892lowast
1119863
1(119866(
1
120583 (119862)120594119862) 0)
10038161003816100381610038161003816100381610038161003816
119909lowastisin 119878
119883lowast 119862 isin Σ
119860 120583 (119862) gt 0
= sup10038171003817100381710038171003817100381710038171003817
1198632(119866(
1
120583 (119862)120594119862) 0)
10038171003817100381710038171003817100381710038171003817
+
10038161003816100381610038161003816100381610038161003816
119892lowast
1119863
1(119866(
1
120583 (119862)120594119862) 0)
10038161003816100381610038161003816100381610038161003816
119862 isin Σ119860 120583 (119862) gt 0
(128)
where Σ119860is the family of measurable subsets of 119860
Hence for any simple function 119904 = sum119899
119894=1((120572
119894120583(119860
119894))120594
119860119894
) isin
1198781198711(120583|119860) where 119860
119894119894is a family of disjoint measurable subsets
with strictly positive measure we have
V (1198782)
⩾
119899
sum
119894=1
10038161003816100381610038161205721198941003816100381610038161003816 (
100381710038171003817100381710038171003817100381710038171003817
1198632(119866(
1
120583 (119860119894)120594119860119894
) 0)
100381710038171003817100381710038171003817100381710038171003817
+
100381610038161003816100381610038161003816100381610038161003816
119892lowast
1119863
1(119866(
1
120583 (119860119894)120594119860119894
) 0)
100381610038161003816100381610038161003816100381610038161003816
)
⩾10038171003817100381710038171198632(119866 (119904) 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119866 (119904) 0)
1003816100381610038161003816
(129)
Since |119892lowast1| = 1119866 is an isometric isomorphism so for each119891 isin
1198781198711(120583|119860)there exists a sequence of norm-one simple functions
(119904119896) such that 119866(119904
119896) converges to 119891 Therefore
V (1198782) ⩾ sup
119891isin1198781198711(120583|119860)
(10038171003817100381710038171198632(119891 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119891 0)
1003816100381610038161003816) ⩾100381710038171003817100381711987831003817100381710038171003817
(130)
On the other hand we see that100381710038171003817100381711987831003817100381710038171003817 ⩾1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
3(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816119909
lowast
1119863
2(119892
1 0) + 119892
lowast
1119863
1(119892
1 0)1003816100381610038161003816 = V (119878
2) = 1
(131)
Therefore 1 = 1198783 = 119878
3(119892
1 0) which proves Claim 2
Finally set 1198783= (0 ) for a suitable 119871
1(120583|
119860)oplus
1119883 rarr
119883 and define the operator 1198791 119871
1(120583) rarr 119883 by
1198791(119891) = 119879
0(119891120594
119860119888) + (119875
119860119891 0) (132)
for every 119891 isin 1198711(120583) Then we have
10038171003817100381710038171198791 (119891)1003817100381710038171003817 ⩽100381710038171003817100381711987901003817100381710038171003817
10038171003817100381710038171198911205941198601198881003817100381710038171003817 +1003817100381710038171003817100381710038171003817100381710038171003817
10038171003817100381710038171198911205941198601003817100381710038171003817 =10038171003817100381710038171198911003817100381710038171003817 (133)
for every 119891 isin 1198711(120583) so 119879
1 ⩽ 1 Also
10038171003817100381710038171198791 (1198691198601198921)1003817100381710038171003817 =10038171003817100381710038171198783 (1198921 0)
1003817100381710038171003817 =100381710038171003817100381711987831003817100381710038171003817 = 1 (134)
so 1198791attains its norm at 119869
1198601198921isin 119871
1(120583) and
10038171003817100381710038171198691198601198921 minus 11989101003817100381710038171003817 =10038171003817100381710038171198921 minus 1198920
1003817100381710038171003817 lt 120576 (135)
We also have that for any 119891 isin 1198781198711(120583)
10038171003817100381710038171198790 (119891) minus 1198791 (119891)1003817100381710038171003817 =100381710038171003817100381710038171198790(119891120594
119860) minus (119875
119860119891 0)10038171003817100381710038171003817
⩽10038171003817100381710038171198790 (119869119860119875119860119891) minus 119879119860 (119875119860119891)
1003817100381710038171003817
+10038171003817100381710038171003817119879119860(119875
119860119891) minus (119875
119860119891 0)10038171003817100381710038171003817
⩽10038171003817100381710038171198790119869119860 minus 119879119860
1003817100381710038171003817 +10038171003817100381710038171198780 minus 1198783
1003817100381710038171003817
lt 120578 (1205760) + 6120576
(136)
Hence 1198790minus 119879
1 ⩽ 120578(120576
0) + 6120576 lt 120576
0
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Abstract and Applied Analysis 15
Acknowledgments
The authors thank Gilles Godefroy and Rafael Paya forfruitful conversations about the content of this paper andVladimir Kadets for providing Lemma 18 They also appre-ciate anonymous referees for careful reading and fruitfulsuggestions about revision The second author was sup-ported by Basic Science Research Program through theNational Research Foundation of Korea (NRF) fundedby the Ministry of Education Science and Technology(NRF-2012R1A1A1006869) The third author was partiallysupported by Spanish MICINN and FEDER project noMTM2012-31755 and by Junta de Andalucıa and FEDERGrants FQM-185 and P09-FQM-4911
References
[1] F F Bonsall and J Duncan Numerical Ranges of Operators onNormed Spaces and of Elements of Normed Algebras vol 2 ofLondon Mathematical Society Lecture Note Series 2 CambridgeUniversity Press London UK 1971
[2] F F Bonsall and J Duncan Numerical Ranges II CambridgeUniversity Press London UK 1973 London MathematicalSociety Lecture Notes Series 10
[3] B Sims On numerical range and its applications to Banachalgebras [PhD thesis] University of Newcastle New SouthWales Australia 1972
[4] I D Berg and B Sims ldquoDenseness of operators which attaintheir numerical radiusrdquoAustralian Mathematical Society A vol36 no 1 pp 130ndash133 1984
[5] C S Cardassi ldquoDensity of numerical radius attaining operatorson some reflexive spacesrdquo Bulletin of the Australian Mathemat-ical Society vol 31 no 1 pp 1ndash3 1985
[6] C S Cardassi ldquoNumerical radius attaining operatorsrdquo inBanach Spaces (Columbia Mo 1984) vol 1166 of Lecture Notesin Mathematics pp 11ndash14 Springer Berlin Germany 1985
[7] C S Cardassi ldquoNumerical radius-attaining operators on119862(119870)rdquoProceedings of the AmericanMathematical Society vol 95 no 4pp 537ndash543 1985
[8] MDAcostaOperadores que alcanzan su radio numerico [PhDthesis] Universidad de Granada Granada Spain 1990
[9] M D Acosta and R Paya ldquoNumerical radius attaining oper-ators and the Radon-Nikodym propertyrdquo The Bulletin of theLondon Mathematical Society vol 25 no 1 pp 67ndash73 1993
[10] R Paya ldquoA counterexample on numerical radius attainingoperatorsrdquo Israel Journal of Mathematics vol 79 no 1 pp 83ndash101 1992
[11] Y S Choi and S G Kim ldquoNorm or numerical radius attainingmultilinear mappings and polynomialsrdquo Journal of the LondonMathematical Society vol 54 no 1 pp 135ndash147 1996
[12] M D Acosta J Becerra Guerrero and M Ruiz GalanldquoNumerical-radius-attaining polynomialsrdquo The Quarterly Jour-nal of Mathematics vol 54 no 1 pp 1ndash10 2003
[13] M D Acosta and S G Kim ldquoDenseness of holomorphicfunctions attaining their numerical radiirdquo Israel Journal ofMathematics vol 161 pp 373ndash386 2007
[14] S G Kim and H J Lee ldquoNumerical peak holomorphicfunctions on Banach spacesrdquo Journal of Mathematical Analysisand Applications vol 364 no 2 pp 437ndash452 2010
[15] M D Acosta R M Aron D Garcıa and M MaestreldquoThe Bishop-Phelps-Bollobas theorem for operatorsrdquo Journal ofFunctional Analysis vol 254 no 11 pp 2780ndash2799 2008
[16] A J Guirao and O Kozhushkina ldquoThe Bishop-Phelps-Bollobasproperty for numerical radius in ℓ
1(119862)rdquo Studia Mathematica
vol 218 no 1 pp 41ndash54 2013[17] J Falco ldquoThe Bishop-Phelps-Bollobas property for numerical
radius on 1198711rdquo Journal of Mathematical Analysis and Applica-
tions vol 414 no 1 pp 125ndash133 2014[18] A Aviles A J Guirao and J Rodriguez ldquoOn the Bishop-
Phelps-Bollobas property for numerical radius in119862(119870)-spacesrdquoPreprint
[19] RM Aron Y S Choi S K Kim H J Lee andMMartin ldquoTheBishop-Phelps-Bollobas version of Lindenstrauss properties Aand Brdquo Preprint
[20] V Kadets M Martın and R Paya ldquoRecent progress and openquestions on the numerical index of Banach spacesrdquo Revista dela Real Academia de Ciencias Exactas Fısicas y Naturales A vol100 no 1-2 pp 155ndash182 2006
[21] M Martın J Merı and M Popov ldquoOn the numerical index ofreal 119871
119901(120583)-spacesrdquo Israel Journal of Mathematics vol 184 pp
183ndash192 2011[22] A Iwanik ldquoNorm attaining operators on Lebesgue spacesrdquo
Pacific Journal of Mathematics vol 83 no 2 pp 381ndash386 1979[23] HH SchaeferBanach Lattices and Positive Operators Springer
New York NY USA 1974[24] J Duncan C M McGregor J D Pryce and A J White ldquoThe
numerical index of a normed spacerdquo Journal of the LondonMathematical Society Second Series vol 2 pp 481ndash488 1970
[25] Y S Choi S K Kim H J Lee and M Martin ldquoThe Bishop-Phelps-Bollobas theorem for operators on 119871
1(120583)rdquo Preprint
[26] H E Lacey The Isometric Theory of Classical Banach SpacesSpringer New York NY USA 1974
[27] G Godefroy ldquoUniversal spaces for strictly convex Banachspacesrdquo Revista de la Real Academia de Ciencias Exactas Fısicasy Naturales A vol 100 no 1-2 pp 137ndash146 2006
[28] C Finet M Martın and R Paya ldquoNumerical index andrenormingrdquo Proceedings of the American Mathematical Societyvol 131 no 3 pp 871ndash877 2003
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Differential EquationsInternational Journal of
Volume 2014
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Abstract and Applied AnalysisHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Algebra
Discrete Dynamics in Nature and Society
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Decision SciencesAdvances in
Discrete MathematicsJournal of
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Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of
2 Abstract and Applied Analysis
It is shown in [16] that the real or complex spaces 1198880and
ℓ1have the BPBp-nu This result has been extended to the
real space 1198711(R) by Falco [17] Aviles et al [18] give sufficient
conditions on a compact space 119870 for the real space 119862(119870) tohave the BPBp-nu which in particular include all metrizablecompact spaces
The content of this paper is the following First weintroduce in Section 2 a modulus of the BPBp-nu analogousto the one introduced in [19] for the Bishop-Phelps-Bollobasproperty for the operator norm and we will use it as a toolin the rest of the paper As easy applications we prove thatfinite-dimensional spaces always have the BPBp-nu and thata reflexive space has the BPBp-nu if and only if its dual doesNext Section 3 is devoted to prove that Banach spaces whichare both uniformly convex and uniformly smooth satisfy aweaker version of the BPBp-nu and to discuss such weakerversion In particular it is shown that 119871
119901(120583) spaces have the
BPBp-nu for every measure 120583 when 1 lt 119901 lt infin 119901 = 2We show in Section 4 that given any measure 120583 the real orcomplex space 119871
1(120583) has the BPBp-nu Finally we prove in
Section 5 that every separable infinite-dimensional Banachspace can be equivalently renormed to fail the BPBp-nu(actually to fail the weaker version) In particular this showsthat reflexivity (or even superreflexivity) is not enough for theBPBp-nu while the Radon-Nikodym property was knownto be sufficient for the density of numerical radius attainingoperators
Let us introduce some notations for later use The 119899-dimensional space with the ℓ
1norm is denoted by ℓ(119899)
1
Given a family 119883119896infin
119896=1of Banach spaces [⨁infin
119896=1119883
119896]1198880
(resp[⨁
infin
119896=1119883
119896]ℓ1
) is the Banach space consisting of all sequences(119909
119896)infin
119896=1such that each 119909
119896is in 119883
119896and lim
119896rarrinfin119909
119896 = 0
(resp suminfin
119896=1119909
119896 lt infin) equipped with the norm (119909
119896)infin
119896=1 =
sup119896119909
119896 (resp (119909
119896)infin
119896=1 = sum
infin
119896=1119909
119896)
2 Modulus of the Bishop-Phelps-Bollobaacutes forNumerical Radius
Analogously to what is done in [19] for the BPBp for theoperator norm we introduce here a modulus to quan-tify the Bishop-Phelps-Bollobas property for numericalradius
Notation 1 Let119883 be a Banach space Consider the set
Πnu (119883) = (119909 119909lowast 119879) (119909 119909
lowast) isin Π (119883)
119879 isinL (119883) V (119879) = 1 = 1003816100381610038161003816119909lowast1198791199091003816100381610038161003816
(3)
which is closed in 119878119883times 119878
119883lowast times L(119883) with respect to the
following metric
dist ((119909 119909lowast 119879) (119910 119910lowast 119878))
= max 1003817100381710038171003817119909 minus 1199101003817100381710038171003817 1003817100381710038171003817119909
lowastminus 119910
lowast1003817100381710038171003817 119879 minus 119878
(4)
The modulus of the Bishop-Phelps-Bollobas property fornumerical radius is the function defined by
120578nu (119883) (120576)
= inf 1 minus 1003816100381610038161003816119909lowast1198791199091003816100381610038161003816 (119909 119909
lowast) isin Π (119883) 119879 isinL (119883)
V (119879) = 1 dist ((119909 119909lowast 119879) Πnu (119883)) ⩾ 120576
(5)
for every 120576 isin (0 1) Equivalently 120578nu(119883)(120576) is the supremumof those scalars 120578 gt 0 such that whenever 119879 isin L(119883) and(119909 119909
lowast) isin Π(119883) satisfy V(119879) = 1 and |119909lowast119879119909| gt 1 minus 120578 there
exist 119878 isinL(119883) and (119910 119910lowast) isin Π(119883) such that
V (119878) = 1003816100381610038161003816119910lowast1198781199101003816100381610038161003816 = 1 119879 minus 119878 lt 120576
1003817100381710038171003817119909 minus 1199101003817100381710038171003817 lt 120576
1003817100381710038171003817119909lowastminus 119910
lowast1003817100381710038171003817 lt 120576
(6)
It is immediate that a Banach space119883 has the BPBp-nu ifand only if 120578nu(120576) gt 0 for every 0 lt 120576 lt 1 By construction ifa function 120576 997891rarr 120578(120576) is valid in the definition of the BPBp-nuthen 120578nu(120576) ⩾ 120578(120576)
An immediate consequence of the compactness of theunit ball of a finite-dimensional space is the following resultIt was previously known to A Guirao (private communica-tion)
Proposition 2 Let 119883 be a finite-dimensional Banach spaceThen119883 has the Bishop-Phelps-Bollobas property for numericalradius
Proof Let 119870 = 119878 isin L(119883) V(119878) = 0 Then 119870 is anorm-closed subspace of L(119883) Hence L(119883)119870 is a finite-dimensional space with two norms
V ([119879]) = inf V (119879 minus 119878) 119878 isin 119870 = V (119879)
[119879] = inf 119879 minus 119878 119878 isin 119870 (7)
where [119879] is the class of 119879 in the quotient space L(119883)119870Hence there is a constant 0 lt 119888 ⩽ 1 such that
119888 [119879] ⩽ V (119879) ⩽ [119879] (8)
Suppose that119883 does not have the BPBp-nuThen there is0 lt 120576 lt 1 such that 120578nu(119883)(120576) = 0That is there are sequences(119909
119899 119909
lowast
119899) isin Π(119883) and (119879
119899) isinL(119883) with V(119879
119899) = 1 such that
dist ((119909119899 119909
lowast
119899 119879
119899) Πnu (119883)) ⩾ 120576 (119899 isin N)
lim119899
1003816100381610038161003816119909lowast
119899119879119899119909119899
1003816100381610038161003816 = 1
(9)
By compactness wemay assume that lim119899[119879
119899]minus[119879
0] = 0 for
some1198790isinL(119883) and V(119879
0) = 1 Hence there exists a sequence
119878119899119899in 119870 such that lim
119899119879
119899minus (119879
0+ 119878
119899) = 0 Observe that
V(1198790+ 119878
119899) = V(119879
0) = 1 for every 119899 isin N
By compactness again we may assume that (119909119899 119909
lowast
119899)
converges to (1199090 119909
lowast
0) isin 119883 times 119883
lowast This implies that (1199090 119909
lowast
0) isin
Π(119883) and |119909lowast0(119879
0+ 119878
119899)119909
0| = V(119879
0+ 119878
119899) = 1 that is
(1199090 119909
lowast
0 119879
0+ 119878
119899) isin Πnu(119883) for all 119899 This is a contradiction
with the fact that0 = lim
119899dist ((119909
119899 119909
lowast
119899 119879
119899) (119909
0 119909
lowast
0 119879
0+ 119878
119899))
⩾ lim119899
dist ((119909119899 119909
lowast
119899 119879
119899) Πnu (119883)) ⩾ 120576
(10)
Abstract and Applied Analysis 3
We may also give the following easy result concerningduality
Proposition 3 Let119883 be a reflexive space Then
120578nu (119883) (120576) = 120578nu (119883lowast) (120576) (11)
for every 120576 isin (0 1) In particular 119883 has the BPBp-nu if andonly if 119883lowast has the BPBp-nu
Wewill use that V(119879lowast) = V(119879) for all 119879 isinL(119883) where 119879lowastdenotes the adjoint operator of 119879 This result can be found in[1] but it is obvious if119883 is reflexive
Proof By reflexivity it is enough to show that 120578nu(119883)(120576) ⩽120578nu(119883
lowast)(120576) Let 120576 isin (0 1) be fixed If 120578nu(119883)(120576) = 0 there is
nothing to prove Otherwise consider 0 lt 120578 lt 120578nu(119883)(120576)Suppose that 119879
1isinL(119883lowast
) and (119909lowast1 119909
1) isin Π(119883
lowast) satisfy
V (1198791) = 1
100381610038161003816100381611990911198791119909lowast
1
1003816100381610038161003816 gt V (1198791) minus 120578 (12)
By considering 119879lowast1isin L(119883) we may find 119878
1isin L(119883) and
(1199101 119910
lowast
1) isin Π(119883) such that
1003816100381610038161003816119910lowast
111987811199101
1003816100381610038161003816 = V (1198781) = 1
10038171003817100381710038171199101 minus 11990911003817100381710038171003817 lt 120576
1003817100381710038171003817119910lowast
1minus 119909
lowast
1
1003817100381710038171003817 lt 1205761003817100381710038171003817119879
lowast
1minus 119878
1
1003817100381710038171003817 lt 120576
(13)
Then 119878lowast1isinL(119883lowast
) and (119910lowast1 119910
1) isin Π(119883
lowast) satisfy
1003816100381610038161003816⟨1199101 119878lowast
1119910lowast
1⟩1003816100381610038161003816 = V (119878
1) = 1
1003817100381710038171003817119910lowast
1minus 119909
lowast
1
1003817100381710038171003817 lt 120576
10038171003817100381710038171199101 minus 11990911003817100381710038171003817 lt 120576
10038171003817100381710038171198791 minus 119878lowast
1
1003817100381710038171003817 lt 120576
(14)
This implies that 120578nu(119883lowast)(120576) ⩾ 120578 We finish by just taking
supremum on 120578
We do not know whether the result above is valid in thenonreflexive case
3 Spaces Which Are Both Uniformly Convexand Uniformly Smooth
For a Banach space which is both uniformly convex anduniformly smooth we get a property which is weaker thanBPBp-nu This result was known to A Guirao (privatecommunication)
Proposition 4 Let 119883 be a uniformly convex and uniformlysmooth Banach space Then given 120576 gt 0 there exists 120578(120576) gt 0such that whenever 119879
0isinL(119883) with V(119879
0) = 1 and (119909
0 119909
lowast
0) isin
Π(119883) satisfy |119909lowast011987901199090| gt 1 minus 120578(120576) there exist 119878 isin L(119883) and
(119910 119910lowast) isin Π(119883) such that
V (119878) = 1003816100381610038161003816119910lowast1198781199101003816100381610038161003816
1003817100381710038171003817119909 minus 1199101003817100381710038171003817 lt 120576
1003817100381710038171003817119909lowastminus 119910
lowast1003817100381710038171003817 lt 1205761003817100381710038171003817119878 minus 1198790
1003817100381710038171003817 lt 120576
(15)
Proof Notice that the uniform smoothness of119883 is equivalentto the uniform convexity of 119883lowast Let 120575
119883(120576) and 120575
119883lowast(120576) be the
moduli of convexity119883 and119883lowast respectively Given 0 lt 120576 lt 1consider
120578 (120576) =120576
4min 120575
119883(120576
4) 120575
119883lowast (120576
4) gt 0 (16)
Consider 1198790isin L(119883) with V(119879
0) = 1 and (119909
0 119909
lowast
0) isin Π(119883)
satisfying |119909lowast011987901199090| gt 1 minus 120578(120576) Define 119879
1isinL(119883) by
1198791119909 = 119879
0119909 + 120582
1(120576
4) 119909
lowast
0(119909) 119909
0(17)
for all 119909 isin 119883 where 1205821is the scalar satisfying |120582
1| = 1 and
|119909lowast
011987901199090+ 120582
1(1205764)| = |119909
lowast
011987901199090| + 1205764 Now choose 119909
1isin 119878
119883
and 119909lowast1isin 119878
119883lowast such that |119909lowast
1(119909
1)| = 1 119909lowast
1(119909
0) = |119909
lowast
1(119909
0)| and
1003816100381610038161003816119909lowast
111987911199091
1003816100381610038161003816 ⩾ V (1198791) minus 120578(
1205762
42) (18)
Now we define a sequence (119909119899 119909
lowast
119899 119879
119899) in 119878
119883times 119878
119883lowast times L(119883)
inductively Indeed suppose that we have a defined sequence(119909
119895 119909
lowast
119895 119879
119895) for 0 ⩽ 119895 ⩽ 119899 and let
119879119899+1119909 = 119879
119899119909 + 120582
119899+1
120576119899+1
4119899+1119909lowast
119899(119909) 119909
119899 (19)
Then choose 119909119899+1
isin 119878119883
and 119909lowast119899+1
isin 119878119883lowast such that
|119909lowast
119899+1(119909
119899+1)| = 1 and |119909lowast
119899+1(119909
119899)| = 119909
lowast
119899+1(119909
119899)
1003816100381610038161003816119909lowast
119899+1119879119899+1119909119899+1
1003816100381610038161003816 ⩾ V (119879119899+1) minus 120578(
120576119899+2
4119899+2) (20)
Notice that for all 119899 ⩾ 0 we have
1003817100381710038171003817119879119899+1 minus 1198791198991003817100381710038171003817 ⩽120576119899+1
4119899+1
1003816100381610038161003816V (119879119899+1) minus V (119879119899)1003816100381610038161003816 ⩽120576119899+1
4119899+1
(21)
This implies that (119879119899) is a Cauchy sequence and assume that
it converges to 119878 isinL(119883) Then we have
lim119899119879119899= 119878
10038171003817100381710038171198790 minus 1198781003817100381710038171003817 lt 120576
lim119899
1003816100381610038161003816119909lowast
119899119879119899119909119899
1003816100381610038161003816 = lim119899V (119879
119899) = V (119878)
(22)
We will show that both sequences (119909119899) and (119909lowast
119899) are Cauchy
From the definition we have
V (119879119899+1) minus 120578(
120576119899+2
4119899+2)
⩽1003816100381610038161003816119909
lowast
119899+1119879119899+1119909119899+1
1003816100381610038161003816
4 Abstract and Applied Analysis
⩽
100381610038161003816100381610038161003816100381610038161003816
119909lowast
119899+1119879119899119909119899+1+ 120582
119899+1
120576119899+1
4119899+1119909lowast
119899(119909
119899+1) 119909
lowast
119899+1(119909
119899)
100381610038161003816100381610038161003816100381610038161003816
⩽ V (119879119899) +120576119899+1
4119899+1119909lowast
119899+1(119909
119899)
V (119879119899+1)
⩾1003816100381610038161003816119909
lowast
119899119879119899+1119909119899
1003816100381610038161003816 =
100381610038161003816100381610038161003816100381610038161003816
119909lowast
119899119879119899119909119899+ 120582
119899+1
120576119899+1
4119899+1
100381610038161003816100381610038161003816100381610038161003816
=1003816100381610038161003816119909
lowast
119899119879119899119909119899
1003816100381610038161003816 +120576119899+1
4119899+1⩾ V (119879
119899) minus 120578(
120576119899+1
4119899+1) +120576119899+1
4119899+1
(23)
In summary we have
V (119879119899) +120576119899+1
4119899+1119909lowast
119899+1(119909
119899)
⩾ V (119879119899) minus 120578(
120576119899+1
4119899+1) +120576119899+1
4119899+1minus 120578(
120576119899+2
4119899+2)
(24)
Hence
119909lowast
119899+1(119909
119899) ⩾ 1 minus 2
4119899+1
120576119899+1120578(120576119899+1
4119899+1)
= 1 minus1
2min120575
119883(120576119899+1
4119899+2) 120575
119883lowast (120576119899+1
4119899+2)
1003817100381710038171003817100381710038171003817
119909119899+ 119909
119899+1
2
1003817100381710038171003817100381710038171003817⩾ 119909
lowast
119899+1(119909119899+ 119909
119899+1
2) ⩾ 1 minus 120575
119883(120576119899+1
4119899+2)
100381710038171003817100381710038171003817100381710038171003817
119909lowast
119899+ 119909
lowast
119899+1
2
100381710038171003817100381710038171003817100381710038171003817
⩾119909lowast
119899+ 119909
lowast
119899+1
2(119909
119899) ⩾ 1 minus 120575
119883lowast (120576119899+1
4119899+2)
(25)
This means that
1003817100381710038171003817119909119899 minus 119909119899+11003817100381710038171003817 ⩽ 120576
119899+14
119899+2
1003817100381710038171003817119909lowast
119899minus 119909
lowast
119899+1
1003817100381710038171003817 ⩽ 120576119899+14
119899+2
(26)
for all 119899 So (119909119899) and (119909lowast
119899) are Cauchy Let 119909
infin= lim
119899119909119899and
119909lowast
infin= lim
119899119909lowast
119899 Then 119909
0minus 119909
infin lt 1205764 and 119909lowast
0minus 119909
lowast
infin lt 1205764
Hence |119909lowastinfin(119909
infin)| = lim
119899|119909
lowast
119899(119909
119899)| = 1 and
V (119878) = lim119899V (119879
119899) = lim
119899
1003816100381610038161003816119909lowast
119899119879119899119909119899
1003816100381610038161003816 =1003816100381610038161003816119909
lowast
infin119878119909
infin
1003816100381610038161003816 (27)
Let 120572 = 119909lowastinfin(119909
infin) 119910lowast = 119909lowast
infin and 119910 = 119909
infin Then we have
119910lowast(119910) = 1 V(119878) = |119910lowast119878119910| and 119910 minus 119909
0 lt 120576 Notice that
|120572 minus 1| =1003816100381610038161003816119909
lowast
infin(119909
infin) minus 119909
lowast
0(119909
0)1003816100381610038161003816
⩽1003816100381610038161003816(119909
lowast
infinminus 119909
lowast
0) (119909
infin)1003816100381610038161003816 +1003816100381610038161003816119909
lowast
0(119909
infin) minus 119909
lowast
0(119909
0)1003816100381610038161003816 lt120576
2
(28)
Therefore
1003817100381710038171003817119910lowastminus 119909
lowast1003817100381710038171003817 ⩽1003817100381710038171003817119910
lowastminus 119910
lowast1003817100381710038171003817 +1003817100381710038171003817119910
lowastminus 119909
lowast1003817100381710038171003817 lt120576
2+120576
4lt 120576 (29)
This completes the proof
Let us discuss a little bit about the equivalence betweenthe property in the result above and the BPBp-nu Forconvenience let us introduce the following definition
Definition 5 A Banach space 119883 has the weak Bishop-Phelps-Bollobas property for the numerical radius (in short weak-BPBp-nu) if given 120576 gt 0 there exists 120578(120576) gt 0 such thatwhenever 119879
0isin L(119883) with V(119879
0) = 1 and (119909
0 119909
lowast
0) isin Π(119883)
satisfy |119909lowast011987901199090| gt 1 minus 120578(120576) there exist 119878 isin L(119883) and
(119910 119910lowast) isin Π(119883) such that
V (119878) = 1003816100381610038161003816119910lowast1198781199101003816100381610038161003816
1003817100381710038171003817119909 minus 1199101003817100381710038171003817 lt 120576
1003817100381710038171003817119909lowastminus 119910
lowast1003817100381710038171003817 lt 120576 119878 minus 119879 lt 120576
(30)
Notice that the only difference between this concept andthe BPBp-nu is the normalization of the operator 119878 by thenumerical radius Of course if the numerical radius andthe operator norm are equivalent these two properties arethe same This equivalence is measured by the so-callednumerical index of the Banach space as follows For a Banachspace119883 the numerical index of119883 is defined by
119899 (119883) = inf V (119879) 119879 isinL (119883) 119879 = 1 (31)
It is clear that 0 ⩽ 119899(119883) ⩽ 1 and 119899(119883)119879 ⩽ V(119879) ⩽ 119879for all 119879 isin L(119883) The value 119899(119883) = 1 means that V equalsthe usual operator norm This is the case of 119883 = 119871
1(120583) and
119883 = 119862(119870) among many others On the other hand 119899(119883) gt 0if and only if the numerical radius is equivalent to the normof L(119883) We refer the reader to [20] for more informationand background
The following result is immediate We include a proof forthe sake of completeness
Proposition 6 Let119883 be a Banach space with 119899(119883) gt 0 Then119883 has the BPBp-nu if and only if119883 has the weak-BPBp-nu
Proof Thenecessity is clear For the converse assume that wehave 120578(120576) gt 0 satisfying the conditions of the weak-BPBp-nufor all 0 lt 120576 lt 1 If 119879 isin L(119883) with V(119879) = 1 and (119909
0 119909
lowast
0) isin
Π(119883) satisfy |119909lowast0119879119909
0| gt 1 minus 120578(120576) for 0 lt 120576 lt 1 then there exist
119878 isinL(119883) and (119910 119910lowast) isin Π(119883) such that
V (119878) = 1003816100381610038161003816119910lowast1198781199101003816100381610038161003816 119878 minus 119879 lt 120576
1003817100381710038171003817119909 minus 1199101003817100381710038171003817 lt 120576
1003817100381710038171003817119909lowastminus 119910
lowast1003817100381710038171003817 lt 120576
(32)
As V(119878) gt 0 by the above let 1198781= (1V(119878))119878 Then we have
1 = V (1198781) =1003816100381610038161003816119910
lowast11987811199101003816100381610038161003816
1003817100381710038171003817119909 minus 1199101003817100381710038171003817 lt 120576
1003817100381710038171003817119909lowastminus 119910
lowast1003817100381710038171003817 lt 120576
(33)
Abstract and Applied Analysis 5
Finally we have
10038171003817100381710038171198781 minus 1198791003817100381710038171003817 ⩽
10038171003817100381710038171003817100381710038171003817
1
V (119878)119878 minus 119878
10038171003817100381710038171003817100381710038171003817
+ 119878 minus 119879
=119878
V (119878)|V (119878) minus 1| + 119878 minus 119879
⩽1
119899 (119883)|V (119878) minus V (119879)| + 119878 minus 119879
⩽ (1
119899 (119883)+ 1) 119878 minus 119879 lt
119899 (119883) + 1
119899 (119883)120576
(34)
An obvious change of parameters finishes the proof
We do not know whether the hypothesis of 119899(119883) gt 0 canbe omitted in the above result
Putting together Propositions 4 and 6 we get the follow-ing
Corollary 7 Let 119883 be a uniformly convex and uniformlysmooth Banach space with 119899(119883) gt 0 Then 119883 has the BPBp-nu
Let us comment that every complex Banach space 119883satisfies 119899(119883) ⩾ 1119890 so the above corollary automaticallyapplies in the complex case In the real case this is no longertrue as the numerical index of a Hilbert space of dimensiongreater than or equal to two is 0 On the other hand it isproved in [21] that real 119871
119901(120583) spaces have nonzero numerical
index for every measure 120583when 119901 = 2Therefore we have thefollowing examples
Example 8 (a) Complex Banach spaces which are uniformlysmooth and uniformly convex satisfy the BPBp-nu
(b) In particular for every measure 120583 the complex spaces119871119901(120583) have the BPBp-nu for 1 lt 119901 lt infin(c) For every measure 120583 the real spaces 119871
119901(120583) have the
BPBp-nu for 1 lt 119901 lt infin 119901 = 2
Note Added in Revision Very recently H J Lee M Martınand J Merı have proved that Proposition 6 can be extendedto some Banach spaces with numerical index zero as forinstance real Hilbert spaces Hence they have shown thatHilbert spaces have the BPBp-nu These results will appearelsewhere
4 L1
Spaces
In this section we will show that 1198711(120583) has the BPBp-nu for
every measure 120583 In the proof we are dealing with complexintegrable functions since the real case is followed easilyby applying the same proof Our main result here is thefollowing
Theorem 9 Let 120583 be a measure Then 1198711(120583) has the Bishop-
Phelps-Bollobas property for numerical radius More preciselygiven 120576 gt 0 there exists 120578(120576) gt 0 (which does not depend on120583) such that whenever 119879
0isin L(119871
1(120583)) with V(119879
0) = 1 and
(1198910 119892
0) isin Π(119871
1(120583)) satisfy |⟨119879
01198910 119892
0⟩| gt 1 minus 120578(120576) then there
exist 119879 isinL(1198711(120583)) (119891
1 119892
1) isin Π(119871
1(120583)) such that
1003816100381610038161003816⟨1198791198911 1198921⟩1003816100381610038161003816 = V (119879) = 1 10038171003817100381710038171198910 minus 1198911
1003817100381710038171003817 lt 120576
10038171003817100381710038171198920 minus 11989211003817100381710038171003817 lt 120576
1003817100381710038171003817119879 minus 11987901003817100381710038171003817 lt 120576
(35)
As a first step we have to start dealing with finite regularpositive Borel measures for which a representation theoremfor operators exists
Proposition 10 Let 119898 be a finite regular positive Borelmeasure on a compact Hausdorff spaceΩ Then 119871
1(119898) has the
Bishop-Phelps-Bollobas property for numerical radius Moreprecisely given 120576 gt 0 there is 120578(120576) gt 0 (which is independent ofthemeasure119898) such that if a norm-one element119879 inL(119871
1(119898))
and (1198910 119892
0) isin Π(119871
1(119898)) satisfy |⟨119879119891
0 119892
0⟩| gt 1 minus 120578(120576) then
there exist an operator 119878 isin L(1198711(119898)) (119891
1 119892
1) isin Π(119871
1(119898))
such that1003816100381610038161003816⟨1198781198911 1198921⟩
1003816100381610038161003816 = 119878 = 110038171003817100381710038171198910 minus 1198911
1003817100381710038171003817 ⩽ 120576
10038171003817100381710038171198920 minus 11989211003817100381710038171003817 ⩽ 120576 119879 minus 119878 ⩽ 120576
(36)
To prove this proposition we need some backgroundon representation of operators on Lebesgue spaces on finiteregular positive Borel measures and several preliminarylemmas
Let 119898 be a finite regular positive Borel measure on acompact Hausdorff space Ω If 120583 is a complex-valued Borelmeasure on the product space Ω times Ω then define theirmarginal measures 120583119894 on Ω (119894 = 1 2) as follows 1205831(119860) =120583(119860 times Ω) and 1205832(119861) = 120583(Ω times 119861) where 119860 and 119861 are Borelmeasurable subsets of Ω
Let 119872(119898) be the complex Banach lattice of measuresconsisting of all complex-valued Borel measures 120583 on theproduct spaceΩ timesΩ such that |120583|119894 are absolutely continuouswith respect to119898 for 119894 = 1 2 endowed with the norm
1003817100381710038171003817100381710038171003817100381710038171003817
11988910038161003816100381610038161205831003816100381610038161003816
1
119889119898
1003817100381710038171003817100381710038171003817100381710038171003817infin
(37)
Each 120583 isin 119872(119898) defines a bounded linear operator 119879120583from
1198711(119898) to itself by
⟨119879120583(119891) 119892⟩ = int
ΩtimesΩ
119891 (119909) 119892 (119910) 119889120583 (119909 119910) (38)
where 119891 isin 1198711(119898) and 119892 isin 119871
infin(119898) Iwanik [22] showed that
the mapping 120583 997891rarr 119879120583is a lattice isometric isomorphism from
119872(119898) onto L(1198711(119898)) Even though he showed this for the
real case it can be easily generalized to the complex caseFor details see [22 Theorem 1] and [23 IV Theorem 15(ii)Corollary 2]
We will also use that given an arbitrary measure 120583 every119879 isin L(119871
1(120583)) satisfies V(119879) = 119879 [24] (that is the space
1198711(120583) has numerical index 1)
Lemma 11 (see [15 Lemma 33]) Let 119888119899 be a sequence of
complex numbers with |119888119899| ⩽ 1 for every 119899 and let 120578 gt 0 such
6 Abstract and Applied Analysis
that for a convex series sum120572119899 Resuminfin
119899=1120572119899119888119899gt 1 minus 120578 Then for
every 0 lt 119903 lt 1 the set 119860 = 119894 isin N Re 119888119894gt 119903 satisfies the
estimate
sum
119894isin119860
120572119894⩾ 1 minus
120578
1 minus 119903 (39)
From now on 119898 will be a finite regular positive Borelmeasure on the compact Hausdorff space Ω
Lemma 12 Suppose that there exist a nonnegative simplefunction 119891 isin 119878
1198711(119898)
and a function 119892 isin 119878119871infin(119898)
such that
Re ⟨119891 119892⟩ gt 1 minus 1205763
16 (40)
Then there exist a nonnegative simple function 1198911isin 119878
1198711(119898)
anda function 119892
1isin 119878
119871infin(119898)
such that
1198921(119909) = 120594supp(119891
1)(119909) + 119892 (119909) 120594
Ωsupp(1198911)(119909)
⟨1198911 119892
1⟩ = 1
1003817100381710038171003817119891 minus 119891110038171003817100381710038171lt 120576
1003817100381710038171003817119892 minus 11989211003817100381710038171003817infinlt radic120576 supp (119891
1) sub supp (119891)
(41)
Proof Let119891 = sum119898
119895=1(120573
119895119898(119861
119895))120594
119861119895
for some (120573119895) such that
120573119895⩾ 0 for all 119895 andsum119898
119895=1120573119895= 1 and119861
119895rsquos aremutually disjoint
By the assumption we have
Re ⟨119891 119892⟩ =119899
sum
119895=1
120573119895
1
119898 (119861119895)
int119861119895
Re119892 (119909) 119889119898 (119909) gt 1 minus 1205763
16
(42)
and letting
119869 = 119895 1 ⩽ 119895 ⩽ 1198991
119898 (119861119895)
int119861119895
Re119892 (119909) 119889119898 (119909) gt 1 minus 1205762
4
(43)
we have by Lemma 11
sum
119895isin119869
120573119895gt 1 minus
120576
4 (44)
For each 119895 isin 119869 we have
1 minus1205762
4lt
1
119898 (119861119895)
int119861119895
Re119892 (119909) 119889119898 (119909)
=1
119898 (119861119895)
int119861119895capRe119892⩽1minus120576
Re119892 (119909) 119889119898 (119909)
+ int119861119895capRe119892gt1minus120576
Re119892 (119909) 119889119898 (119909)
⩽1
119898 (119861119895)
((1 minus 120576)119898 (119861119895cap Re119892 ⩽ 1 minus 120576)
+ 119898 (119861119895cap Re119892 gt 1 minus 120576))
= 1 minus 120576
119898 (119861119895cap Re119892 ⩽ 1 minus 120576)
119898 (119861119895)
(45)
This implies that
119898(119861119895cap Re119892 ⩽ 1 minus 120576)
119898 (119861119895)
lt120576
4 (46)
Define 119861119895= 119861
119895cap Re119892 gt 1 minus 120576 for all 119895 isin 119869
1198911= (1sum
119895isin119869
120573119895)sum
119895isin119869
120573119895(120594
119861119895
119898 (119861119895)) (47)
and 1198921(119909) = 1 on supp(119891
1) and 119892
1(119909) = 119892(119909) elsewhereThen
it is clear that supp(1198911) sub supp(119891) 119892 minus 119892
1infinlt radic120576 and
⟨1198911 119892
1⟩ = 1 Finally we will show that 119891 minus 119891
1 lt 120576 Notice
first that10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
⩽
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
= 2sum
119895isin119869
120573119895
119898(119861119895 119861
119895)
119898 (119861119895)
lt120576
2
(48)
Hence1003817100381710038171003817119891 minus 1198911
1003817100381710038171003817
⩽
10038171003817100381710038171003817100381710038171003817100381710038171003817
1
sum119895isin119869120573119895
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus 119891
10038171003817100381710038171003817100381710038171003817100381710038171003817
⩽
1 minus sum119895isin119869120573119895
sum119895isin119869120573119895
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+120576
4
= (1 minussum
119895isin119869
120573119895) +
120576
2+120576
4
⩽120576
4+120576
2+120576
4= 120576
(49)
Lemma 13 (see [25 Lemma 33]) Suppose that 119879120583is a norm-
one element in L(1198711(119898)) for some 120583 isin 119872(119898) and there is
a nonnegative simple function 1198910such that 119891
0is a norm-one
element of 1198711(119898) and 119879
1205831198910 ⩾ 1 minus 120576
32
6 for some 0 lt 120576 lt 1
Abstract and Applied Analysis 7
Then there exist a norm-one bounded linear operator 119879] forsome ] isin 119872(119898119898) and a nonnegative simple function 119891
1
in 1198781198711(119898)
such that 119879120583minus 119879] ⩽ 120576 1198911 minus 1198910 ⩽ 3120576 and
(119889|]|1119889119898)(119909) = 1 for all 119909 isin supp(1198911)
Lemma 14 Suppose that 119879] isin L(1198711(119898)) is a norm-one
operator 119891 = sum119899
119894=1120573119894(120594
119861119894
119898(119861119894)) where 119898(119861
119895) gt 0 for all
1 ⩽ 119895 ⩽ 119899 and 119861119895119899
119895=1are mutually disjoint Borel subsets ofΩ
is a norm-one nonnegative simple function and 119892 is an elementof 119878
119871infin(119898)
such that
Re ⟨119892 119879]119891⟩ ⩾ 1 minus1205766
27(50)
for some 0 lt 120576 lt 1 and
119889|]|1
119889119898(119909) = 1 119892 (119909) = 1 (51)
for all x in the support of 119891Then there exist a nonnegative simple function 119891 isin 119878
1198711(119898)
a function119892 isin 119878
119871infin(119898)
and an operator119879] inL(1198711(119898) 1198711(119898))such that
⟨119892 119879]119891⟩ =1003817100381710038171003817119879]1003817100381710038171003817 = 1
1003817100381710038171003817119879] minus 119879]1003817100381710038171003817 ⩽ 2120576
10038171003817100381710038171003817119891 minus 119891
10038171003817100381710038171003817⩽ 3120576
1003817100381710038171003817119892 minus 1198921003817100381710038171003817 ⩽radic120576 ⟨119891 119892⟩ = 1
(52)
Proof Since
Re ⟨119892 119879]119891⟩ ⩾ 1 minus1205766
27 (53)
we have
1 minus1205766
27lt Re ⟨119892 119879]119891⟩
= intΩtimesΩ
119891 (119909)Re119892 (119910) 119889120583 (119909 119910)
=
119899
sum
119895=1
120573119895intΩtimesΩ
120594119861119895
(119909)
119898 (119861119895)
Re119892 (119910) 119889] (119909 119910)
(54)
Let
119869 = 119895 intΩtimesΩ
(
120594119861119895
(119909)
119898 (119861119895)
)Re119892 (119910) 119889] (119909 119910) gt 1 minus 1205763
26
(55)
Then from Lemma 11 we have sum119895isin119869120573119895gt 1 minus 120576
32 Let 119891
1=
sum119895isin119869120573119895(120594
119861119895
119898(119861119895)) where 120573
119895= 120573
119895(sum
119895isin119869120573119895) for all 119895 isin 119869
Then
10038171003817100381710038171198911 minus 1198911003817100381710038171003817 ⩽
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
(120573119895minus 120573
119895)
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+ sum
119895isin119869
120573119895⩽ 120576
3⩽ 120576 (56)
Note that there is a Borel measurable function ℎ on Ω times Ωsuch that 119889](119909 119910) = ℎ(119909 119910) 119889|]|(119909 119910) and |ℎ(119909 119910)| = 1 forall (119909 119910) isin Ω times Ω Let
119862 = (119909 119910) 1003816100381610038161003816119892 (119910) ℎ (119909 119910) minus 1
1003816100381610038161003816 ltradic120576
232 (57)
Define two measures ]119891and ]
119888as follows
]119891(119860) = ] (119860 119862) ]
119888(119860) = ] (119860 cap 119862) (58)
for every Borel subset 119860 ofΩ times Ω It is clear that
119889] = 119889]119891+ 119889]
119888 119889
10038161003816100381610038161003816]119891
10038161003816100381610038161003816= ℎ 119889]
119891
1198891003816100381610038161003816]1198881003816100381610038161003816 = ℎ 119889]119888 119889 |]| = 119889
10038161003816100381610038161003816]119891
10038161003816100381610038161003816+ 1198891003816100381610038161003816]1198881003816100381610038161003816
(59)
Since (119889|]|11198891198981)(119909) = 1 for all 119909 isin ⋃119899
119895=1119861119895 we have
1 =119889|]|1
1198891198981
(119909) =
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909) +1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909) (60)
for all 119909 isin 119861 = ⋃119899
119895=1119861119895 and we deduce that |]|1(119861
119895) = 119898
1(119861
119895)
for all 1 ⩽ 119895 ⩽ 119899We claim that |]
119891|1(119861
119895)119898
1(119861
119895) ⩽ 120576
22
2 for all 119895 isin 119869Indeed if |119892(119910)ℎ(119909 119910) minus 1| ⩾ radic120576232 then
Re (119892 (119910) ℎ (119909 119910)) ⩽ 1 minus 12057624 (61)
So we have
1 minus1205763
26
⩽1
1198981(119861
119895)
ReintΩtimesΩ
120594119861119895(119909)119892 (119910) 119889] (119909 119910)
=1
1198981(119861
119895)
intΩtimesΩ
120594119861119895(119909)
Re (119892 (119910) ℎ (119909 119910)) 119889 |]| (119909 119910)
=1
1198981(119861
119895)
intΩtimesΩ
120594119861119895(119909)
Re (119892 (119910) ℎ (119909 119910)) 119889 10038161003816100381610038161003816]11989110038161003816100381610038161003816(119909 119910)
+1
1198981(119861
119895)
intΩtimesΩ
120594119861119895(119909)
Re (119892 (119910) ℎ (119909 119910)) 119889 1003816100381610038161003816]1198881003816100381610038161003816 (119909 119910)
⩽1
1198981(119861
119895)
((1 minus120576
24)10038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
(119861119895) +1003816100381610038161003816]1198881003816100381610038161003816
1
(119861119895))
= 1 minus120576
24
10038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
(119861119895)
1198981(119861
119895)
(62)
This proves our claimWe also claim that for each 119895 isin 119869 there exists a Borel
subset 119861119895of 119861
119895such that
(1 minus120576
2)119898
1(119861
119895) ⩽ 119898
1(119861
119895) ⩽ 119898
1(119861
119895)
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909) ⩽120576
2
(63)
8 Abstract and Applied Analysis
for all 119909 isin 119861119895 Indeed set 119861
119895= 119861
119895cap 119909 isin Ω (119889|]
119891|1119889119898
1)
(119909) ⩽ 1205762 Then
int119861119895119861119895
120576
2119889119898
1(119909) ⩽ int
119861119895
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909) 1198891198981(119909)
=10038161003816100381610038161003816]1119891
10038161003816100381610038161003816(119861
119895) ⩽1205762
22119898
1(119861
119895)
(64)
This shows that 1198981(119861
119895 119861
119895) ⩽ (1205762)119898
1(119861
119895) This proves our
second claimNow we define 119892 by 119892(119910) = 119892(119910)|119892(119910)| if |119892(119910)| ⩾ 1 minus
radic120576232 and 119892(119910) = 119892(119910) if |119892(119910)| lt 1 minus radic120576232 and we
write 119891 = sum119895isin119869120573119895(120594
119861119895
1198981(119861
119895)) It is clear that 119892 isin 119878
119871infin(119898)
119892 minus 119892 lt radic120576 and 119892(119910) = 1 for all 119909 isin supp119891
Finally we define the measure
119889] (119909 119910)
= sum
119895isin119869
120594119861119895
(119909) 119892 (119910)ℎ (119909 119910)119889]119888(119909 119910)(
1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+ 1205941198691119861(119909) 119889] (119909 119910)
(65)
where 119861 = ⋃119895isin119869119861119895 It is easy to see that (119889|]|1119889119898
1)(119909) = 1
on 119861 and (119889|]|11198891198981)(119909) ⩽ 1 elsewhere Note that
119889 (] minus ]) (119909 119910)
= sum
119895isin119869
120594119861119895
(119909) [
[
119892 (119910)ℎ (119909 119910)(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1]
]
119889]119888(119909 119910)
minus sum
119895isin119869
120594119861119895
(119909) 119889]119891(119909 119910)
(66)
If (119909 119910) isin 119862 then |119892(119910)| ⩾ 1 minus radic120576232 ⩾ 1 minus 1232 and
100381610038161003816100381610038161003816119892 (119910)ℎ (119909 119910) minus 1
100381610038161003816100381610038161003816
=
100381610038161003816100381610038161003816100381610038161003816
119892 (119910)
1003816100381610038161003816119892 (119910)1003816100381610038161003816
ℎ (119909 119910) minus 1
100381610038161003816100381610038161003816100381610038161003816
⩽
1003816100381610038161003816119892 (119910) ℎ (119909 119910) minus 11003816100381610038161003816
1003816100381610038161003816119892 (119910)1003816100381610038161003816
+
10038161003816100381610038161 minus1003816100381610038161003816119892 (119910)
1003816100381610038161003816
10038161003816100381610038161003816100381610038161003816119892 (119910)
1003816100381610038161003816
⩽ 2
1003816100381610038161003816119892 (119910) ℎ (119909 119910) minus 11003816100381610038161003816
1003816100381610038161003816119892 (119910)1003816100381610038161003816
⩽ 2radic120576
232
232
232 minus 1⩽ 2radic120576
(67)
Hence for all (119909 119910) isin 119862 we have
10038161003816100381610038161003816100381610038161003816100381610038161003816
119892 (119910)ℎ (119909 119910)(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
⩽100381610038161003816100381610038161003816119892 (119910)ℎ (119909 119910) minus 1
100381610038161003816100381610038161003816(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+
10038161003816100381610038161003816100381610038161003816100381610038161003816
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
⩽ 2radic120576(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+
10038161003816100381610038161003816100381610038161003816100381610038161003816
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
(68)
So we have for all 119909 isin 1198691
119889|] minus ]|1
1198891198981
(119909)
⩽ sum
119895isin119869
120594119861119895
(119909) [
[
2radic120576(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+
10038161003816100381610038161003816100381610038161003816100381610038161003816
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
]
]
1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909)
+ sum
119895isin119869
120594119861119895
(119909)
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909)
⩽ sum
119895isin119869
120594119861119895
(119909)(2radic120576 + (1 minus1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909)))
+sum
119895isin119869
120594119861119895
(119909)(
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909))
⩽ 2radic120576 + 120576 lt 3radic120576
(69)
This gives that 119879] minus 119879] lt 3radic120576 Note also that for all 119895 isin 119869
⟨119879]
120594119861119895
1198981(119861
119895)
119892⟩
= intΩtimesΩ
120594119861119895
(119909)
1198981(119861
119895)
119892 (119910) 119889] (119909 119910)
= intΩtimesΩ
120594119861119895
(119909)
1198981(119861
119895)
ℎ (119909 119910)(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
119889]119888(119909 119910)
Abstract and Applied Analysis 9
= intΩ
120594119861119895
(119909)
1198981(119861
119895)
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
1198891003816100381610038161003816]1198881003816100381610038161003816
1
(119909)
= intΩ
120594119861119895
(119909)
1198981(119861
119895)
1198891198981(119909) = 1
(70)
Hence we get ⟨119879]119891 119892⟩ = 1 which implies that 119879]119891 =119879] = 1 Finally10038171003817100381710038171003817119891 minus 119891
10038171003817100381710038171003817
⩽10038171003817100381710038171003817119891 minus 119891
1
10038171003817100381710038171003817+10038171003817100381710038171198911 minus 119891
1003817100381710038171003817
=
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
1198981(119861
119895)
minus sum
119895isin119869
120573119895
120594119861119895
1198981(119861
119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+ 120576
⩽ sum
119895isin119869
120573119895(
10038171003817100381710038171003817100381710038171003817100381710038171003817
120594119861119895
1198981(119861
119895)
minus
120594119861119895
1198981(119861
119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+
10038171003817100381710038171003817100381710038171003817100381710038171003817
120594119861119895
1198981(119861
119895)
minus
120594119861119895
1198981(119861
119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
) + 120576
= 2sum
119895isin119869
120573119895
1198981(119861
119895 119861
119895)
1198981(119861
119895)
+ 120576
⩽ 2sum
119895isin119869
120573119895
(1205762)1198981(119861
119895)
1198981(119861
119895)
+ 120576 ⩽120576
1 minus 1205762+ 120576 lt 3120576
(71)
We are now ready to present the proof of the main resultin the case of finite regular positive Borel measures
Proof of Proposition 10 Let 1205751= 120575
3
2(5sdot2
4) 120575
2= 120575
12
3(3
2sdot2
14)
and 1205753= (12057610)
2 for some 0 lt 120576 lt 1 Suppose that 119879 isinL(119871
1(119898)) with 119879 = 1 and that there is an 119891
0isin 119878
1198711(119898)
and1198920isin 119878
119871infin(119898)
such that ⟨1198910 119892
0⟩ = 1 and |⟨119879119891
0 119892
0⟩| gt 1minus120575
3
12
6Then there is an isometric isomorphism Ψ from 119871
1(119898) onto
itself such thatΨ(1198910) = |119891
0| and there is a scalar number 120572 in
119878R such that |⟨1198791198910 119892
0⟩| = ⟨120572119879119891
0 119892
0⟩ Then letting 119891
1= Ψ119891
0
1198921= (Ψ
minus1)lowast1198920 and 119879
1= 120572Ψ119879Ψ
minus1 we have
⟨1198781198911 119892
1⟩ = ⟨120572Ψ119879
0Ψ
minus1Ψ119891
0 (Ψ
minus1)lowast
1198920⟩
= ⟨1205721198791198910 119892
0⟩ gt 1 minus
1205753
1
26
⟨1198911 119892
1⟩ = ⟨Ψ119891
0 (Ψ
minus1)lowast
1198920⟩ = 1
(72)
Since 11987911198911 gt 1 minus 120575(120575
3
12
6) by Lemma 13 there exists a
norm-one bounded operator 119879] and a nonnegative simplefunction119891
2isin 119878
1198711(119898)
such that 1198791minus119879] ⩽ 1205751 1198912minus1198911 ⩽ 31205751
and (119889|]|11198891198981)(119909) = 1 for all 119909 isin supp(119891
2) Then
⟨119879]1198912 1198921⟩
= ⟨11987911198911 119892
1⟩ minus ⟨119879
11198911minus 119879
11198912 119892
1⟩ minus ⟨119879
11198912minus 119879]1198912 1198921⟩
⩾ ⟨11987911198911 119892
1⟩ minus10038171003817100381710038171198911 minus 1198912
1003817100381710038171003817 minus10038171003817100381710038171198791 minus 119879]
1003817100381710038171003817
⩾ 1 minus1205753
1
26minus 3120575
1minus 120575
1⩾ 1 minus 5120575
1= 1 minus
1205753
2
16
(73)
Notice also that
⟨1198912 119892
1⟩ = ⟨119891
1 119892
1⟩ minus ⟨119891
1minus 119891
2 119892
1⟩ ⩾ 1 minus
10038171003817100381710038171198911 minus 11989121003817100381710038171003817
⩾ 1 minus 31205751⩾ 1 minus 5120575
1= 1 minus
1205753
2
16
(74)
By Lemma 12 there are a nonnegative simple function 1198913isin
1198781198711(119898)
and a function 1198923isin 119878
119871infin(119898)
such that
1198923(119909) = 120594supp119891
3
(119909) + 1198922(119909) 120594
Ωsupp1198913
(119909)
10038171003817100381710038171198912 minus 11989131003817100381710038171003817 ⩽ 1205752
10038171003817100381710038171198923 minus 11989211003817100381710038171003817 ⩽radic120575
2 ⟨119891
3 119892
3⟩ = 1
(75)
So we have
⟨119879]1198913 1198923⟩ = ⟨119879]1198912 1198921⟩ minus ⟨119879]1198912 minus 119879]1198913 1198921⟩
minus ⟨119879]1198913 1198921 minus 1198923⟩
⩾ 1 minus1205753
2
16minus 2radic120575
2⩾ 1 minus 3radic120575
2= 1 minus
1205756
3
27
(76)
By Lemma 14 there exist 1198914isin 119878
1198711(119898)
and 1198924isin 119878
119871infin(119898)
and anoperator 119879
4such that
⟨1198924 119879
41198914⟩ = 1 =
100381710038171003817100381711987941003817100381710038171003817
10038171003817100381710038171198794 minus 119879]1003817100381710038171003817 ⩽ 21205753
10038171003817100381710038171198914 minus 11989131003817100381710038171003817 ⩽ 31205753
10038171003817100381710038171198924 minus 11989231003817100381710038171003817 ⩽radic120575
3 ⟨119891
4 119892
4⟩ = 1
(77)
So we have
10038171003817100381710038171198794 minus 11987911003817100381710038171003817 ⩽10038171003817100381710038171198794 minus 119879]
1003817100381710038171003817 +1003817100381710038171003817119879] minus 1198791
1003817100381710038171003817
⩽ 1205751+ 2120575
3⩽ 3120575
3
10038171003817100381710038171198911 minus 11989141003817100381710038171003817 ⩽10038171003817100381710038171198911 minus 1198912
1003817100381710038171003817 +10038171003817100381710038171198912 minus 1198913
1003817100381710038171003817 +10038171003817100381710038171198913 minus 1198914
1003817100381710038171003817
⩽ 31205751+ 120575
2+ 3120575
3⩽ 10120575
3
10038171003817100381710038171198921 minus 11989241003817100381710038171003817 ⩽10038171003817100381710038171198921 minus 1198923
1003817100381710038171003817 + 1198923 minus 1198924
⩽ 1205752+ radic120575
3⩽ 2radic120575
3
(78)
10 Abstract and Applied Analysis
Let 119878 = 120572Ψminus11198794Ψ 119891 = Ψminus1
1198914 and 119892 = Ψlowast
1198924 then we have
119879 minus 119878 =10038171003817100381710038171003817119879 minus 120572Ψ
minus11198794Ψ10038171003817100381710038171003817=10038171003817100381710038171003817120572Ψ119879Ψ
minus1minus 119879
4
10038171003817100381710038171003817
=10038171003817100381710038171198791 minus 1198794
1003817100381710038171003817 ⩽ 31205753
100381710038171003817100381710038171198910minus 11989110038171003817100381710038171003817=100381710038171003817100381710038171198910minus Ψ
minus11198914
10038171003817100381710038171003817=10038171003817100381710038171198911 minus 1198914
1003817100381710038171003817 ⩽ 101205753
100381710038171003817100381710038171198920minus 11989110038171003817100381710038171003817=10038171003817100381710038171198920 minus Ψ
lowast1198924
1003817100381710038171003817 =100381710038171003817100381710038171003817(Ψ
minus1)lowast
1198920minus 119892
4
100381710038171003817100381710038171003817
=10038171003817100381710038171198921 minus 1198924
1003817100381710038171003817 ⩽ 2radic120575
3
⟨119891 119892⟩ = ⟨Ψminus11198914 Ψ
lowast1198924⟩ = ⟨119891
4 119892
4⟩ = 1
10038161003816100381610038161003816⟨119878119891 119892⟩
10038161003816100381610038161003816=10038161003816100381610038161003816⟨120572Ψ
minus11198794ΨΨ
minus11198914 Ψ
lowast1198924⟩10038161003816100381610038161003816= |120572| = 1
(79)
This completes the proof
Finally we may give the proof of the main result in fullgenerality
Proof of Theorem 9 Notice that the Kakutani representationtheorem (see [26] for a reference) says that for every 120590-finitemeasure ] the space 119871
1(]) is isometrically isomorphic to
1198711(119898) for some positive Borel regular measure on a compact
Hausdorff space Then by Proposition 10 there is a universalfunction 120576 997891rarr 120578(120576) gt 0 which gives the BPBp-nu for 119871
1(]) for
every 120590-finite measure ]Fix 120576 gt 0 Suppose that 119879
0isin L(119871
1(120583)) with V(119879
0) = 1
and (1198910 119891
lowast
0) isin Π(119871
1(120583)) satisfy
1003816100381610038161003816⟨119891lowast
0 119879
01198910⟩1003816100381610038161003816 gt 1 minus 120578 (120576) (80)
Choose a sequence 119891119899 in 119871
1(120583) such that sup
119899119879
0119891119899 = 1
and let 119866 be the closed linear span of
119879119899
0119891119898 119899 119898 isin N cup 0 (81)
As 119866 is separable there is a dense subset 119892119899 119899 isin N of 119866
and let 119864 = ⋃infin
119899=1supp119892
119899 where supp119892
119899is the support of 119892
119899
Then the measure 120583|119864is 120590-finite Let
119884 = 119891 isin 1198711(120583) supp (119891) sub 119864 (82)
be a closed subspace of 1198711(120583) It is clear that 119871
1(120583) = 119884oplus
1119885
and 119884 is isometrically isomorphic to 1198711(120583|
119864) So 119884 has the
BPBp-nu with 120578(120576)Now write 119878
0= 119879
0|119884 119884 rarr 119884 consider 119910
0= 119891
0isin
119878119884 119910lowast
0= 119891
lowast
0|119884isin 119878
119884lowast and observe that 119910lowast
0(119910
0) = 1 and
|119910lowast
0(119878
01199100)| = |119891
lowast
0(119879
01198910)| gt 1 minus 120578(120576) Hence there exist 119878 isin
L(119884) and (1199100 119910
lowast
0) isin Π(119884) such that
1003816100381610038161003816119910lowast
0(119878119910
0)1003816100381610038161003816 = 1 = V (119878) 1003817100381710038171003817119878 minus 1198780
1003817100381710038171003817 lt 120576
10038171003817100381710038171199100 minus 11991001003817100381710038171003817 lt 120576
1003817100381710038171003817119910lowast
0minus 119910
lowast
0
1003817100381710038171003817 lt 120576
(83)
Finally consider the operator 119879 isinL(1198711(120583)) given by
119879 (119910 119911)
= (119878119910 0) + 1198790(0 119911) ((119910 119911) isin 119871
1(120583) equiv 119884oplus
1119885)
(84)
We have 119879 = 1 (and so V(119879) = 1) Indeed1003817100381710038171003817119879 (119910 119911)
1003817100381710038171003817 =1003817100381710038171003817(119878119910 0)
1003817100381710038171003817 +10038171003817100381710038171198790 (0 119911)
1003817100381710038171003817 ⩽10038171003817100381710038171199101003817100381710038171003817 + 119911 =
1003817100381710038171003817(119910 119911)1003817100381710038171003817
(85)
for all (119910 119911) isin 1198711(120583) and 119879(119910
0 0) = (119878119910
0 0) = 119878119910
0 = 1
Let 119909 = (1199100 0) and 119909lowast = (119910lowast
0 119891
0|119885) Then (119909 119909lowast) isin Π(119871
1(120583))
Moreover we have1003816100381610038161003816119909
lowast1198791199091003816100381610038161003816 =1003816100381610038161003816119910
lowast
0119878119910
0
1003816100381610038161003816 = 1 = V (119879) 1003817100381710038171003817119909 minus 1198910
1003817100381710038171003817 =1003817100381710038171003817119910 minus 1199100
1003817100381710038171003817 lt 120576
1003817100381710038171003817119909lowast
0minus 119891
lowast
0
1003817100381710038171003817
= max 1003817100381710038171003817119910 minus 119891lowast
0|119884
1003817100381710038171003817 1003817100381710038171003817119891
lowast
0|119885minus 119891
lowast
0|119885
1003817100381710038171003817
=1003817100381710038171003817119910
lowastminus 119910
lowast
0
1003817100381710038171003817 lt 120576
1003817100381710038171003817119879 minus 11987901003817100381710038171003817 = sup119910+119911⩽1
1003817100381710038171003817119879 (119910 119911) minus 1198790 (119910 119911)1003817100381710038171003817
= sup119910⩽1
1003817100381710038171003817119878119910 minus 11987801199101003817100381710038171003817 =1003817100381710038171003817119878 minus 1198780
1003817100381710038171003817 lt 120576
(86)
This completes the proof
5 Examples of Spaces Failing theBishop-Phelps-Bollobaacutes Property forNumerical Radius
Our goal here is to prove that the density of numerical radiusattaining operators does not imply the BPBp-nu Actuallywe will show that among separable spaces there is noisomorphic property implying the BPBp-nu other than finite-dimensionality
We need to relate the BPBp-nu to the Bishop-Phelps-Bollobas property for operators which as mentioned in theintroduction was introduced in [15] A pair (119883 119884) of Banachspaces has the Bishop-Phelps-Bollobas property for operators(in short BPBp) if given 120576 gt 0 there exists 120578(120576) gt 0 suchthat given 119879 isin L(119883 119884) with 119879 = 1 and 119909 isin 119878
119883such that
119879119909 gt 1 minus 120578(120576) then there exist 119911 isin 119878119883and 119878 isin L(119883 119884)
satisfying
119878 = 119878119911 = 1 119909 minus 119911 lt 120576
119879 minus 119878 lt 120576
(87)
We refer the reader to [15 19 25] and references therein formore information and background Among the interestingresults on the BPBp we emphasize that a pair (119883 119884) when119883is finite-dimensional does not necessarily have the BPBp Forinstance if119884 is a strictly convex space which is not uniformlyconvex then the pair (ℓ(2)
1 119884) fails to have the BPBp (this is
contained in [15] see [19 Section 3])
Abstract and Applied Analysis 11
The next result relates the BPBp-nu to the BPBp foroperators in a particular case We will deduce our examplefrom it
Theorem 15 If 1198711(120583)oplus
1119883 has the BPBp-119899119906 then the pair
(1198711(120583) 119883) has the BPBp for operators
Before proving this proposition we will use it to get themain examples of this section The first example shows thatthe density of numerical radius attaining operators does notimply the BPBp-nu
Example 16 There is a reflexive space (and so numerical radiusattaining operators on it are dense) which fails to have theBPBp-nu Indeed let 119884 be a reflexive separable space whichis not superreflexive and we may suppose that 119884 is strictlyconvex Observe that 119884 cannot be uniformly convex since itis not superreflexive Now 119883 = ℓ(2)
1oplus1119884 is reflexive but the
pair (ℓ(2)1 119884) fails the BPBp since 119884 is strictly convex but not
uniformly convex [19 Corollary 33] Therefore Theorem 15gives us that119883 does not have the BPBp-nu
The example above can be extended to get the result thatevery infinite-dimensional separable Banach space can berenormed to fail the BPBp-nu This follows from the factthat every infinite-dimensional separable Banach space canbe renormed to be strictly convex but not uniformly convex(this result can be proved ldquoby handrdquo an alternative categoricalargument for it can be found in [27] and references therein)With a little more of effort we may get the main result of thesection
Theorem 17 Every infinite-dimensional separable Banachspace can be renormed to fail the weak-BPBp-nu (and so inparticular to fail the BPBp-nu)
We need the following result which is surely well knownAs we have not found a reference we include a nice and easyproof kindly given to us by Vladimir Kadets We recall thatgiven a Banach space 119884 the set of all equivalent norms on119884 can be viewed as a metric space using the Banach-Mazurdistance
Lemma 18 Let119884 be an infinite-dimensional separable Banachspace Then the set of equivalent norms on 119884 which are strictlyconvex and are not (locally) uniformly convex is dense in the setof all equivalent norms on119884 (with respect to the Banach-Mazurdistance)
Proof Fix 119890 isin 119878119884and 119890lowast
1isin 119878
119884lowast such that 119890lowast
1(119890) = 1 For a fixed
120576 isin (0 12) denote
119902 (119910) = max (1 minus 120576) 10038171003817100381710038171199101003817100381710038171003817 1003816100381610038161003816119890lowast
1(119910)1003816100381610038161003816 (119910 isin 119884) (88)
Evidently (1 minus 120576)119910 ⩽ 119902(119910) ⩽ 119910 for every 119910 isin 119884 Fix asequence 119890lowast
119896 119896 ⩾ 2 of norm-one functionals separating
the points of 119884 and denote
119901 (119910) = radic
infin
sum
119896=1
1
2119896
1003816100381610038161003816119890lowast
119896(119910)1003816100381610038161003816
2
(119910 isin 119884) (89)
Then 119901 is a strictly convex norm on 119884 119901(119890) ⩾ 1radic2 and119901(119910) ⩽ 119910 for all 119910 isin 119883 Finally write
100381710038171003817100381711991010038171003817100381710038171= (1 minus 120576) 119902 (119910) + 120576
119901 (119910)
119901 (119890)(119910 isin 119884) (90)
Then sdot 1is a strictly convex norm on 119884 and
(1 minus 120576)2 10038171003817100381710038171199101003817100381710038171003817 ⩽100381710038171003817100381711991010038171003817100381710038171⩽ (1 + 120576)
10038171003817100381710038171199101003817100381710038171003817 (119910 isin 119884) (91)
Wewill finish the proof by showing that sdot 1is not uniformly
convex (actually it is not locally uniformly convex) Indeedfor each 119899 isin N we select 119910
119899isin ⋂
119899
119896=1ker 119890lowast
119896with 119910
119899 = 1 and
consider 119890119899= 119890 + (1205764)119910
119899 Then 119902(119890) = 1 119902(119890
119899) = 1 and
119902(119890+119890119899) = 2 At the same time 119901(119910
119899) rarr 0 so 119901(119890
119899) rarr 119901(119890)
and 119901(119890 + 119890119899) rarr 2119901(119890) Consequently
1198901 = 1100381710038171003817100381711989011989910038171003817100381710038171997888rarr 1
1003817100381710038171003817119890 + 11989011989910038171003817100381710038171997888rarr 2 (92)
but 119890 minus 1198901198991= (1205764)119910
1198991⩾ (1 minus 120576)
2(1205764) which means the
absence of local uniform convexity at 119890
Proof of Theorem 17 Let 119883 be an infinite-dimensional sepa-rable Banach space Take a closed subspace 119884 of 119883 of codi-mension two By [28 Proposition 2] the map carrying everyequivalent norm on 119884 to its numerical index is continuousand so the set of values of the numerical index of 119884 upto reforming is a nontrivial interval [28 Theorem 9] ThenLemma 18 allows us to find an equivalent norm | sdot | on 119884 insuch a way that (119884 | sdot |) is strictly convex and is not uniformlyconvex and 119899(119884 | sdot |) gt 0 Now the space 119883 = ℓ(2)
1oplus1(119884 | sdot |)
is an equivalent renorming of 119883 which does not have theBPBp-119899119906 (indeed otherwise the pair (ℓ(2)
1 (119884 | sdot |)) would
have the BPBp for the operator norm and so (119884 | sdot |) wouldbe uniformly convex by [19 Corollary 33] a contradiction)Moreover as
119899 (119883) = min 119899 (ℓ(2)1) 119899 (119884 |sdot|) gt 0 (93)
(see [20 Proposition 2] for instance) 119883 also fails the weak-BPBp-nu by Proposition 6
To finish the section with the promised proof ofTheorem 15 we first see the following stability result
Lemma 19 Let119883 = [⨁infin
119896=1119883
119896]1198880
or [⨁infin
119896=1119883
119896]ℓ1
If119883 has theBishop-Phelps-Bollobas property for numerical radius with afunction 120578 then each Banach space 119883
119894has the Bishop-Phelps-
Bollobas property for numerical radius with 120578nu(119883119894) ⩾ 120578 That
is inf119894120578nu(119883119894
)(120576) ⩾ 120578nu(119883)(120576) for all 0 lt 120576 lt 1
Proof Let 119875119894 119883 rarr 119883
119894and 1198751015840
119894 119883
lowastrarr 119883
lowast
119894be the natural
projections and let 119876119894 119883
119894rarr 119883 and 1198761015840
119894 119883
lowast
119894rarr 119883
lowast be thenatural embeddings
Assume that an operator 119879119894 119883
119894rarr 119883
119894and a pair
(119909119894 119909
lowast
119894) isin Π(119883
119894) satisfy that
V (119879119894) = 1
1003816100381610038161003816119909lowast
119894119879119894119909119894
1003816100381610038161003816 gt 1 minus 120578 (120576) (94)
We define an operator 119879 119883 rarr 119883 and (119909 119909lowast) isin Π(119883) by
119879 = 119876119894∘ 119879
119894∘ 119875
119894 (119909 119909
lowast) = (119876
119894119909119894 119876
1015840
119894119909lowast
119894) (95)
12 Abstract and Applied Analysis
then clearly we see that1003816100381610038161003816119909
lowast1198791199091003816100381610038161003816 =1003816100381610038161003816119909
lowast
119894119879119894119909119894
1003816100381610038161003816 gt 1 minus 120578 (120576) (96)
From the assumption there exist 119878 119883 rarr 119883 and a pair(119910 119910
lowast) isin Π(119883) such that
1003816100381610038161003816119910lowast1198781199101003816100381610038161003816 = 1 = V (119878) 119878 minus 119879 lt 120576
1003817100381710038171003817119910lowastminus 119909
lowast1003817100381710038171003817 lt 1205761003817100381710038171003817119910 minus 119909
1003817100381710038171003817 lt 120576
(97)
Since this clearly shows that1003817100381710038171003817119875119894 ∘ 119878 ∘ 119876119894
minus 119879119894
1003817100381710038171003817 lt 120576100381710038171003817100381710038171198751015840
119894119910lowastminus 119909
lowast
119894
10038171003817100381710038171003817lt 120576
1003817100381710038171003817119875119894119910 minus 1199091198941003817100381710038171003817 lt 120576
(98)
we only need to show that |1198751015840119894119910lowast(119875
119894∘ 119878 ∘ 119876
119894)119875
119894119910| = 1
We first show the case of 1198880sum Since
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817=10038171003817100381710038171003817119875119895119910 minus 119875
11989511990910038171003817100381710038171003817⩽1003817100381710038171003817119910 minus 119909
1003817100381710038171003817 lt 120576 (99)
for every 119895 = 119894 we have
1 = 119910lowast(119910) = sum
119895isinN
1198751015840
119895119910lowast(119875
119895119910) ⩽ sum
119895isinN
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817
⩽100381710038171003817100381710038171198751015840
119894119910lowast10038171003817100381710038171003817+ 120576 sum
119895isinN119895 = 119894
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817⩽1003817100381710038171003817119910
lowast1003817100381710038171003817 = 1
(100)
This shows that 1198751015840119894119910lowast = 1 and 1198751015840
119895119910lowast= 0 for every 119895 = 119894
So 119910lowast = 1198761015840
1198941198751015840
119894119910lowast and 1198751015840
119894119910lowast(119875
119894119910) = 1 This and the fact that
119910 minus 119876119894119875119894119910 lt 120576 imply that
(119876119894119875119894119910 + (
1
120576) (119910 minus 119876
119894119875119894119910) 119876
1015840
1198941198751015840
119894119910lowast) isin Π (119883) (101)
So we get that (1198761015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910 + (1120576)(119910 minus 119876
119894119875119894119910))V(119878) = 1
Hence we have
1 =1003816100381610038161003816119910
lowast1198781199101003816100381610038161003816 =10038161003816100381610038161003816(119876
1015840
1198941198751015840
119894119910lowast) 11987811991010038161003816100381610038161003816
=
1003816100381610038161003816100381610038161003816(1 minus 120576) (119876
1015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910)
+120576 (1198761015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910 +1
120576(119910 minus 119876
119894119875119894119910))
1003816100381610038161003816100381610038161003816⩽ 1
(102)
and so we get |1198751015840119894119910lowast(119875
119894∘119878 ∘119876
119894)119875
119894119910| = |(119876
1015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910)| = 1
We next show the case of ℓ1sum The proof is almost
the same as that of the 1198880case However for the sake of
completeness we provide it hereSince 1198751015840
119895119910lowast = 119875
1015840
119895119910lowastminus 119875
1015840
119895119909lowast ⩽ 119910
lowastminus 119909
lowast lt 120576 for every
119895 = 119894 we have
1 = 119910lowast(119910) = sum
119895isinN
1198751015840
119895119910lowast(119875
119895119910) ⩽ sum
119895isinN
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817
⩽10038171003817100381710038171198751198941199101003817100381710038171003817 + 120576 sum
119895isinN119895 = 119894
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817⩽10038171003817100381710038171199101003817100381710038171003817 = 1
(103)
which shows 119875119894119910 = 1 and 119875
119895119910 = 0 for every 119895 = 119894 Since this
implies (119876119894119875119894119910119876
1015840
1198941198751015840
119894119910lowast+(1120576)(119910
lowastminus119876
1015840
1198941198751015840
119894119910lowast)) isin Π(119883) we get
that |(1198761015840
1198941198751015840
119894119910lowast+ (1120576)(119910
lowastminus 119876
1015840
1198941198751015840
119894119910lowast))119878(119876
119894119875119894119910)| ⩽ V(119878) = 1
Hence we have
1 =1003816100381610038161003816119910
lowast1198781199101003816100381610038161003816 =1003816100381610038161003816119910
lowast119878 (119876
119894119875119894119910)1003816100381610038161003816
=
1003816100381610038161003816100381610038161003816(1 minus 120576) (119876
1015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910)
+120576 (1198761015840
1198941198751015840
119894119910lowast+1
120576(119910
lowastminus 119876
1015840
1198941198751015840
119894119910lowast)) 119878 (119876
119894119875119894119910)
1003816100381610038161003816100381610038161003816⩽ 1
(104)
and so |1198751015840119894119910lowast(119875
119894∘ 119878 ∘ 119876
119894)119875
119894119910| = |(119876
1015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910)| = 1
Proof of Theorem 15 Note that 120578nu(1198711(120583)oplus1119883)(120576) rarr 0 as120576 rarr 0 Fix 0 lt 120576
0lt 1 and choose 0 lt 120576 lt 1 such that
6120576 + 120578nu(1198711(120583)oplus1119883)(120576) lt 1205760 Let 120578(1205760) = 120578nu(1198711(120583)oplus1119883)(120576)Suppose that 119879
0isin L(119871
1(120583) 119883) with 119879
0 = 1 and 119891
0isin
1198781198711(120583)
satisfy
1003817100381710038171003817119879011989101003817100381710038171003817 gt 1 minus 120578 (1205760) (105)
For any measurable subset 119861 let
1198711(120583|
119861) = 119891|
119861 119891 isin 119871
1(120583) (106)
with the norm 119891|119861 = 119891120594
1198611 Then it is easy to see
that 1198711(120583|
119861) is isometrically isomorphic to a complemented
subspace of 1198711(120583) Let 119875
119861 119871
1(120583) rarr 119871
1(120583|
119861) be the
restriction defined by 119875119861(119891) = 119891|
119861for all 119891 isin 119871
1(120583) and
let 119869119861 119871
1(120583|
119861) rarr 119871
1(120583) be the extension defined by
119869119861(119891)(120596) = 119891(120596) if 120596 isin 119861 and 119869
119861(119891)(120596) = 0 otherwise
It is clear that 119875119861119869119861= Id
1198711(120583|119861)and 119869
119861119875119861(119891) = 119891120594
119861for all
119891 isin 1198711(120583) Notice also that 119871
1(120583) is isometrically isomorphic
to 1198711(120583|
119861)oplus
11198711(120583|
119861119888)
Let 119860 = supp1198910and 119892
0= 119875
1198601198910 Then
1003817100381710038171003817119879011986911986011989201003817100381710038171003817 =100381710038171003817100381711987901198910
1003817100381710038171003817 gt 1 minus 120578 (1205760) gt 0 (107)
and define the operator 119879119860 119871
1(120583|
119860) rarr 119883 by 119879
119860119891 =
1198790119869119860119891119879
0119869119860 for every 119891 isin 119871
1(120583|
119860) Then
100381710038171003817100381711987911986011989201003817100381710038171003817 ⩾100381710038171003817100381711987901198910
1003817100381710038171003817 gt 1 minus 120578 (1205760) (108)
Since 120583|119860is 120590-finite 119871
1(120583|
119860)lowast= 119871
infin(120583|
119860) Let 119892lowast
0isin 119878
119871infin(120583|119860)
be a function such that ⟨119892lowast0 119892
0⟩ = 1 and choose 119909lowast
0isin 119878
119883lowast
such that 119909lowast0(119879
1198601198920) = 119879
1198601198920 Define the operator 119878
0isin
L(1198711(120583|
119860)oplus
1119883) by
1198780(119891 119909) = (0 119879
119860119891) ((119891 119909) isin 119871
1(120583|
119860) oplus
1119883) (109)
and observe that 1198780 = V(119878
0) = 1 Indeed
100381710038171003817100381711987801003817100381710038171003817 ⩽ 1 =
10038171003817100381710038171198791198601003817100381710038171003817
= sup 1003816100381610038161003816119909lowast1198791198601198911003816100381610038161003816 119909
lowastisin 119878
119883lowast 119891 isin 119878
1198711(120583|119860)
= sup 1003816100381610038161003816(119891lowast 119909
lowast) 119878
0(119891 119909)
1003816100381610038161003816
((119891lowast 119909
lowast) (119891 119909)) isin Π (119871
1(120583|
119860) oplus
1119883)
= V (1198780) ⩽100381710038171003817100381711987801003817100381710038171003817
(110)
Abstract and Applied Analysis 13
It is immediate that
(119892lowast
0 119909
lowast
0) 119878
0(119892
0 0) = 119909
lowast
0(119879
1198601198920) =10038171003817100381710038171198791198601198920
1003817100381710038171003817 gt 1 minus 120578 (1205760)
(111)
By Lemma 19 1198711(120583|
119860)oplus
1119883 has the BPBp-nu with the func-
tion 120578 Therefore there exist 1198781isin L(119871
1(120583|
119860)oplus
1119883) (119892
1 119909
1) isin
1198781198711(120583|119860)oplus1119883 and (119892lowast
1 119909
lowast
1) isin 119878
119871infin(120583|119860)oplusinfin119883lowast such that
1003817100381710038171003817(1198921 1199091) minus (1198920 0)1003817100381710038171003817 lt 120576
1003817100381710038171003817(119892lowast
1 119909
lowast
1) minus (119891
lowast
0 119909
lowast
0)1003817100381710038171003817 lt 120576
10038171003817100381710038171198781 minus 11987801003817100381710038171003817 lt 120576 ⟨(119892
lowast
1 119909
lowast
1) (119892
1 119909
1)⟩ = 1
1003816100381610038161003816(119892lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)1003816100381610038161003816 = V (119878
1) = 1
(112)
Claim 1 We claim that 1199091= 0
Otherwise
1 = Re ⟨(119892lowast1 119909
lowast
1) (119892
1 119909
1)⟩
=100381710038171003817100381711989211003817100381710038171003817Re⟨(119892
lowast
1 119909
lowast
1) (119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
⟩
+100381710038171003817100381711990911003817100381710038171003817Re⟨(119892
lowast
1 119909
lowast
1) (0 119909
1)
100381710038171003817100381711990911003817100381710038171003817
⟩ ⩽ 1
(113)
We deduce that
((119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
(119892lowast
1 119909
lowast
1)) (
(0 1199091)
100381710038171003817100381711990911003817100381710038171003817
(119892lowast
1 119909
lowast
1))
isin Π (1198711(120583|
119860) oplus
1119883)
(114)
Since 1198781((0 119909
1)119909
1) = (119878
1minus 119878
0)((0 119909
1)119909
1) lt 120576 we
get that
1 =1003816100381610038161003816⟨(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)⟩1003816100381610038161003816
=
100381610038161003816100381610038161003816100381610038161003816
100381710038171003817100381711989211003817100381710038171003817⟨(119892
lowast
1 119909
lowast
1) 119878
1((119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
)⟩
+100381710038171003817100381711990911003817100381710038171003817⟨(119892
lowast
1 119909
lowast
1) 119878
1((0 119909
1)
100381710038171003817100381711990911003817100381710038171003817
)⟩
100381610038161003816100381610038161003816100381610038161003816
⩽100381710038171003817100381711989211003817100381710038171003817 V (1198781) + 120576
100381710038171003817100381711990911003817100381710038171003817 lt100381710038171003817100381711989211003817100381710038171003817 +100381710038171003817100381711990911003817100381710038171003817 = 1
(115)
a contradiction This proves the claimWe define the operator 119878
2 119871
1(120583|
119860)oplus
1119883 rarr 119871
1(120583|
119860)oplus
1119883
by 1198782(119891 119909) = 119878
1(119891 0) for every 119891 isin 119871
1(120583|
119860) and for every
119909 isin 119883 Then we have
V (1198782) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816 = 1
10038171003817100381710038171198781 minus 11987821003817100381710038171003817 ⩽ 120576 (116)
Indeed from Claim 1 we have
V (1198781) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)1003816100381610038161003816 =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816 ⩽ V (119878
2)
(117)
On the other hand we see that1003816100381610038161003816(119891
lowast 119909
lowast) 119878
2(119891 119909)
1003816100381610038161003816
=1003816100381610038161003816(119891
lowast 119909
lowast) 119878
1(119891 0)
1003816100381610038161003816 ⩽10038171003817100381710038171198911003817100381710038171003817 V (1198781) ⩽ V (119878
1)
(118)
for every ((119891lowast 119909lowast) (119891 119909)) isin Π(1198711(120583|
119860)oplus
1119883) Therefore
V(1198782) ⩽ V(119878
1) Also
10038171003817100381710038171198781 minus 11987821003817100381710038171003817 ⩽ sup
119909isin119878119883
10038171003817100381710038171198781 (0 119909)1003817100381710038171003817
= sup119909isin119878119883
10038171003817100381710038171198781 (0 119909) minus 1198780 (0 119909)1003817100381710038171003817 ⩽ 120576
(119)
Claim 2 There exists 1198783 119871
1(120583|
119860)oplus
1119883 rarr 119871
1(120583|
119860)oplus
1119883 such
that 1198783(119892
1 0) = 119878
3 = 1 119878
3(0 119909) = 0 119878
3(119891 119909) isin 0oplus
1119883
for every (119891 119909) isin 1198711(120583|
119860)oplus
1119883 and 119878
3minus 119878
2 lt 4120576
Indeed write 1198781= (119863
1 119863
2) where 119863
1 119871
1(120583|
119860)oplus
1119883 rarr
1198711(120583|
119860) and119863
2 119871
1(120583|
119860)oplus
1119883 rarr 119883 We have that
sup 1003816100381610038161003816119892lowast119863
1(119892
1 0) + 119909
lowast119863
2(119892
1 0)1003816100381610038161003816
119909lowastisin 119878
119883lowast ⟨119892
lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860)
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816 +10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
⟨119892lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860)
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816
⟨119892lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860) +10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
⩽ V (1198782) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816119892
lowast
1119863
1(119892
1 0) + 119909
lowast
1119863
2(119892
1 0)1003816100381610038161003816
(120)
This implies that
1003816100381610038161003816119909lowast
1119863
2(119892
1 0)1003816100381610038161003816 =10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
1003816100381610038161003816119892lowast
1119863
1(119892
1 0)1003816100381610038161003816
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816 ⟨119892
lowast 119892
1⟩ = 1 119892
lowastisin 119871
infin(120583|
119860)
(121)
Therefore |119892lowast1| equals 1 on the support of 119863
1(119892
1 0) As
|⟨119892lowast
1 119892
1⟩| = 1 we also have that |119892lowast
1| equals 1 on the support
of 1198921 Changing the values of 119892lowast
1by the ones of 119891lowast
0on 119860
(supp(1198631(119892
1 0)) cup supp(g
1)) we may and do suppose that
|119892lowast
1| = 1 on the whole 119860We also have 119863
2(119892
1 0) gt 0 Indeed
10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198920 0)1003817100381710038171003817
⩽10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198921 0)
1003817100381710038171003817 +10038171003817100381710038171198780 (1198921 0) minus 1198780 (1198920 0)
1003817100381710038171003817
lt 2120576 + 120576 = 3120576
(122)
So we have
10038171003817100381710038171198632(119892
1 0) minus 119879
1198601198920
1003817100381710038171003817
⩽10038171003817100381710038171198631(119892
1 0)1003817100381710038171003817 +10038171003817100381710038171198632(119892
1 0) minus 119879
1198601198920
1003817100381710038171003817
14 Abstract and Applied Analysis
=1003817100381710038171003817(1198631
(1198921 0) 119863
2(119892
1 0)) minus (0 119879
1198601198920)1003817100381710038171003817
=10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198920 0)
1003817100381710038171003817 lt 3120576
(123)
and 1198632(119892
1 0) gt 119879
1198601198920 minus 3120576 ⩾ 1 minus 120578(120576
0) minus 3120576 gt 0
Finally define the operator 1198783by
1198783(119891 119909) = (0119863
2(119891 0) + 119892
lowast
1(119863
1(119891 0))
1198632(119892
1 0)
119909lowast
1119863
2(119892
1 0))
(124)
for (119891 119909) isin 1198711(120583|
119860)oplus
1119883 It is clear that
100381710038171003817100381711987831003817100381710038171003817 ⩽ sup
119891isin1198781198711(120583|119860)
(10038171003817100381710038171198632(119891 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119891 0)
1003816100381610038161003816) (125)
Notice also that
10038171003817100381710038171198631(119891 0)
1003817100381710038171003817
⩽10038171003817100381710038171198631(119891 0)
1003817100381710038171003817 +10038171003817100381710038171198632(119891 0) minus 119879
1198601198911003817100381710038171003817
=1003817100381710038171003817(1198631
(119891 0) 1198632(119891 0)) minus (0 119879
119860119891)1003817100381710038171003817
=10038171003817100381710038171198782 (119891 119909) minus 1198780 (119891 119909)
1003817100381710038171003817
(126)
for all (119891 119909) isin 1198711(120583|
119860)oplus
1119883 Hence we have
10038171003817100381710038171198783 minus 11987821003817100381710038171003817
= 2 sup119891isin1198781198711(120583|119860)
10038171003817100381710038171198631(119891 0)
1003817100381710038171003817 ⩽ 210038171003817100381710038171198782 minus 1198780
1003817100381710038171003817 lt 4120576(127)
On the other hand let 119866 1198711(120583|
119860) rarr 119871
1(120583|
119860) be defined by
119866(119891) = 119892lowast
1119891 for every 119891 isin 119871
1(120583|
119860) Then we have
V (1198782)
= sup 1003816100381610038161003816119911lowast11987821199111003816100381610038161003816 (119911 119911
lowast) isin Π (119871
1(120583|
119860) oplus
1119883)
⩾ sup10038161003816100381610038161003816100381610038161003816
119909lowast119863
2(119866(
1
120583 (119862)120594119862) 0)
+119892lowast
1119863
1(119866(
1
120583 (119862)120594119862) 0)
10038161003816100381610038161003816100381610038161003816
119909lowastisin 119878
119883lowast 119862 isin Σ
119860 120583 (119862) gt 0
= sup10038171003817100381710038171003817100381710038171003817
1198632(119866(
1
120583 (119862)120594119862) 0)
10038171003817100381710038171003817100381710038171003817
+
10038161003816100381610038161003816100381610038161003816
119892lowast
1119863
1(119866(
1
120583 (119862)120594119862) 0)
10038161003816100381610038161003816100381610038161003816
119862 isin Σ119860 120583 (119862) gt 0
(128)
where Σ119860is the family of measurable subsets of 119860
Hence for any simple function 119904 = sum119899
119894=1((120572
119894120583(119860
119894))120594
119860119894
) isin
1198781198711(120583|119860) where 119860
119894119894is a family of disjoint measurable subsets
with strictly positive measure we have
V (1198782)
⩾
119899
sum
119894=1
10038161003816100381610038161205721198941003816100381610038161003816 (
100381710038171003817100381710038171003817100381710038171003817
1198632(119866(
1
120583 (119860119894)120594119860119894
) 0)
100381710038171003817100381710038171003817100381710038171003817
+
100381610038161003816100381610038161003816100381610038161003816
119892lowast
1119863
1(119866(
1
120583 (119860119894)120594119860119894
) 0)
100381610038161003816100381610038161003816100381610038161003816
)
⩾10038171003817100381710038171198632(119866 (119904) 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119866 (119904) 0)
1003816100381610038161003816
(129)
Since |119892lowast1| = 1119866 is an isometric isomorphism so for each119891 isin
1198781198711(120583|119860)there exists a sequence of norm-one simple functions
(119904119896) such that 119866(119904
119896) converges to 119891 Therefore
V (1198782) ⩾ sup
119891isin1198781198711(120583|119860)
(10038171003817100381710038171198632(119891 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119891 0)
1003816100381610038161003816) ⩾100381710038171003817100381711987831003817100381710038171003817
(130)
On the other hand we see that100381710038171003817100381711987831003817100381710038171003817 ⩾1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
3(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816119909
lowast
1119863
2(119892
1 0) + 119892
lowast
1119863
1(119892
1 0)1003816100381610038161003816 = V (119878
2) = 1
(131)
Therefore 1 = 1198783 = 119878
3(119892
1 0) which proves Claim 2
Finally set 1198783= (0 ) for a suitable 119871
1(120583|
119860)oplus
1119883 rarr
119883 and define the operator 1198791 119871
1(120583) rarr 119883 by
1198791(119891) = 119879
0(119891120594
119860119888) + (119875
119860119891 0) (132)
for every 119891 isin 1198711(120583) Then we have
10038171003817100381710038171198791 (119891)1003817100381710038171003817 ⩽100381710038171003817100381711987901003817100381710038171003817
10038171003817100381710038171198911205941198601198881003817100381710038171003817 +1003817100381710038171003817100381710038171003817100381710038171003817
10038171003817100381710038171198911205941198601003817100381710038171003817 =10038171003817100381710038171198911003817100381710038171003817 (133)
for every 119891 isin 1198711(120583) so 119879
1 ⩽ 1 Also
10038171003817100381710038171198791 (1198691198601198921)1003817100381710038171003817 =10038171003817100381710038171198783 (1198921 0)
1003817100381710038171003817 =100381710038171003817100381711987831003817100381710038171003817 = 1 (134)
so 1198791attains its norm at 119869
1198601198921isin 119871
1(120583) and
10038171003817100381710038171198691198601198921 minus 11989101003817100381710038171003817 =10038171003817100381710038171198921 minus 1198920
1003817100381710038171003817 lt 120576 (135)
We also have that for any 119891 isin 1198781198711(120583)
10038171003817100381710038171198790 (119891) minus 1198791 (119891)1003817100381710038171003817 =100381710038171003817100381710038171198790(119891120594
119860) minus (119875
119860119891 0)10038171003817100381710038171003817
⩽10038171003817100381710038171198790 (119869119860119875119860119891) minus 119879119860 (119875119860119891)
1003817100381710038171003817
+10038171003817100381710038171003817119879119860(119875
119860119891) minus (119875
119860119891 0)10038171003817100381710038171003817
⩽10038171003817100381710038171198790119869119860 minus 119879119860
1003817100381710038171003817 +10038171003817100381710038171198780 minus 1198783
1003817100381710038171003817
lt 120578 (1205760) + 6120576
(136)
Hence 1198790minus 119879
1 ⩽ 120578(120576
0) + 6120576 lt 120576
0
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Abstract and Applied Analysis 15
Acknowledgments
The authors thank Gilles Godefroy and Rafael Paya forfruitful conversations about the content of this paper andVladimir Kadets for providing Lemma 18 They also appre-ciate anonymous referees for careful reading and fruitfulsuggestions about revision The second author was sup-ported by Basic Science Research Program through theNational Research Foundation of Korea (NRF) fundedby the Ministry of Education Science and Technology(NRF-2012R1A1A1006869) The third author was partiallysupported by Spanish MICINN and FEDER project noMTM2012-31755 and by Junta de Andalucıa and FEDERGrants FQM-185 and P09-FQM-4911
References
[1] F F Bonsall and J Duncan Numerical Ranges of Operators onNormed Spaces and of Elements of Normed Algebras vol 2 ofLondon Mathematical Society Lecture Note Series 2 CambridgeUniversity Press London UK 1971
[2] F F Bonsall and J Duncan Numerical Ranges II CambridgeUniversity Press London UK 1973 London MathematicalSociety Lecture Notes Series 10
[3] B Sims On numerical range and its applications to Banachalgebras [PhD thesis] University of Newcastle New SouthWales Australia 1972
[4] I D Berg and B Sims ldquoDenseness of operators which attaintheir numerical radiusrdquoAustralian Mathematical Society A vol36 no 1 pp 130ndash133 1984
[5] C S Cardassi ldquoDensity of numerical radius attaining operatorson some reflexive spacesrdquo Bulletin of the Australian Mathemat-ical Society vol 31 no 1 pp 1ndash3 1985
[6] C S Cardassi ldquoNumerical radius attaining operatorsrdquo inBanach Spaces (Columbia Mo 1984) vol 1166 of Lecture Notesin Mathematics pp 11ndash14 Springer Berlin Germany 1985
[7] C S Cardassi ldquoNumerical radius-attaining operators on119862(119870)rdquoProceedings of the AmericanMathematical Society vol 95 no 4pp 537ndash543 1985
[8] MDAcostaOperadores que alcanzan su radio numerico [PhDthesis] Universidad de Granada Granada Spain 1990
[9] M D Acosta and R Paya ldquoNumerical radius attaining oper-ators and the Radon-Nikodym propertyrdquo The Bulletin of theLondon Mathematical Society vol 25 no 1 pp 67ndash73 1993
[10] R Paya ldquoA counterexample on numerical radius attainingoperatorsrdquo Israel Journal of Mathematics vol 79 no 1 pp 83ndash101 1992
[11] Y S Choi and S G Kim ldquoNorm or numerical radius attainingmultilinear mappings and polynomialsrdquo Journal of the LondonMathematical Society vol 54 no 1 pp 135ndash147 1996
[12] M D Acosta J Becerra Guerrero and M Ruiz GalanldquoNumerical-radius-attaining polynomialsrdquo The Quarterly Jour-nal of Mathematics vol 54 no 1 pp 1ndash10 2003
[13] M D Acosta and S G Kim ldquoDenseness of holomorphicfunctions attaining their numerical radiirdquo Israel Journal ofMathematics vol 161 pp 373ndash386 2007
[14] S G Kim and H J Lee ldquoNumerical peak holomorphicfunctions on Banach spacesrdquo Journal of Mathematical Analysisand Applications vol 364 no 2 pp 437ndash452 2010
[15] M D Acosta R M Aron D Garcıa and M MaestreldquoThe Bishop-Phelps-Bollobas theorem for operatorsrdquo Journal ofFunctional Analysis vol 254 no 11 pp 2780ndash2799 2008
[16] A J Guirao and O Kozhushkina ldquoThe Bishop-Phelps-Bollobasproperty for numerical radius in ℓ
1(119862)rdquo Studia Mathematica
vol 218 no 1 pp 41ndash54 2013[17] J Falco ldquoThe Bishop-Phelps-Bollobas property for numerical
radius on 1198711rdquo Journal of Mathematical Analysis and Applica-
tions vol 414 no 1 pp 125ndash133 2014[18] A Aviles A J Guirao and J Rodriguez ldquoOn the Bishop-
Phelps-Bollobas property for numerical radius in119862(119870)-spacesrdquoPreprint
[19] RM Aron Y S Choi S K Kim H J Lee andMMartin ldquoTheBishop-Phelps-Bollobas version of Lindenstrauss properties Aand Brdquo Preprint
[20] V Kadets M Martın and R Paya ldquoRecent progress and openquestions on the numerical index of Banach spacesrdquo Revista dela Real Academia de Ciencias Exactas Fısicas y Naturales A vol100 no 1-2 pp 155ndash182 2006
[21] M Martın J Merı and M Popov ldquoOn the numerical index ofreal 119871
119901(120583)-spacesrdquo Israel Journal of Mathematics vol 184 pp
183ndash192 2011[22] A Iwanik ldquoNorm attaining operators on Lebesgue spacesrdquo
Pacific Journal of Mathematics vol 83 no 2 pp 381ndash386 1979[23] HH SchaeferBanach Lattices and Positive Operators Springer
New York NY USA 1974[24] J Duncan C M McGregor J D Pryce and A J White ldquoThe
numerical index of a normed spacerdquo Journal of the LondonMathematical Society Second Series vol 2 pp 481ndash488 1970
[25] Y S Choi S K Kim H J Lee and M Martin ldquoThe Bishop-Phelps-Bollobas theorem for operators on 119871
1(120583)rdquo Preprint
[26] H E Lacey The Isometric Theory of Classical Banach SpacesSpringer New York NY USA 1974
[27] G Godefroy ldquoUniversal spaces for strictly convex Banachspacesrdquo Revista de la Real Academia de Ciencias Exactas Fısicasy Naturales A vol 100 no 1-2 pp 137ndash146 2006
[28] C Finet M Martın and R Paya ldquoNumerical index andrenormingrdquo Proceedings of the American Mathematical Societyvol 131 no 3 pp 871ndash877 2003
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical Problems in Engineering
Hindawi Publishing Corporationhttpwwwhindawicom
Differential EquationsInternational Journal of
Volume 2014
Applied MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Probability and StatisticsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical PhysicsAdvances in
Complex AnalysisJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
OptimizationJournal of
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CombinatoricsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Operations ResearchAdvances in
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Function Spaces
Abstract and Applied AnalysisHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of Mathematics and Mathematical Sciences
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Algebra
Discrete Dynamics in Nature and Society
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Decision SciencesAdvances in
Discrete MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom
Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of
Abstract and Applied Analysis 3
We may also give the following easy result concerningduality
Proposition 3 Let119883 be a reflexive space Then
120578nu (119883) (120576) = 120578nu (119883lowast) (120576) (11)
for every 120576 isin (0 1) In particular 119883 has the BPBp-nu if andonly if 119883lowast has the BPBp-nu
Wewill use that V(119879lowast) = V(119879) for all 119879 isinL(119883) where 119879lowastdenotes the adjoint operator of 119879 This result can be found in[1] but it is obvious if119883 is reflexive
Proof By reflexivity it is enough to show that 120578nu(119883)(120576) ⩽120578nu(119883
lowast)(120576) Let 120576 isin (0 1) be fixed If 120578nu(119883)(120576) = 0 there is
nothing to prove Otherwise consider 0 lt 120578 lt 120578nu(119883)(120576)Suppose that 119879
1isinL(119883lowast
) and (119909lowast1 119909
1) isin Π(119883
lowast) satisfy
V (1198791) = 1
100381610038161003816100381611990911198791119909lowast
1
1003816100381610038161003816 gt V (1198791) minus 120578 (12)
By considering 119879lowast1isin L(119883) we may find 119878
1isin L(119883) and
(1199101 119910
lowast
1) isin Π(119883) such that
1003816100381610038161003816119910lowast
111987811199101
1003816100381610038161003816 = V (1198781) = 1
10038171003817100381710038171199101 minus 11990911003817100381710038171003817 lt 120576
1003817100381710038171003817119910lowast
1minus 119909
lowast
1
1003817100381710038171003817 lt 1205761003817100381710038171003817119879
lowast
1minus 119878
1
1003817100381710038171003817 lt 120576
(13)
Then 119878lowast1isinL(119883lowast
) and (119910lowast1 119910
1) isin Π(119883
lowast) satisfy
1003816100381610038161003816⟨1199101 119878lowast
1119910lowast
1⟩1003816100381610038161003816 = V (119878
1) = 1
1003817100381710038171003817119910lowast
1minus 119909
lowast
1
1003817100381710038171003817 lt 120576
10038171003817100381710038171199101 minus 11990911003817100381710038171003817 lt 120576
10038171003817100381710038171198791 minus 119878lowast
1
1003817100381710038171003817 lt 120576
(14)
This implies that 120578nu(119883lowast)(120576) ⩾ 120578 We finish by just taking
supremum on 120578
We do not know whether the result above is valid in thenonreflexive case
3 Spaces Which Are Both Uniformly Convexand Uniformly Smooth
For a Banach space which is both uniformly convex anduniformly smooth we get a property which is weaker thanBPBp-nu This result was known to A Guirao (privatecommunication)
Proposition 4 Let 119883 be a uniformly convex and uniformlysmooth Banach space Then given 120576 gt 0 there exists 120578(120576) gt 0such that whenever 119879
0isinL(119883) with V(119879
0) = 1 and (119909
0 119909
lowast
0) isin
Π(119883) satisfy |119909lowast011987901199090| gt 1 minus 120578(120576) there exist 119878 isin L(119883) and
(119910 119910lowast) isin Π(119883) such that
V (119878) = 1003816100381610038161003816119910lowast1198781199101003816100381610038161003816
1003817100381710038171003817119909 minus 1199101003817100381710038171003817 lt 120576
1003817100381710038171003817119909lowastminus 119910
lowast1003817100381710038171003817 lt 1205761003817100381710038171003817119878 minus 1198790
1003817100381710038171003817 lt 120576
(15)
Proof Notice that the uniform smoothness of119883 is equivalentto the uniform convexity of 119883lowast Let 120575
119883(120576) and 120575
119883lowast(120576) be the
moduli of convexity119883 and119883lowast respectively Given 0 lt 120576 lt 1consider
120578 (120576) =120576
4min 120575
119883(120576
4) 120575
119883lowast (120576
4) gt 0 (16)
Consider 1198790isin L(119883) with V(119879
0) = 1 and (119909
0 119909
lowast
0) isin Π(119883)
satisfying |119909lowast011987901199090| gt 1 minus 120578(120576) Define 119879
1isinL(119883) by
1198791119909 = 119879
0119909 + 120582
1(120576
4) 119909
lowast
0(119909) 119909
0(17)
for all 119909 isin 119883 where 1205821is the scalar satisfying |120582
1| = 1 and
|119909lowast
011987901199090+ 120582
1(1205764)| = |119909
lowast
011987901199090| + 1205764 Now choose 119909
1isin 119878
119883
and 119909lowast1isin 119878
119883lowast such that |119909lowast
1(119909
1)| = 1 119909lowast
1(119909
0) = |119909
lowast
1(119909
0)| and
1003816100381610038161003816119909lowast
111987911199091
1003816100381610038161003816 ⩾ V (1198791) minus 120578(
1205762
42) (18)
Now we define a sequence (119909119899 119909
lowast
119899 119879
119899) in 119878
119883times 119878
119883lowast times L(119883)
inductively Indeed suppose that we have a defined sequence(119909
119895 119909
lowast
119895 119879
119895) for 0 ⩽ 119895 ⩽ 119899 and let
119879119899+1119909 = 119879
119899119909 + 120582
119899+1
120576119899+1
4119899+1119909lowast
119899(119909) 119909
119899 (19)
Then choose 119909119899+1
isin 119878119883
and 119909lowast119899+1
isin 119878119883lowast such that
|119909lowast
119899+1(119909
119899+1)| = 1 and |119909lowast
119899+1(119909
119899)| = 119909
lowast
119899+1(119909
119899)
1003816100381610038161003816119909lowast
119899+1119879119899+1119909119899+1
1003816100381610038161003816 ⩾ V (119879119899+1) minus 120578(
120576119899+2
4119899+2) (20)
Notice that for all 119899 ⩾ 0 we have
1003817100381710038171003817119879119899+1 minus 1198791198991003817100381710038171003817 ⩽120576119899+1
4119899+1
1003816100381610038161003816V (119879119899+1) minus V (119879119899)1003816100381610038161003816 ⩽120576119899+1
4119899+1
(21)
This implies that (119879119899) is a Cauchy sequence and assume that
it converges to 119878 isinL(119883) Then we have
lim119899119879119899= 119878
10038171003817100381710038171198790 minus 1198781003817100381710038171003817 lt 120576
lim119899
1003816100381610038161003816119909lowast
119899119879119899119909119899
1003816100381610038161003816 = lim119899V (119879
119899) = V (119878)
(22)
We will show that both sequences (119909119899) and (119909lowast
119899) are Cauchy
From the definition we have
V (119879119899+1) minus 120578(
120576119899+2
4119899+2)
⩽1003816100381610038161003816119909
lowast
119899+1119879119899+1119909119899+1
1003816100381610038161003816
4 Abstract and Applied Analysis
⩽
100381610038161003816100381610038161003816100381610038161003816
119909lowast
119899+1119879119899119909119899+1+ 120582
119899+1
120576119899+1
4119899+1119909lowast
119899(119909
119899+1) 119909
lowast
119899+1(119909
119899)
100381610038161003816100381610038161003816100381610038161003816
⩽ V (119879119899) +120576119899+1
4119899+1119909lowast
119899+1(119909
119899)
V (119879119899+1)
⩾1003816100381610038161003816119909
lowast
119899119879119899+1119909119899
1003816100381610038161003816 =
100381610038161003816100381610038161003816100381610038161003816
119909lowast
119899119879119899119909119899+ 120582
119899+1
120576119899+1
4119899+1
100381610038161003816100381610038161003816100381610038161003816
=1003816100381610038161003816119909
lowast
119899119879119899119909119899
1003816100381610038161003816 +120576119899+1
4119899+1⩾ V (119879
119899) minus 120578(
120576119899+1
4119899+1) +120576119899+1
4119899+1
(23)
In summary we have
V (119879119899) +120576119899+1
4119899+1119909lowast
119899+1(119909
119899)
⩾ V (119879119899) minus 120578(
120576119899+1
4119899+1) +120576119899+1
4119899+1minus 120578(
120576119899+2
4119899+2)
(24)
Hence
119909lowast
119899+1(119909
119899) ⩾ 1 minus 2
4119899+1
120576119899+1120578(120576119899+1
4119899+1)
= 1 minus1
2min120575
119883(120576119899+1
4119899+2) 120575
119883lowast (120576119899+1
4119899+2)
1003817100381710038171003817100381710038171003817
119909119899+ 119909
119899+1
2
1003817100381710038171003817100381710038171003817⩾ 119909
lowast
119899+1(119909119899+ 119909
119899+1
2) ⩾ 1 minus 120575
119883(120576119899+1
4119899+2)
100381710038171003817100381710038171003817100381710038171003817
119909lowast
119899+ 119909
lowast
119899+1
2
100381710038171003817100381710038171003817100381710038171003817
⩾119909lowast
119899+ 119909
lowast
119899+1
2(119909
119899) ⩾ 1 minus 120575
119883lowast (120576119899+1
4119899+2)
(25)
This means that
1003817100381710038171003817119909119899 minus 119909119899+11003817100381710038171003817 ⩽ 120576
119899+14
119899+2
1003817100381710038171003817119909lowast
119899minus 119909
lowast
119899+1
1003817100381710038171003817 ⩽ 120576119899+14
119899+2
(26)
for all 119899 So (119909119899) and (119909lowast
119899) are Cauchy Let 119909
infin= lim
119899119909119899and
119909lowast
infin= lim
119899119909lowast
119899 Then 119909
0minus 119909
infin lt 1205764 and 119909lowast
0minus 119909
lowast
infin lt 1205764
Hence |119909lowastinfin(119909
infin)| = lim
119899|119909
lowast
119899(119909
119899)| = 1 and
V (119878) = lim119899V (119879
119899) = lim
119899
1003816100381610038161003816119909lowast
119899119879119899119909119899
1003816100381610038161003816 =1003816100381610038161003816119909
lowast
infin119878119909
infin
1003816100381610038161003816 (27)
Let 120572 = 119909lowastinfin(119909
infin) 119910lowast = 119909lowast
infin and 119910 = 119909
infin Then we have
119910lowast(119910) = 1 V(119878) = |119910lowast119878119910| and 119910 minus 119909
0 lt 120576 Notice that
|120572 minus 1| =1003816100381610038161003816119909
lowast
infin(119909
infin) minus 119909
lowast
0(119909
0)1003816100381610038161003816
⩽1003816100381610038161003816(119909
lowast
infinminus 119909
lowast
0) (119909
infin)1003816100381610038161003816 +1003816100381610038161003816119909
lowast
0(119909
infin) minus 119909
lowast
0(119909
0)1003816100381610038161003816 lt120576
2
(28)
Therefore
1003817100381710038171003817119910lowastminus 119909
lowast1003817100381710038171003817 ⩽1003817100381710038171003817119910
lowastminus 119910
lowast1003817100381710038171003817 +1003817100381710038171003817119910
lowastminus 119909
lowast1003817100381710038171003817 lt120576
2+120576
4lt 120576 (29)
This completes the proof
Let us discuss a little bit about the equivalence betweenthe property in the result above and the BPBp-nu Forconvenience let us introduce the following definition
Definition 5 A Banach space 119883 has the weak Bishop-Phelps-Bollobas property for the numerical radius (in short weak-BPBp-nu) if given 120576 gt 0 there exists 120578(120576) gt 0 such thatwhenever 119879
0isin L(119883) with V(119879
0) = 1 and (119909
0 119909
lowast
0) isin Π(119883)
satisfy |119909lowast011987901199090| gt 1 minus 120578(120576) there exist 119878 isin L(119883) and
(119910 119910lowast) isin Π(119883) such that
V (119878) = 1003816100381610038161003816119910lowast1198781199101003816100381610038161003816
1003817100381710038171003817119909 minus 1199101003817100381710038171003817 lt 120576
1003817100381710038171003817119909lowastminus 119910
lowast1003817100381710038171003817 lt 120576 119878 minus 119879 lt 120576
(30)
Notice that the only difference between this concept andthe BPBp-nu is the normalization of the operator 119878 by thenumerical radius Of course if the numerical radius andthe operator norm are equivalent these two properties arethe same This equivalence is measured by the so-callednumerical index of the Banach space as follows For a Banachspace119883 the numerical index of119883 is defined by
119899 (119883) = inf V (119879) 119879 isinL (119883) 119879 = 1 (31)
It is clear that 0 ⩽ 119899(119883) ⩽ 1 and 119899(119883)119879 ⩽ V(119879) ⩽ 119879for all 119879 isin L(119883) The value 119899(119883) = 1 means that V equalsthe usual operator norm This is the case of 119883 = 119871
1(120583) and
119883 = 119862(119870) among many others On the other hand 119899(119883) gt 0if and only if the numerical radius is equivalent to the normof L(119883) We refer the reader to [20] for more informationand background
The following result is immediate We include a proof forthe sake of completeness
Proposition 6 Let119883 be a Banach space with 119899(119883) gt 0 Then119883 has the BPBp-nu if and only if119883 has the weak-BPBp-nu
Proof Thenecessity is clear For the converse assume that wehave 120578(120576) gt 0 satisfying the conditions of the weak-BPBp-nufor all 0 lt 120576 lt 1 If 119879 isin L(119883) with V(119879) = 1 and (119909
0 119909
lowast
0) isin
Π(119883) satisfy |119909lowast0119879119909
0| gt 1 minus 120578(120576) for 0 lt 120576 lt 1 then there exist
119878 isinL(119883) and (119910 119910lowast) isin Π(119883) such that
V (119878) = 1003816100381610038161003816119910lowast1198781199101003816100381610038161003816 119878 minus 119879 lt 120576
1003817100381710038171003817119909 minus 1199101003817100381710038171003817 lt 120576
1003817100381710038171003817119909lowastminus 119910
lowast1003817100381710038171003817 lt 120576
(32)
As V(119878) gt 0 by the above let 1198781= (1V(119878))119878 Then we have
1 = V (1198781) =1003816100381610038161003816119910
lowast11987811199101003816100381610038161003816
1003817100381710038171003817119909 minus 1199101003817100381710038171003817 lt 120576
1003817100381710038171003817119909lowastminus 119910
lowast1003817100381710038171003817 lt 120576
(33)
Abstract and Applied Analysis 5
Finally we have
10038171003817100381710038171198781 minus 1198791003817100381710038171003817 ⩽
10038171003817100381710038171003817100381710038171003817
1
V (119878)119878 minus 119878
10038171003817100381710038171003817100381710038171003817
+ 119878 minus 119879
=119878
V (119878)|V (119878) minus 1| + 119878 minus 119879
⩽1
119899 (119883)|V (119878) minus V (119879)| + 119878 minus 119879
⩽ (1
119899 (119883)+ 1) 119878 minus 119879 lt
119899 (119883) + 1
119899 (119883)120576
(34)
An obvious change of parameters finishes the proof
We do not know whether the hypothesis of 119899(119883) gt 0 canbe omitted in the above result
Putting together Propositions 4 and 6 we get the follow-ing
Corollary 7 Let 119883 be a uniformly convex and uniformlysmooth Banach space with 119899(119883) gt 0 Then 119883 has the BPBp-nu
Let us comment that every complex Banach space 119883satisfies 119899(119883) ⩾ 1119890 so the above corollary automaticallyapplies in the complex case In the real case this is no longertrue as the numerical index of a Hilbert space of dimensiongreater than or equal to two is 0 On the other hand it isproved in [21] that real 119871
119901(120583) spaces have nonzero numerical
index for every measure 120583when 119901 = 2Therefore we have thefollowing examples
Example 8 (a) Complex Banach spaces which are uniformlysmooth and uniformly convex satisfy the BPBp-nu
(b) In particular for every measure 120583 the complex spaces119871119901(120583) have the BPBp-nu for 1 lt 119901 lt infin(c) For every measure 120583 the real spaces 119871
119901(120583) have the
BPBp-nu for 1 lt 119901 lt infin 119901 = 2
Note Added in Revision Very recently H J Lee M Martınand J Merı have proved that Proposition 6 can be extendedto some Banach spaces with numerical index zero as forinstance real Hilbert spaces Hence they have shown thatHilbert spaces have the BPBp-nu These results will appearelsewhere
4 L1
Spaces
In this section we will show that 1198711(120583) has the BPBp-nu for
every measure 120583 In the proof we are dealing with complexintegrable functions since the real case is followed easilyby applying the same proof Our main result here is thefollowing
Theorem 9 Let 120583 be a measure Then 1198711(120583) has the Bishop-
Phelps-Bollobas property for numerical radius More preciselygiven 120576 gt 0 there exists 120578(120576) gt 0 (which does not depend on120583) such that whenever 119879
0isin L(119871
1(120583)) with V(119879
0) = 1 and
(1198910 119892
0) isin Π(119871
1(120583)) satisfy |⟨119879
01198910 119892
0⟩| gt 1 minus 120578(120576) then there
exist 119879 isinL(1198711(120583)) (119891
1 119892
1) isin Π(119871
1(120583)) such that
1003816100381610038161003816⟨1198791198911 1198921⟩1003816100381610038161003816 = V (119879) = 1 10038171003817100381710038171198910 minus 1198911
1003817100381710038171003817 lt 120576
10038171003817100381710038171198920 minus 11989211003817100381710038171003817 lt 120576
1003817100381710038171003817119879 minus 11987901003817100381710038171003817 lt 120576
(35)
As a first step we have to start dealing with finite regularpositive Borel measures for which a representation theoremfor operators exists
Proposition 10 Let 119898 be a finite regular positive Borelmeasure on a compact Hausdorff spaceΩ Then 119871
1(119898) has the
Bishop-Phelps-Bollobas property for numerical radius Moreprecisely given 120576 gt 0 there is 120578(120576) gt 0 (which is independent ofthemeasure119898) such that if a norm-one element119879 inL(119871
1(119898))
and (1198910 119892
0) isin Π(119871
1(119898)) satisfy |⟨119879119891
0 119892
0⟩| gt 1 minus 120578(120576) then
there exist an operator 119878 isin L(1198711(119898)) (119891
1 119892
1) isin Π(119871
1(119898))
such that1003816100381610038161003816⟨1198781198911 1198921⟩
1003816100381610038161003816 = 119878 = 110038171003817100381710038171198910 minus 1198911
1003817100381710038171003817 ⩽ 120576
10038171003817100381710038171198920 minus 11989211003817100381710038171003817 ⩽ 120576 119879 minus 119878 ⩽ 120576
(36)
To prove this proposition we need some backgroundon representation of operators on Lebesgue spaces on finiteregular positive Borel measures and several preliminarylemmas
Let 119898 be a finite regular positive Borel measure on acompact Hausdorff space Ω If 120583 is a complex-valued Borelmeasure on the product space Ω times Ω then define theirmarginal measures 120583119894 on Ω (119894 = 1 2) as follows 1205831(119860) =120583(119860 times Ω) and 1205832(119861) = 120583(Ω times 119861) where 119860 and 119861 are Borelmeasurable subsets of Ω
Let 119872(119898) be the complex Banach lattice of measuresconsisting of all complex-valued Borel measures 120583 on theproduct spaceΩ timesΩ such that |120583|119894 are absolutely continuouswith respect to119898 for 119894 = 1 2 endowed with the norm
1003817100381710038171003817100381710038171003817100381710038171003817
11988910038161003816100381610038161205831003816100381610038161003816
1
119889119898
1003817100381710038171003817100381710038171003817100381710038171003817infin
(37)
Each 120583 isin 119872(119898) defines a bounded linear operator 119879120583from
1198711(119898) to itself by
⟨119879120583(119891) 119892⟩ = int
ΩtimesΩ
119891 (119909) 119892 (119910) 119889120583 (119909 119910) (38)
where 119891 isin 1198711(119898) and 119892 isin 119871
infin(119898) Iwanik [22] showed that
the mapping 120583 997891rarr 119879120583is a lattice isometric isomorphism from
119872(119898) onto L(1198711(119898)) Even though he showed this for the
real case it can be easily generalized to the complex caseFor details see [22 Theorem 1] and [23 IV Theorem 15(ii)Corollary 2]
We will also use that given an arbitrary measure 120583 every119879 isin L(119871
1(120583)) satisfies V(119879) = 119879 [24] (that is the space
1198711(120583) has numerical index 1)
Lemma 11 (see [15 Lemma 33]) Let 119888119899 be a sequence of
complex numbers with |119888119899| ⩽ 1 for every 119899 and let 120578 gt 0 such
6 Abstract and Applied Analysis
that for a convex series sum120572119899 Resuminfin
119899=1120572119899119888119899gt 1 minus 120578 Then for
every 0 lt 119903 lt 1 the set 119860 = 119894 isin N Re 119888119894gt 119903 satisfies the
estimate
sum
119894isin119860
120572119894⩾ 1 minus
120578
1 minus 119903 (39)
From now on 119898 will be a finite regular positive Borelmeasure on the compact Hausdorff space Ω
Lemma 12 Suppose that there exist a nonnegative simplefunction 119891 isin 119878
1198711(119898)
and a function 119892 isin 119878119871infin(119898)
such that
Re ⟨119891 119892⟩ gt 1 minus 1205763
16 (40)
Then there exist a nonnegative simple function 1198911isin 119878
1198711(119898)
anda function 119892
1isin 119878
119871infin(119898)
such that
1198921(119909) = 120594supp(119891
1)(119909) + 119892 (119909) 120594
Ωsupp(1198911)(119909)
⟨1198911 119892
1⟩ = 1
1003817100381710038171003817119891 minus 119891110038171003817100381710038171lt 120576
1003817100381710038171003817119892 minus 11989211003817100381710038171003817infinlt radic120576 supp (119891
1) sub supp (119891)
(41)
Proof Let119891 = sum119898
119895=1(120573
119895119898(119861
119895))120594
119861119895
for some (120573119895) such that
120573119895⩾ 0 for all 119895 andsum119898
119895=1120573119895= 1 and119861
119895rsquos aremutually disjoint
By the assumption we have
Re ⟨119891 119892⟩ =119899
sum
119895=1
120573119895
1
119898 (119861119895)
int119861119895
Re119892 (119909) 119889119898 (119909) gt 1 minus 1205763
16
(42)
and letting
119869 = 119895 1 ⩽ 119895 ⩽ 1198991
119898 (119861119895)
int119861119895
Re119892 (119909) 119889119898 (119909) gt 1 minus 1205762
4
(43)
we have by Lemma 11
sum
119895isin119869
120573119895gt 1 minus
120576
4 (44)
For each 119895 isin 119869 we have
1 minus1205762
4lt
1
119898 (119861119895)
int119861119895
Re119892 (119909) 119889119898 (119909)
=1
119898 (119861119895)
int119861119895capRe119892⩽1minus120576
Re119892 (119909) 119889119898 (119909)
+ int119861119895capRe119892gt1minus120576
Re119892 (119909) 119889119898 (119909)
⩽1
119898 (119861119895)
((1 minus 120576)119898 (119861119895cap Re119892 ⩽ 1 minus 120576)
+ 119898 (119861119895cap Re119892 gt 1 minus 120576))
= 1 minus 120576
119898 (119861119895cap Re119892 ⩽ 1 minus 120576)
119898 (119861119895)
(45)
This implies that
119898(119861119895cap Re119892 ⩽ 1 minus 120576)
119898 (119861119895)
lt120576
4 (46)
Define 119861119895= 119861
119895cap Re119892 gt 1 minus 120576 for all 119895 isin 119869
1198911= (1sum
119895isin119869
120573119895)sum
119895isin119869
120573119895(120594
119861119895
119898 (119861119895)) (47)
and 1198921(119909) = 1 on supp(119891
1) and 119892
1(119909) = 119892(119909) elsewhereThen
it is clear that supp(1198911) sub supp(119891) 119892 minus 119892
1infinlt radic120576 and
⟨1198911 119892
1⟩ = 1 Finally we will show that 119891 minus 119891
1 lt 120576 Notice
first that10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
⩽
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
= 2sum
119895isin119869
120573119895
119898(119861119895 119861
119895)
119898 (119861119895)
lt120576
2
(48)
Hence1003817100381710038171003817119891 minus 1198911
1003817100381710038171003817
⩽
10038171003817100381710038171003817100381710038171003817100381710038171003817
1
sum119895isin119869120573119895
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus 119891
10038171003817100381710038171003817100381710038171003817100381710038171003817
⩽
1 minus sum119895isin119869120573119895
sum119895isin119869120573119895
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+120576
4
= (1 minussum
119895isin119869
120573119895) +
120576
2+120576
4
⩽120576
4+120576
2+120576
4= 120576
(49)
Lemma 13 (see [25 Lemma 33]) Suppose that 119879120583is a norm-
one element in L(1198711(119898)) for some 120583 isin 119872(119898) and there is
a nonnegative simple function 1198910such that 119891
0is a norm-one
element of 1198711(119898) and 119879
1205831198910 ⩾ 1 minus 120576
32
6 for some 0 lt 120576 lt 1
Abstract and Applied Analysis 7
Then there exist a norm-one bounded linear operator 119879] forsome ] isin 119872(119898119898) and a nonnegative simple function 119891
1
in 1198781198711(119898)
such that 119879120583minus 119879] ⩽ 120576 1198911 minus 1198910 ⩽ 3120576 and
(119889|]|1119889119898)(119909) = 1 for all 119909 isin supp(1198911)
Lemma 14 Suppose that 119879] isin L(1198711(119898)) is a norm-one
operator 119891 = sum119899
119894=1120573119894(120594
119861119894
119898(119861119894)) where 119898(119861
119895) gt 0 for all
1 ⩽ 119895 ⩽ 119899 and 119861119895119899
119895=1are mutually disjoint Borel subsets ofΩ
is a norm-one nonnegative simple function and 119892 is an elementof 119878
119871infin(119898)
such that
Re ⟨119892 119879]119891⟩ ⩾ 1 minus1205766
27(50)
for some 0 lt 120576 lt 1 and
119889|]|1
119889119898(119909) = 1 119892 (119909) = 1 (51)
for all x in the support of 119891Then there exist a nonnegative simple function 119891 isin 119878
1198711(119898)
a function119892 isin 119878
119871infin(119898)
and an operator119879] inL(1198711(119898) 1198711(119898))such that
⟨119892 119879]119891⟩ =1003817100381710038171003817119879]1003817100381710038171003817 = 1
1003817100381710038171003817119879] minus 119879]1003817100381710038171003817 ⩽ 2120576
10038171003817100381710038171003817119891 minus 119891
10038171003817100381710038171003817⩽ 3120576
1003817100381710038171003817119892 minus 1198921003817100381710038171003817 ⩽radic120576 ⟨119891 119892⟩ = 1
(52)
Proof Since
Re ⟨119892 119879]119891⟩ ⩾ 1 minus1205766
27 (53)
we have
1 minus1205766
27lt Re ⟨119892 119879]119891⟩
= intΩtimesΩ
119891 (119909)Re119892 (119910) 119889120583 (119909 119910)
=
119899
sum
119895=1
120573119895intΩtimesΩ
120594119861119895
(119909)
119898 (119861119895)
Re119892 (119910) 119889] (119909 119910)
(54)
Let
119869 = 119895 intΩtimesΩ
(
120594119861119895
(119909)
119898 (119861119895)
)Re119892 (119910) 119889] (119909 119910) gt 1 minus 1205763
26
(55)
Then from Lemma 11 we have sum119895isin119869120573119895gt 1 minus 120576
32 Let 119891
1=
sum119895isin119869120573119895(120594
119861119895
119898(119861119895)) where 120573
119895= 120573
119895(sum
119895isin119869120573119895) for all 119895 isin 119869
Then
10038171003817100381710038171198911 minus 1198911003817100381710038171003817 ⩽
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
(120573119895minus 120573
119895)
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+ sum
119895isin119869
120573119895⩽ 120576
3⩽ 120576 (56)
Note that there is a Borel measurable function ℎ on Ω times Ωsuch that 119889](119909 119910) = ℎ(119909 119910) 119889|]|(119909 119910) and |ℎ(119909 119910)| = 1 forall (119909 119910) isin Ω times Ω Let
119862 = (119909 119910) 1003816100381610038161003816119892 (119910) ℎ (119909 119910) minus 1
1003816100381610038161003816 ltradic120576
232 (57)
Define two measures ]119891and ]
119888as follows
]119891(119860) = ] (119860 119862) ]
119888(119860) = ] (119860 cap 119862) (58)
for every Borel subset 119860 ofΩ times Ω It is clear that
119889] = 119889]119891+ 119889]
119888 119889
10038161003816100381610038161003816]119891
10038161003816100381610038161003816= ℎ 119889]
119891
1198891003816100381610038161003816]1198881003816100381610038161003816 = ℎ 119889]119888 119889 |]| = 119889
10038161003816100381610038161003816]119891
10038161003816100381610038161003816+ 1198891003816100381610038161003816]1198881003816100381610038161003816
(59)
Since (119889|]|11198891198981)(119909) = 1 for all 119909 isin ⋃119899
119895=1119861119895 we have
1 =119889|]|1
1198891198981
(119909) =
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909) +1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909) (60)
for all 119909 isin 119861 = ⋃119899
119895=1119861119895 and we deduce that |]|1(119861
119895) = 119898
1(119861
119895)
for all 1 ⩽ 119895 ⩽ 119899We claim that |]
119891|1(119861
119895)119898
1(119861
119895) ⩽ 120576
22
2 for all 119895 isin 119869Indeed if |119892(119910)ℎ(119909 119910) minus 1| ⩾ radic120576232 then
Re (119892 (119910) ℎ (119909 119910)) ⩽ 1 minus 12057624 (61)
So we have
1 minus1205763
26
⩽1
1198981(119861
119895)
ReintΩtimesΩ
120594119861119895(119909)119892 (119910) 119889] (119909 119910)
=1
1198981(119861
119895)
intΩtimesΩ
120594119861119895(119909)
Re (119892 (119910) ℎ (119909 119910)) 119889 |]| (119909 119910)
=1
1198981(119861
119895)
intΩtimesΩ
120594119861119895(119909)
Re (119892 (119910) ℎ (119909 119910)) 119889 10038161003816100381610038161003816]11989110038161003816100381610038161003816(119909 119910)
+1
1198981(119861
119895)
intΩtimesΩ
120594119861119895(119909)
Re (119892 (119910) ℎ (119909 119910)) 119889 1003816100381610038161003816]1198881003816100381610038161003816 (119909 119910)
⩽1
1198981(119861
119895)
((1 minus120576
24)10038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
(119861119895) +1003816100381610038161003816]1198881003816100381610038161003816
1
(119861119895))
= 1 minus120576
24
10038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
(119861119895)
1198981(119861
119895)
(62)
This proves our claimWe also claim that for each 119895 isin 119869 there exists a Borel
subset 119861119895of 119861
119895such that
(1 minus120576
2)119898
1(119861
119895) ⩽ 119898
1(119861
119895) ⩽ 119898
1(119861
119895)
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909) ⩽120576
2
(63)
8 Abstract and Applied Analysis
for all 119909 isin 119861119895 Indeed set 119861
119895= 119861
119895cap 119909 isin Ω (119889|]
119891|1119889119898
1)
(119909) ⩽ 1205762 Then
int119861119895119861119895
120576
2119889119898
1(119909) ⩽ int
119861119895
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909) 1198891198981(119909)
=10038161003816100381610038161003816]1119891
10038161003816100381610038161003816(119861
119895) ⩽1205762
22119898
1(119861
119895)
(64)
This shows that 1198981(119861
119895 119861
119895) ⩽ (1205762)119898
1(119861
119895) This proves our
second claimNow we define 119892 by 119892(119910) = 119892(119910)|119892(119910)| if |119892(119910)| ⩾ 1 minus
radic120576232 and 119892(119910) = 119892(119910) if |119892(119910)| lt 1 minus radic120576232 and we
write 119891 = sum119895isin119869120573119895(120594
119861119895
1198981(119861
119895)) It is clear that 119892 isin 119878
119871infin(119898)
119892 minus 119892 lt radic120576 and 119892(119910) = 1 for all 119909 isin supp119891
Finally we define the measure
119889] (119909 119910)
= sum
119895isin119869
120594119861119895
(119909) 119892 (119910)ℎ (119909 119910)119889]119888(119909 119910)(
1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+ 1205941198691119861(119909) 119889] (119909 119910)
(65)
where 119861 = ⋃119895isin119869119861119895 It is easy to see that (119889|]|1119889119898
1)(119909) = 1
on 119861 and (119889|]|11198891198981)(119909) ⩽ 1 elsewhere Note that
119889 (] minus ]) (119909 119910)
= sum
119895isin119869
120594119861119895
(119909) [
[
119892 (119910)ℎ (119909 119910)(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1]
]
119889]119888(119909 119910)
minus sum
119895isin119869
120594119861119895
(119909) 119889]119891(119909 119910)
(66)
If (119909 119910) isin 119862 then |119892(119910)| ⩾ 1 minus radic120576232 ⩾ 1 minus 1232 and
100381610038161003816100381610038161003816119892 (119910)ℎ (119909 119910) minus 1
100381610038161003816100381610038161003816
=
100381610038161003816100381610038161003816100381610038161003816
119892 (119910)
1003816100381610038161003816119892 (119910)1003816100381610038161003816
ℎ (119909 119910) minus 1
100381610038161003816100381610038161003816100381610038161003816
⩽
1003816100381610038161003816119892 (119910) ℎ (119909 119910) minus 11003816100381610038161003816
1003816100381610038161003816119892 (119910)1003816100381610038161003816
+
10038161003816100381610038161 minus1003816100381610038161003816119892 (119910)
1003816100381610038161003816
10038161003816100381610038161003816100381610038161003816119892 (119910)
1003816100381610038161003816
⩽ 2
1003816100381610038161003816119892 (119910) ℎ (119909 119910) minus 11003816100381610038161003816
1003816100381610038161003816119892 (119910)1003816100381610038161003816
⩽ 2radic120576
232
232
232 minus 1⩽ 2radic120576
(67)
Hence for all (119909 119910) isin 119862 we have
10038161003816100381610038161003816100381610038161003816100381610038161003816
119892 (119910)ℎ (119909 119910)(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
⩽100381610038161003816100381610038161003816119892 (119910)ℎ (119909 119910) minus 1
100381610038161003816100381610038161003816(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+
10038161003816100381610038161003816100381610038161003816100381610038161003816
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
⩽ 2radic120576(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+
10038161003816100381610038161003816100381610038161003816100381610038161003816
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
(68)
So we have for all 119909 isin 1198691
119889|] minus ]|1
1198891198981
(119909)
⩽ sum
119895isin119869
120594119861119895
(119909) [
[
2radic120576(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+
10038161003816100381610038161003816100381610038161003816100381610038161003816
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
]
]
1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909)
+ sum
119895isin119869
120594119861119895
(119909)
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909)
⩽ sum
119895isin119869
120594119861119895
(119909)(2radic120576 + (1 minus1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909)))
+sum
119895isin119869
120594119861119895
(119909)(
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909))
⩽ 2radic120576 + 120576 lt 3radic120576
(69)
This gives that 119879] minus 119879] lt 3radic120576 Note also that for all 119895 isin 119869
⟨119879]
120594119861119895
1198981(119861
119895)
119892⟩
= intΩtimesΩ
120594119861119895
(119909)
1198981(119861
119895)
119892 (119910) 119889] (119909 119910)
= intΩtimesΩ
120594119861119895
(119909)
1198981(119861
119895)
ℎ (119909 119910)(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
119889]119888(119909 119910)
Abstract and Applied Analysis 9
= intΩ
120594119861119895
(119909)
1198981(119861
119895)
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
1198891003816100381610038161003816]1198881003816100381610038161003816
1
(119909)
= intΩ
120594119861119895
(119909)
1198981(119861
119895)
1198891198981(119909) = 1
(70)
Hence we get ⟨119879]119891 119892⟩ = 1 which implies that 119879]119891 =119879] = 1 Finally10038171003817100381710038171003817119891 minus 119891
10038171003817100381710038171003817
⩽10038171003817100381710038171003817119891 minus 119891
1
10038171003817100381710038171003817+10038171003817100381710038171198911 minus 119891
1003817100381710038171003817
=
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
1198981(119861
119895)
minus sum
119895isin119869
120573119895
120594119861119895
1198981(119861
119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+ 120576
⩽ sum
119895isin119869
120573119895(
10038171003817100381710038171003817100381710038171003817100381710038171003817
120594119861119895
1198981(119861
119895)
minus
120594119861119895
1198981(119861
119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+
10038171003817100381710038171003817100381710038171003817100381710038171003817
120594119861119895
1198981(119861
119895)
minus
120594119861119895
1198981(119861
119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
) + 120576
= 2sum
119895isin119869
120573119895
1198981(119861
119895 119861
119895)
1198981(119861
119895)
+ 120576
⩽ 2sum
119895isin119869
120573119895
(1205762)1198981(119861
119895)
1198981(119861
119895)
+ 120576 ⩽120576
1 minus 1205762+ 120576 lt 3120576
(71)
We are now ready to present the proof of the main resultin the case of finite regular positive Borel measures
Proof of Proposition 10 Let 1205751= 120575
3
2(5sdot2
4) 120575
2= 120575
12
3(3
2sdot2
14)
and 1205753= (12057610)
2 for some 0 lt 120576 lt 1 Suppose that 119879 isinL(119871
1(119898)) with 119879 = 1 and that there is an 119891
0isin 119878
1198711(119898)
and1198920isin 119878
119871infin(119898)
such that ⟨1198910 119892
0⟩ = 1 and |⟨119879119891
0 119892
0⟩| gt 1minus120575
3
12
6Then there is an isometric isomorphism Ψ from 119871
1(119898) onto
itself such thatΨ(1198910) = |119891
0| and there is a scalar number 120572 in
119878R such that |⟨1198791198910 119892
0⟩| = ⟨120572119879119891
0 119892
0⟩ Then letting 119891
1= Ψ119891
0
1198921= (Ψ
minus1)lowast1198920 and 119879
1= 120572Ψ119879Ψ
minus1 we have
⟨1198781198911 119892
1⟩ = ⟨120572Ψ119879
0Ψ
minus1Ψ119891
0 (Ψ
minus1)lowast
1198920⟩
= ⟨1205721198791198910 119892
0⟩ gt 1 minus
1205753
1
26
⟨1198911 119892
1⟩ = ⟨Ψ119891
0 (Ψ
minus1)lowast
1198920⟩ = 1
(72)
Since 11987911198911 gt 1 minus 120575(120575
3
12
6) by Lemma 13 there exists a
norm-one bounded operator 119879] and a nonnegative simplefunction119891
2isin 119878
1198711(119898)
such that 1198791minus119879] ⩽ 1205751 1198912minus1198911 ⩽ 31205751
and (119889|]|11198891198981)(119909) = 1 for all 119909 isin supp(119891
2) Then
⟨119879]1198912 1198921⟩
= ⟨11987911198911 119892
1⟩ minus ⟨119879
11198911minus 119879
11198912 119892
1⟩ minus ⟨119879
11198912minus 119879]1198912 1198921⟩
⩾ ⟨11987911198911 119892
1⟩ minus10038171003817100381710038171198911 minus 1198912
1003817100381710038171003817 minus10038171003817100381710038171198791 minus 119879]
1003817100381710038171003817
⩾ 1 minus1205753
1
26minus 3120575
1minus 120575
1⩾ 1 minus 5120575
1= 1 minus
1205753
2
16
(73)
Notice also that
⟨1198912 119892
1⟩ = ⟨119891
1 119892
1⟩ minus ⟨119891
1minus 119891
2 119892
1⟩ ⩾ 1 minus
10038171003817100381710038171198911 minus 11989121003817100381710038171003817
⩾ 1 minus 31205751⩾ 1 minus 5120575
1= 1 minus
1205753
2
16
(74)
By Lemma 12 there are a nonnegative simple function 1198913isin
1198781198711(119898)
and a function 1198923isin 119878
119871infin(119898)
such that
1198923(119909) = 120594supp119891
3
(119909) + 1198922(119909) 120594
Ωsupp1198913
(119909)
10038171003817100381710038171198912 minus 11989131003817100381710038171003817 ⩽ 1205752
10038171003817100381710038171198923 minus 11989211003817100381710038171003817 ⩽radic120575
2 ⟨119891
3 119892
3⟩ = 1
(75)
So we have
⟨119879]1198913 1198923⟩ = ⟨119879]1198912 1198921⟩ minus ⟨119879]1198912 minus 119879]1198913 1198921⟩
minus ⟨119879]1198913 1198921 minus 1198923⟩
⩾ 1 minus1205753
2
16minus 2radic120575
2⩾ 1 minus 3radic120575
2= 1 minus
1205756
3
27
(76)
By Lemma 14 there exist 1198914isin 119878
1198711(119898)
and 1198924isin 119878
119871infin(119898)
and anoperator 119879
4such that
⟨1198924 119879
41198914⟩ = 1 =
100381710038171003817100381711987941003817100381710038171003817
10038171003817100381710038171198794 minus 119879]1003817100381710038171003817 ⩽ 21205753
10038171003817100381710038171198914 minus 11989131003817100381710038171003817 ⩽ 31205753
10038171003817100381710038171198924 minus 11989231003817100381710038171003817 ⩽radic120575
3 ⟨119891
4 119892
4⟩ = 1
(77)
So we have
10038171003817100381710038171198794 minus 11987911003817100381710038171003817 ⩽10038171003817100381710038171198794 minus 119879]
1003817100381710038171003817 +1003817100381710038171003817119879] minus 1198791
1003817100381710038171003817
⩽ 1205751+ 2120575
3⩽ 3120575
3
10038171003817100381710038171198911 minus 11989141003817100381710038171003817 ⩽10038171003817100381710038171198911 minus 1198912
1003817100381710038171003817 +10038171003817100381710038171198912 minus 1198913
1003817100381710038171003817 +10038171003817100381710038171198913 minus 1198914
1003817100381710038171003817
⩽ 31205751+ 120575
2+ 3120575
3⩽ 10120575
3
10038171003817100381710038171198921 minus 11989241003817100381710038171003817 ⩽10038171003817100381710038171198921 minus 1198923
1003817100381710038171003817 + 1198923 minus 1198924
⩽ 1205752+ radic120575
3⩽ 2radic120575
3
(78)
10 Abstract and Applied Analysis
Let 119878 = 120572Ψminus11198794Ψ 119891 = Ψminus1
1198914 and 119892 = Ψlowast
1198924 then we have
119879 minus 119878 =10038171003817100381710038171003817119879 minus 120572Ψ
minus11198794Ψ10038171003817100381710038171003817=10038171003817100381710038171003817120572Ψ119879Ψ
minus1minus 119879
4
10038171003817100381710038171003817
=10038171003817100381710038171198791 minus 1198794
1003817100381710038171003817 ⩽ 31205753
100381710038171003817100381710038171198910minus 11989110038171003817100381710038171003817=100381710038171003817100381710038171198910minus Ψ
minus11198914
10038171003817100381710038171003817=10038171003817100381710038171198911 minus 1198914
1003817100381710038171003817 ⩽ 101205753
100381710038171003817100381710038171198920minus 11989110038171003817100381710038171003817=10038171003817100381710038171198920 minus Ψ
lowast1198924
1003817100381710038171003817 =100381710038171003817100381710038171003817(Ψ
minus1)lowast
1198920minus 119892
4
100381710038171003817100381710038171003817
=10038171003817100381710038171198921 minus 1198924
1003817100381710038171003817 ⩽ 2radic120575
3
⟨119891 119892⟩ = ⟨Ψminus11198914 Ψ
lowast1198924⟩ = ⟨119891
4 119892
4⟩ = 1
10038161003816100381610038161003816⟨119878119891 119892⟩
10038161003816100381610038161003816=10038161003816100381610038161003816⟨120572Ψ
minus11198794ΨΨ
minus11198914 Ψ
lowast1198924⟩10038161003816100381610038161003816= |120572| = 1
(79)
This completes the proof
Finally we may give the proof of the main result in fullgenerality
Proof of Theorem 9 Notice that the Kakutani representationtheorem (see [26] for a reference) says that for every 120590-finitemeasure ] the space 119871
1(]) is isometrically isomorphic to
1198711(119898) for some positive Borel regular measure on a compact
Hausdorff space Then by Proposition 10 there is a universalfunction 120576 997891rarr 120578(120576) gt 0 which gives the BPBp-nu for 119871
1(]) for
every 120590-finite measure ]Fix 120576 gt 0 Suppose that 119879
0isin L(119871
1(120583)) with V(119879
0) = 1
and (1198910 119891
lowast
0) isin Π(119871
1(120583)) satisfy
1003816100381610038161003816⟨119891lowast
0 119879
01198910⟩1003816100381610038161003816 gt 1 minus 120578 (120576) (80)
Choose a sequence 119891119899 in 119871
1(120583) such that sup
119899119879
0119891119899 = 1
and let 119866 be the closed linear span of
119879119899
0119891119898 119899 119898 isin N cup 0 (81)
As 119866 is separable there is a dense subset 119892119899 119899 isin N of 119866
and let 119864 = ⋃infin
119899=1supp119892
119899 where supp119892
119899is the support of 119892
119899
Then the measure 120583|119864is 120590-finite Let
119884 = 119891 isin 1198711(120583) supp (119891) sub 119864 (82)
be a closed subspace of 1198711(120583) It is clear that 119871
1(120583) = 119884oplus
1119885
and 119884 is isometrically isomorphic to 1198711(120583|
119864) So 119884 has the
BPBp-nu with 120578(120576)Now write 119878
0= 119879
0|119884 119884 rarr 119884 consider 119910
0= 119891
0isin
119878119884 119910lowast
0= 119891
lowast
0|119884isin 119878
119884lowast and observe that 119910lowast
0(119910
0) = 1 and
|119910lowast
0(119878
01199100)| = |119891
lowast
0(119879
01198910)| gt 1 minus 120578(120576) Hence there exist 119878 isin
L(119884) and (1199100 119910
lowast
0) isin Π(119884) such that
1003816100381610038161003816119910lowast
0(119878119910
0)1003816100381610038161003816 = 1 = V (119878) 1003817100381710038171003817119878 minus 1198780
1003817100381710038171003817 lt 120576
10038171003817100381710038171199100 minus 11991001003817100381710038171003817 lt 120576
1003817100381710038171003817119910lowast
0minus 119910
lowast
0
1003817100381710038171003817 lt 120576
(83)
Finally consider the operator 119879 isinL(1198711(120583)) given by
119879 (119910 119911)
= (119878119910 0) + 1198790(0 119911) ((119910 119911) isin 119871
1(120583) equiv 119884oplus
1119885)
(84)
We have 119879 = 1 (and so V(119879) = 1) Indeed1003817100381710038171003817119879 (119910 119911)
1003817100381710038171003817 =1003817100381710038171003817(119878119910 0)
1003817100381710038171003817 +10038171003817100381710038171198790 (0 119911)
1003817100381710038171003817 ⩽10038171003817100381710038171199101003817100381710038171003817 + 119911 =
1003817100381710038171003817(119910 119911)1003817100381710038171003817
(85)
for all (119910 119911) isin 1198711(120583) and 119879(119910
0 0) = (119878119910
0 0) = 119878119910
0 = 1
Let 119909 = (1199100 0) and 119909lowast = (119910lowast
0 119891
0|119885) Then (119909 119909lowast) isin Π(119871
1(120583))
Moreover we have1003816100381610038161003816119909
lowast1198791199091003816100381610038161003816 =1003816100381610038161003816119910
lowast
0119878119910
0
1003816100381610038161003816 = 1 = V (119879) 1003817100381710038171003817119909 minus 1198910
1003817100381710038171003817 =1003817100381710038171003817119910 minus 1199100
1003817100381710038171003817 lt 120576
1003817100381710038171003817119909lowast
0minus 119891
lowast
0
1003817100381710038171003817
= max 1003817100381710038171003817119910 minus 119891lowast
0|119884
1003817100381710038171003817 1003817100381710038171003817119891
lowast
0|119885minus 119891
lowast
0|119885
1003817100381710038171003817
=1003817100381710038171003817119910
lowastminus 119910
lowast
0
1003817100381710038171003817 lt 120576
1003817100381710038171003817119879 minus 11987901003817100381710038171003817 = sup119910+119911⩽1
1003817100381710038171003817119879 (119910 119911) minus 1198790 (119910 119911)1003817100381710038171003817
= sup119910⩽1
1003817100381710038171003817119878119910 minus 11987801199101003817100381710038171003817 =1003817100381710038171003817119878 minus 1198780
1003817100381710038171003817 lt 120576
(86)
This completes the proof
5 Examples of Spaces Failing theBishop-Phelps-Bollobaacutes Property forNumerical Radius
Our goal here is to prove that the density of numerical radiusattaining operators does not imply the BPBp-nu Actuallywe will show that among separable spaces there is noisomorphic property implying the BPBp-nu other than finite-dimensionality
We need to relate the BPBp-nu to the Bishop-Phelps-Bollobas property for operators which as mentioned in theintroduction was introduced in [15] A pair (119883 119884) of Banachspaces has the Bishop-Phelps-Bollobas property for operators(in short BPBp) if given 120576 gt 0 there exists 120578(120576) gt 0 suchthat given 119879 isin L(119883 119884) with 119879 = 1 and 119909 isin 119878
119883such that
119879119909 gt 1 minus 120578(120576) then there exist 119911 isin 119878119883and 119878 isin L(119883 119884)
satisfying
119878 = 119878119911 = 1 119909 minus 119911 lt 120576
119879 minus 119878 lt 120576
(87)
We refer the reader to [15 19 25] and references therein formore information and background Among the interestingresults on the BPBp we emphasize that a pair (119883 119884) when119883is finite-dimensional does not necessarily have the BPBp Forinstance if119884 is a strictly convex space which is not uniformlyconvex then the pair (ℓ(2)
1 119884) fails to have the BPBp (this is
contained in [15] see [19 Section 3])
Abstract and Applied Analysis 11
The next result relates the BPBp-nu to the BPBp foroperators in a particular case We will deduce our examplefrom it
Theorem 15 If 1198711(120583)oplus
1119883 has the BPBp-119899119906 then the pair
(1198711(120583) 119883) has the BPBp for operators
Before proving this proposition we will use it to get themain examples of this section The first example shows thatthe density of numerical radius attaining operators does notimply the BPBp-nu
Example 16 There is a reflexive space (and so numerical radiusattaining operators on it are dense) which fails to have theBPBp-nu Indeed let 119884 be a reflexive separable space whichis not superreflexive and we may suppose that 119884 is strictlyconvex Observe that 119884 cannot be uniformly convex since itis not superreflexive Now 119883 = ℓ(2)
1oplus1119884 is reflexive but the
pair (ℓ(2)1 119884) fails the BPBp since 119884 is strictly convex but not
uniformly convex [19 Corollary 33] Therefore Theorem 15gives us that119883 does not have the BPBp-nu
The example above can be extended to get the result thatevery infinite-dimensional separable Banach space can berenormed to fail the BPBp-nu This follows from the factthat every infinite-dimensional separable Banach space canbe renormed to be strictly convex but not uniformly convex(this result can be proved ldquoby handrdquo an alternative categoricalargument for it can be found in [27] and references therein)With a little more of effort we may get the main result of thesection
Theorem 17 Every infinite-dimensional separable Banachspace can be renormed to fail the weak-BPBp-nu (and so inparticular to fail the BPBp-nu)
We need the following result which is surely well knownAs we have not found a reference we include a nice and easyproof kindly given to us by Vladimir Kadets We recall thatgiven a Banach space 119884 the set of all equivalent norms on119884 can be viewed as a metric space using the Banach-Mazurdistance
Lemma 18 Let119884 be an infinite-dimensional separable Banachspace Then the set of equivalent norms on 119884 which are strictlyconvex and are not (locally) uniformly convex is dense in the setof all equivalent norms on119884 (with respect to the Banach-Mazurdistance)
Proof Fix 119890 isin 119878119884and 119890lowast
1isin 119878
119884lowast such that 119890lowast
1(119890) = 1 For a fixed
120576 isin (0 12) denote
119902 (119910) = max (1 minus 120576) 10038171003817100381710038171199101003817100381710038171003817 1003816100381610038161003816119890lowast
1(119910)1003816100381610038161003816 (119910 isin 119884) (88)
Evidently (1 minus 120576)119910 ⩽ 119902(119910) ⩽ 119910 for every 119910 isin 119884 Fix asequence 119890lowast
119896 119896 ⩾ 2 of norm-one functionals separating
the points of 119884 and denote
119901 (119910) = radic
infin
sum
119896=1
1
2119896
1003816100381610038161003816119890lowast
119896(119910)1003816100381610038161003816
2
(119910 isin 119884) (89)
Then 119901 is a strictly convex norm on 119884 119901(119890) ⩾ 1radic2 and119901(119910) ⩽ 119910 for all 119910 isin 119883 Finally write
100381710038171003817100381711991010038171003817100381710038171= (1 minus 120576) 119902 (119910) + 120576
119901 (119910)
119901 (119890)(119910 isin 119884) (90)
Then sdot 1is a strictly convex norm on 119884 and
(1 minus 120576)2 10038171003817100381710038171199101003817100381710038171003817 ⩽100381710038171003817100381711991010038171003817100381710038171⩽ (1 + 120576)
10038171003817100381710038171199101003817100381710038171003817 (119910 isin 119884) (91)
Wewill finish the proof by showing that sdot 1is not uniformly
convex (actually it is not locally uniformly convex) Indeedfor each 119899 isin N we select 119910
119899isin ⋂
119899
119896=1ker 119890lowast
119896with 119910
119899 = 1 and
consider 119890119899= 119890 + (1205764)119910
119899 Then 119902(119890) = 1 119902(119890
119899) = 1 and
119902(119890+119890119899) = 2 At the same time 119901(119910
119899) rarr 0 so 119901(119890
119899) rarr 119901(119890)
and 119901(119890 + 119890119899) rarr 2119901(119890) Consequently
1198901 = 1100381710038171003817100381711989011989910038171003817100381710038171997888rarr 1
1003817100381710038171003817119890 + 11989011989910038171003817100381710038171997888rarr 2 (92)
but 119890 minus 1198901198991= (1205764)119910
1198991⩾ (1 minus 120576)
2(1205764) which means the
absence of local uniform convexity at 119890
Proof of Theorem 17 Let 119883 be an infinite-dimensional sepa-rable Banach space Take a closed subspace 119884 of 119883 of codi-mension two By [28 Proposition 2] the map carrying everyequivalent norm on 119884 to its numerical index is continuousand so the set of values of the numerical index of 119884 upto reforming is a nontrivial interval [28 Theorem 9] ThenLemma 18 allows us to find an equivalent norm | sdot | on 119884 insuch a way that (119884 | sdot |) is strictly convex and is not uniformlyconvex and 119899(119884 | sdot |) gt 0 Now the space 119883 = ℓ(2)
1oplus1(119884 | sdot |)
is an equivalent renorming of 119883 which does not have theBPBp-119899119906 (indeed otherwise the pair (ℓ(2)
1 (119884 | sdot |)) would
have the BPBp for the operator norm and so (119884 | sdot |) wouldbe uniformly convex by [19 Corollary 33] a contradiction)Moreover as
119899 (119883) = min 119899 (ℓ(2)1) 119899 (119884 |sdot|) gt 0 (93)
(see [20 Proposition 2] for instance) 119883 also fails the weak-BPBp-nu by Proposition 6
To finish the section with the promised proof ofTheorem 15 we first see the following stability result
Lemma 19 Let119883 = [⨁infin
119896=1119883
119896]1198880
or [⨁infin
119896=1119883
119896]ℓ1
If119883 has theBishop-Phelps-Bollobas property for numerical radius with afunction 120578 then each Banach space 119883
119894has the Bishop-Phelps-
Bollobas property for numerical radius with 120578nu(119883119894) ⩾ 120578 That
is inf119894120578nu(119883119894
)(120576) ⩾ 120578nu(119883)(120576) for all 0 lt 120576 lt 1
Proof Let 119875119894 119883 rarr 119883
119894and 1198751015840
119894 119883
lowastrarr 119883
lowast
119894be the natural
projections and let 119876119894 119883
119894rarr 119883 and 1198761015840
119894 119883
lowast
119894rarr 119883
lowast be thenatural embeddings
Assume that an operator 119879119894 119883
119894rarr 119883
119894and a pair
(119909119894 119909
lowast
119894) isin Π(119883
119894) satisfy that
V (119879119894) = 1
1003816100381610038161003816119909lowast
119894119879119894119909119894
1003816100381610038161003816 gt 1 minus 120578 (120576) (94)
We define an operator 119879 119883 rarr 119883 and (119909 119909lowast) isin Π(119883) by
119879 = 119876119894∘ 119879
119894∘ 119875
119894 (119909 119909
lowast) = (119876
119894119909119894 119876
1015840
119894119909lowast
119894) (95)
12 Abstract and Applied Analysis
then clearly we see that1003816100381610038161003816119909
lowast1198791199091003816100381610038161003816 =1003816100381610038161003816119909
lowast
119894119879119894119909119894
1003816100381610038161003816 gt 1 minus 120578 (120576) (96)
From the assumption there exist 119878 119883 rarr 119883 and a pair(119910 119910
lowast) isin Π(119883) such that
1003816100381610038161003816119910lowast1198781199101003816100381610038161003816 = 1 = V (119878) 119878 minus 119879 lt 120576
1003817100381710038171003817119910lowastminus 119909
lowast1003817100381710038171003817 lt 1205761003817100381710038171003817119910 minus 119909
1003817100381710038171003817 lt 120576
(97)
Since this clearly shows that1003817100381710038171003817119875119894 ∘ 119878 ∘ 119876119894
minus 119879119894
1003817100381710038171003817 lt 120576100381710038171003817100381710038171198751015840
119894119910lowastminus 119909
lowast
119894
10038171003817100381710038171003817lt 120576
1003817100381710038171003817119875119894119910 minus 1199091198941003817100381710038171003817 lt 120576
(98)
we only need to show that |1198751015840119894119910lowast(119875
119894∘ 119878 ∘ 119876
119894)119875
119894119910| = 1
We first show the case of 1198880sum Since
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817=10038171003817100381710038171003817119875119895119910 minus 119875
11989511990910038171003817100381710038171003817⩽1003817100381710038171003817119910 minus 119909
1003817100381710038171003817 lt 120576 (99)
for every 119895 = 119894 we have
1 = 119910lowast(119910) = sum
119895isinN
1198751015840
119895119910lowast(119875
119895119910) ⩽ sum
119895isinN
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817
⩽100381710038171003817100381710038171198751015840
119894119910lowast10038171003817100381710038171003817+ 120576 sum
119895isinN119895 = 119894
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817⩽1003817100381710038171003817119910
lowast1003817100381710038171003817 = 1
(100)
This shows that 1198751015840119894119910lowast = 1 and 1198751015840
119895119910lowast= 0 for every 119895 = 119894
So 119910lowast = 1198761015840
1198941198751015840
119894119910lowast and 1198751015840
119894119910lowast(119875
119894119910) = 1 This and the fact that
119910 minus 119876119894119875119894119910 lt 120576 imply that
(119876119894119875119894119910 + (
1
120576) (119910 minus 119876
119894119875119894119910) 119876
1015840
1198941198751015840
119894119910lowast) isin Π (119883) (101)
So we get that (1198761015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910 + (1120576)(119910 minus 119876
119894119875119894119910))V(119878) = 1
Hence we have
1 =1003816100381610038161003816119910
lowast1198781199101003816100381610038161003816 =10038161003816100381610038161003816(119876
1015840
1198941198751015840
119894119910lowast) 11987811991010038161003816100381610038161003816
=
1003816100381610038161003816100381610038161003816(1 minus 120576) (119876
1015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910)
+120576 (1198761015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910 +1
120576(119910 minus 119876
119894119875119894119910))
1003816100381610038161003816100381610038161003816⩽ 1
(102)
and so we get |1198751015840119894119910lowast(119875
119894∘119878 ∘119876
119894)119875
119894119910| = |(119876
1015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910)| = 1
We next show the case of ℓ1sum The proof is almost
the same as that of the 1198880case However for the sake of
completeness we provide it hereSince 1198751015840
119895119910lowast = 119875
1015840
119895119910lowastminus 119875
1015840
119895119909lowast ⩽ 119910
lowastminus 119909
lowast lt 120576 for every
119895 = 119894 we have
1 = 119910lowast(119910) = sum
119895isinN
1198751015840
119895119910lowast(119875
119895119910) ⩽ sum
119895isinN
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817
⩽10038171003817100381710038171198751198941199101003817100381710038171003817 + 120576 sum
119895isinN119895 = 119894
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817⩽10038171003817100381710038171199101003817100381710038171003817 = 1
(103)
which shows 119875119894119910 = 1 and 119875
119895119910 = 0 for every 119895 = 119894 Since this
implies (119876119894119875119894119910119876
1015840
1198941198751015840
119894119910lowast+(1120576)(119910
lowastminus119876
1015840
1198941198751015840
119894119910lowast)) isin Π(119883) we get
that |(1198761015840
1198941198751015840
119894119910lowast+ (1120576)(119910
lowastminus 119876
1015840
1198941198751015840
119894119910lowast))119878(119876
119894119875119894119910)| ⩽ V(119878) = 1
Hence we have
1 =1003816100381610038161003816119910
lowast1198781199101003816100381610038161003816 =1003816100381610038161003816119910
lowast119878 (119876
119894119875119894119910)1003816100381610038161003816
=
1003816100381610038161003816100381610038161003816(1 minus 120576) (119876
1015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910)
+120576 (1198761015840
1198941198751015840
119894119910lowast+1
120576(119910
lowastminus 119876
1015840
1198941198751015840
119894119910lowast)) 119878 (119876
119894119875119894119910)
1003816100381610038161003816100381610038161003816⩽ 1
(104)
and so |1198751015840119894119910lowast(119875
119894∘ 119878 ∘ 119876
119894)119875
119894119910| = |(119876
1015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910)| = 1
Proof of Theorem 15 Note that 120578nu(1198711(120583)oplus1119883)(120576) rarr 0 as120576 rarr 0 Fix 0 lt 120576
0lt 1 and choose 0 lt 120576 lt 1 such that
6120576 + 120578nu(1198711(120583)oplus1119883)(120576) lt 1205760 Let 120578(1205760) = 120578nu(1198711(120583)oplus1119883)(120576)Suppose that 119879
0isin L(119871
1(120583) 119883) with 119879
0 = 1 and 119891
0isin
1198781198711(120583)
satisfy
1003817100381710038171003817119879011989101003817100381710038171003817 gt 1 minus 120578 (1205760) (105)
For any measurable subset 119861 let
1198711(120583|
119861) = 119891|
119861 119891 isin 119871
1(120583) (106)
with the norm 119891|119861 = 119891120594
1198611 Then it is easy to see
that 1198711(120583|
119861) is isometrically isomorphic to a complemented
subspace of 1198711(120583) Let 119875
119861 119871
1(120583) rarr 119871
1(120583|
119861) be the
restriction defined by 119875119861(119891) = 119891|
119861for all 119891 isin 119871
1(120583) and
let 119869119861 119871
1(120583|
119861) rarr 119871
1(120583) be the extension defined by
119869119861(119891)(120596) = 119891(120596) if 120596 isin 119861 and 119869
119861(119891)(120596) = 0 otherwise
It is clear that 119875119861119869119861= Id
1198711(120583|119861)and 119869
119861119875119861(119891) = 119891120594
119861for all
119891 isin 1198711(120583) Notice also that 119871
1(120583) is isometrically isomorphic
to 1198711(120583|
119861)oplus
11198711(120583|
119861119888)
Let 119860 = supp1198910and 119892
0= 119875
1198601198910 Then
1003817100381710038171003817119879011986911986011989201003817100381710038171003817 =100381710038171003817100381711987901198910
1003817100381710038171003817 gt 1 minus 120578 (1205760) gt 0 (107)
and define the operator 119879119860 119871
1(120583|
119860) rarr 119883 by 119879
119860119891 =
1198790119869119860119891119879
0119869119860 for every 119891 isin 119871
1(120583|
119860) Then
100381710038171003817100381711987911986011989201003817100381710038171003817 ⩾100381710038171003817100381711987901198910
1003817100381710038171003817 gt 1 minus 120578 (1205760) (108)
Since 120583|119860is 120590-finite 119871
1(120583|
119860)lowast= 119871
infin(120583|
119860) Let 119892lowast
0isin 119878
119871infin(120583|119860)
be a function such that ⟨119892lowast0 119892
0⟩ = 1 and choose 119909lowast
0isin 119878
119883lowast
such that 119909lowast0(119879
1198601198920) = 119879
1198601198920 Define the operator 119878
0isin
L(1198711(120583|
119860)oplus
1119883) by
1198780(119891 119909) = (0 119879
119860119891) ((119891 119909) isin 119871
1(120583|
119860) oplus
1119883) (109)
and observe that 1198780 = V(119878
0) = 1 Indeed
100381710038171003817100381711987801003817100381710038171003817 ⩽ 1 =
10038171003817100381710038171198791198601003817100381710038171003817
= sup 1003816100381610038161003816119909lowast1198791198601198911003816100381610038161003816 119909
lowastisin 119878
119883lowast 119891 isin 119878
1198711(120583|119860)
= sup 1003816100381610038161003816(119891lowast 119909
lowast) 119878
0(119891 119909)
1003816100381610038161003816
((119891lowast 119909
lowast) (119891 119909)) isin Π (119871
1(120583|
119860) oplus
1119883)
= V (1198780) ⩽100381710038171003817100381711987801003817100381710038171003817
(110)
Abstract and Applied Analysis 13
It is immediate that
(119892lowast
0 119909
lowast
0) 119878
0(119892
0 0) = 119909
lowast
0(119879
1198601198920) =10038171003817100381710038171198791198601198920
1003817100381710038171003817 gt 1 minus 120578 (1205760)
(111)
By Lemma 19 1198711(120583|
119860)oplus
1119883 has the BPBp-nu with the func-
tion 120578 Therefore there exist 1198781isin L(119871
1(120583|
119860)oplus
1119883) (119892
1 119909
1) isin
1198781198711(120583|119860)oplus1119883 and (119892lowast
1 119909
lowast
1) isin 119878
119871infin(120583|119860)oplusinfin119883lowast such that
1003817100381710038171003817(1198921 1199091) minus (1198920 0)1003817100381710038171003817 lt 120576
1003817100381710038171003817(119892lowast
1 119909
lowast
1) minus (119891
lowast
0 119909
lowast
0)1003817100381710038171003817 lt 120576
10038171003817100381710038171198781 minus 11987801003817100381710038171003817 lt 120576 ⟨(119892
lowast
1 119909
lowast
1) (119892
1 119909
1)⟩ = 1
1003816100381610038161003816(119892lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)1003816100381610038161003816 = V (119878
1) = 1
(112)
Claim 1 We claim that 1199091= 0
Otherwise
1 = Re ⟨(119892lowast1 119909
lowast
1) (119892
1 119909
1)⟩
=100381710038171003817100381711989211003817100381710038171003817Re⟨(119892
lowast
1 119909
lowast
1) (119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
⟩
+100381710038171003817100381711990911003817100381710038171003817Re⟨(119892
lowast
1 119909
lowast
1) (0 119909
1)
100381710038171003817100381711990911003817100381710038171003817
⟩ ⩽ 1
(113)
We deduce that
((119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
(119892lowast
1 119909
lowast
1)) (
(0 1199091)
100381710038171003817100381711990911003817100381710038171003817
(119892lowast
1 119909
lowast
1))
isin Π (1198711(120583|
119860) oplus
1119883)
(114)
Since 1198781((0 119909
1)119909
1) = (119878
1minus 119878
0)((0 119909
1)119909
1) lt 120576 we
get that
1 =1003816100381610038161003816⟨(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)⟩1003816100381610038161003816
=
100381610038161003816100381610038161003816100381610038161003816
100381710038171003817100381711989211003817100381710038171003817⟨(119892
lowast
1 119909
lowast
1) 119878
1((119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
)⟩
+100381710038171003817100381711990911003817100381710038171003817⟨(119892
lowast
1 119909
lowast
1) 119878
1((0 119909
1)
100381710038171003817100381711990911003817100381710038171003817
)⟩
100381610038161003816100381610038161003816100381610038161003816
⩽100381710038171003817100381711989211003817100381710038171003817 V (1198781) + 120576
100381710038171003817100381711990911003817100381710038171003817 lt100381710038171003817100381711989211003817100381710038171003817 +100381710038171003817100381711990911003817100381710038171003817 = 1
(115)
a contradiction This proves the claimWe define the operator 119878
2 119871
1(120583|
119860)oplus
1119883 rarr 119871
1(120583|
119860)oplus
1119883
by 1198782(119891 119909) = 119878
1(119891 0) for every 119891 isin 119871
1(120583|
119860) and for every
119909 isin 119883 Then we have
V (1198782) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816 = 1
10038171003817100381710038171198781 minus 11987821003817100381710038171003817 ⩽ 120576 (116)
Indeed from Claim 1 we have
V (1198781) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)1003816100381610038161003816 =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816 ⩽ V (119878
2)
(117)
On the other hand we see that1003816100381610038161003816(119891
lowast 119909
lowast) 119878
2(119891 119909)
1003816100381610038161003816
=1003816100381610038161003816(119891
lowast 119909
lowast) 119878
1(119891 0)
1003816100381610038161003816 ⩽10038171003817100381710038171198911003817100381710038171003817 V (1198781) ⩽ V (119878
1)
(118)
for every ((119891lowast 119909lowast) (119891 119909)) isin Π(1198711(120583|
119860)oplus
1119883) Therefore
V(1198782) ⩽ V(119878
1) Also
10038171003817100381710038171198781 minus 11987821003817100381710038171003817 ⩽ sup
119909isin119878119883
10038171003817100381710038171198781 (0 119909)1003817100381710038171003817
= sup119909isin119878119883
10038171003817100381710038171198781 (0 119909) minus 1198780 (0 119909)1003817100381710038171003817 ⩽ 120576
(119)
Claim 2 There exists 1198783 119871
1(120583|
119860)oplus
1119883 rarr 119871
1(120583|
119860)oplus
1119883 such
that 1198783(119892
1 0) = 119878
3 = 1 119878
3(0 119909) = 0 119878
3(119891 119909) isin 0oplus
1119883
for every (119891 119909) isin 1198711(120583|
119860)oplus
1119883 and 119878
3minus 119878
2 lt 4120576
Indeed write 1198781= (119863
1 119863
2) where 119863
1 119871
1(120583|
119860)oplus
1119883 rarr
1198711(120583|
119860) and119863
2 119871
1(120583|
119860)oplus
1119883 rarr 119883 We have that
sup 1003816100381610038161003816119892lowast119863
1(119892
1 0) + 119909
lowast119863
2(119892
1 0)1003816100381610038161003816
119909lowastisin 119878
119883lowast ⟨119892
lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860)
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816 +10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
⟨119892lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860)
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816
⟨119892lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860) +10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
⩽ V (1198782) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816119892
lowast
1119863
1(119892
1 0) + 119909
lowast
1119863
2(119892
1 0)1003816100381610038161003816
(120)
This implies that
1003816100381610038161003816119909lowast
1119863
2(119892
1 0)1003816100381610038161003816 =10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
1003816100381610038161003816119892lowast
1119863
1(119892
1 0)1003816100381610038161003816
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816 ⟨119892
lowast 119892
1⟩ = 1 119892
lowastisin 119871
infin(120583|
119860)
(121)
Therefore |119892lowast1| equals 1 on the support of 119863
1(119892
1 0) As
|⟨119892lowast
1 119892
1⟩| = 1 we also have that |119892lowast
1| equals 1 on the support
of 1198921 Changing the values of 119892lowast
1by the ones of 119891lowast
0on 119860
(supp(1198631(119892
1 0)) cup supp(g
1)) we may and do suppose that
|119892lowast
1| = 1 on the whole 119860We also have 119863
2(119892
1 0) gt 0 Indeed
10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198920 0)1003817100381710038171003817
⩽10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198921 0)
1003817100381710038171003817 +10038171003817100381710038171198780 (1198921 0) minus 1198780 (1198920 0)
1003817100381710038171003817
lt 2120576 + 120576 = 3120576
(122)
So we have
10038171003817100381710038171198632(119892
1 0) minus 119879
1198601198920
1003817100381710038171003817
⩽10038171003817100381710038171198631(119892
1 0)1003817100381710038171003817 +10038171003817100381710038171198632(119892
1 0) minus 119879
1198601198920
1003817100381710038171003817
14 Abstract and Applied Analysis
=1003817100381710038171003817(1198631
(1198921 0) 119863
2(119892
1 0)) minus (0 119879
1198601198920)1003817100381710038171003817
=10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198920 0)
1003817100381710038171003817 lt 3120576
(123)
and 1198632(119892
1 0) gt 119879
1198601198920 minus 3120576 ⩾ 1 minus 120578(120576
0) minus 3120576 gt 0
Finally define the operator 1198783by
1198783(119891 119909) = (0119863
2(119891 0) + 119892
lowast
1(119863
1(119891 0))
1198632(119892
1 0)
119909lowast
1119863
2(119892
1 0))
(124)
for (119891 119909) isin 1198711(120583|
119860)oplus
1119883 It is clear that
100381710038171003817100381711987831003817100381710038171003817 ⩽ sup
119891isin1198781198711(120583|119860)
(10038171003817100381710038171198632(119891 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119891 0)
1003816100381610038161003816) (125)
Notice also that
10038171003817100381710038171198631(119891 0)
1003817100381710038171003817
⩽10038171003817100381710038171198631(119891 0)
1003817100381710038171003817 +10038171003817100381710038171198632(119891 0) minus 119879
1198601198911003817100381710038171003817
=1003817100381710038171003817(1198631
(119891 0) 1198632(119891 0)) minus (0 119879
119860119891)1003817100381710038171003817
=10038171003817100381710038171198782 (119891 119909) minus 1198780 (119891 119909)
1003817100381710038171003817
(126)
for all (119891 119909) isin 1198711(120583|
119860)oplus
1119883 Hence we have
10038171003817100381710038171198783 minus 11987821003817100381710038171003817
= 2 sup119891isin1198781198711(120583|119860)
10038171003817100381710038171198631(119891 0)
1003817100381710038171003817 ⩽ 210038171003817100381710038171198782 minus 1198780
1003817100381710038171003817 lt 4120576(127)
On the other hand let 119866 1198711(120583|
119860) rarr 119871
1(120583|
119860) be defined by
119866(119891) = 119892lowast
1119891 for every 119891 isin 119871
1(120583|
119860) Then we have
V (1198782)
= sup 1003816100381610038161003816119911lowast11987821199111003816100381610038161003816 (119911 119911
lowast) isin Π (119871
1(120583|
119860) oplus
1119883)
⩾ sup10038161003816100381610038161003816100381610038161003816
119909lowast119863
2(119866(
1
120583 (119862)120594119862) 0)
+119892lowast
1119863
1(119866(
1
120583 (119862)120594119862) 0)
10038161003816100381610038161003816100381610038161003816
119909lowastisin 119878
119883lowast 119862 isin Σ
119860 120583 (119862) gt 0
= sup10038171003817100381710038171003817100381710038171003817
1198632(119866(
1
120583 (119862)120594119862) 0)
10038171003817100381710038171003817100381710038171003817
+
10038161003816100381610038161003816100381610038161003816
119892lowast
1119863
1(119866(
1
120583 (119862)120594119862) 0)
10038161003816100381610038161003816100381610038161003816
119862 isin Σ119860 120583 (119862) gt 0
(128)
where Σ119860is the family of measurable subsets of 119860
Hence for any simple function 119904 = sum119899
119894=1((120572
119894120583(119860
119894))120594
119860119894
) isin
1198781198711(120583|119860) where 119860
119894119894is a family of disjoint measurable subsets
with strictly positive measure we have
V (1198782)
⩾
119899
sum
119894=1
10038161003816100381610038161205721198941003816100381610038161003816 (
100381710038171003817100381710038171003817100381710038171003817
1198632(119866(
1
120583 (119860119894)120594119860119894
) 0)
100381710038171003817100381710038171003817100381710038171003817
+
100381610038161003816100381610038161003816100381610038161003816
119892lowast
1119863
1(119866(
1
120583 (119860119894)120594119860119894
) 0)
100381610038161003816100381610038161003816100381610038161003816
)
⩾10038171003817100381710038171198632(119866 (119904) 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119866 (119904) 0)
1003816100381610038161003816
(129)
Since |119892lowast1| = 1119866 is an isometric isomorphism so for each119891 isin
1198781198711(120583|119860)there exists a sequence of norm-one simple functions
(119904119896) such that 119866(119904
119896) converges to 119891 Therefore
V (1198782) ⩾ sup
119891isin1198781198711(120583|119860)
(10038171003817100381710038171198632(119891 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119891 0)
1003816100381610038161003816) ⩾100381710038171003817100381711987831003817100381710038171003817
(130)
On the other hand we see that100381710038171003817100381711987831003817100381710038171003817 ⩾1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
3(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816119909
lowast
1119863
2(119892
1 0) + 119892
lowast
1119863
1(119892
1 0)1003816100381610038161003816 = V (119878
2) = 1
(131)
Therefore 1 = 1198783 = 119878
3(119892
1 0) which proves Claim 2
Finally set 1198783= (0 ) for a suitable 119871
1(120583|
119860)oplus
1119883 rarr
119883 and define the operator 1198791 119871
1(120583) rarr 119883 by
1198791(119891) = 119879
0(119891120594
119860119888) + (119875
119860119891 0) (132)
for every 119891 isin 1198711(120583) Then we have
10038171003817100381710038171198791 (119891)1003817100381710038171003817 ⩽100381710038171003817100381711987901003817100381710038171003817
10038171003817100381710038171198911205941198601198881003817100381710038171003817 +1003817100381710038171003817100381710038171003817100381710038171003817
10038171003817100381710038171198911205941198601003817100381710038171003817 =10038171003817100381710038171198911003817100381710038171003817 (133)
for every 119891 isin 1198711(120583) so 119879
1 ⩽ 1 Also
10038171003817100381710038171198791 (1198691198601198921)1003817100381710038171003817 =10038171003817100381710038171198783 (1198921 0)
1003817100381710038171003817 =100381710038171003817100381711987831003817100381710038171003817 = 1 (134)
so 1198791attains its norm at 119869
1198601198921isin 119871
1(120583) and
10038171003817100381710038171198691198601198921 minus 11989101003817100381710038171003817 =10038171003817100381710038171198921 minus 1198920
1003817100381710038171003817 lt 120576 (135)
We also have that for any 119891 isin 1198781198711(120583)
10038171003817100381710038171198790 (119891) minus 1198791 (119891)1003817100381710038171003817 =100381710038171003817100381710038171198790(119891120594
119860) minus (119875
119860119891 0)10038171003817100381710038171003817
⩽10038171003817100381710038171198790 (119869119860119875119860119891) minus 119879119860 (119875119860119891)
1003817100381710038171003817
+10038171003817100381710038171003817119879119860(119875
119860119891) minus (119875
119860119891 0)10038171003817100381710038171003817
⩽10038171003817100381710038171198790119869119860 minus 119879119860
1003817100381710038171003817 +10038171003817100381710038171198780 minus 1198783
1003817100381710038171003817
lt 120578 (1205760) + 6120576
(136)
Hence 1198790minus 119879
1 ⩽ 120578(120576
0) + 6120576 lt 120576
0
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Abstract and Applied Analysis 15
Acknowledgments
The authors thank Gilles Godefroy and Rafael Paya forfruitful conversations about the content of this paper andVladimir Kadets for providing Lemma 18 They also appre-ciate anonymous referees for careful reading and fruitfulsuggestions about revision The second author was sup-ported by Basic Science Research Program through theNational Research Foundation of Korea (NRF) fundedby the Ministry of Education Science and Technology(NRF-2012R1A1A1006869) The third author was partiallysupported by Spanish MICINN and FEDER project noMTM2012-31755 and by Junta de Andalucıa and FEDERGrants FQM-185 and P09-FQM-4911
References
[1] F F Bonsall and J Duncan Numerical Ranges of Operators onNormed Spaces and of Elements of Normed Algebras vol 2 ofLondon Mathematical Society Lecture Note Series 2 CambridgeUniversity Press London UK 1971
[2] F F Bonsall and J Duncan Numerical Ranges II CambridgeUniversity Press London UK 1973 London MathematicalSociety Lecture Notes Series 10
[3] B Sims On numerical range and its applications to Banachalgebras [PhD thesis] University of Newcastle New SouthWales Australia 1972
[4] I D Berg and B Sims ldquoDenseness of operators which attaintheir numerical radiusrdquoAustralian Mathematical Society A vol36 no 1 pp 130ndash133 1984
[5] C S Cardassi ldquoDensity of numerical radius attaining operatorson some reflexive spacesrdquo Bulletin of the Australian Mathemat-ical Society vol 31 no 1 pp 1ndash3 1985
[6] C S Cardassi ldquoNumerical radius attaining operatorsrdquo inBanach Spaces (Columbia Mo 1984) vol 1166 of Lecture Notesin Mathematics pp 11ndash14 Springer Berlin Germany 1985
[7] C S Cardassi ldquoNumerical radius-attaining operators on119862(119870)rdquoProceedings of the AmericanMathematical Society vol 95 no 4pp 537ndash543 1985
[8] MDAcostaOperadores que alcanzan su radio numerico [PhDthesis] Universidad de Granada Granada Spain 1990
[9] M D Acosta and R Paya ldquoNumerical radius attaining oper-ators and the Radon-Nikodym propertyrdquo The Bulletin of theLondon Mathematical Society vol 25 no 1 pp 67ndash73 1993
[10] R Paya ldquoA counterexample on numerical radius attainingoperatorsrdquo Israel Journal of Mathematics vol 79 no 1 pp 83ndash101 1992
[11] Y S Choi and S G Kim ldquoNorm or numerical radius attainingmultilinear mappings and polynomialsrdquo Journal of the LondonMathematical Society vol 54 no 1 pp 135ndash147 1996
[12] M D Acosta J Becerra Guerrero and M Ruiz GalanldquoNumerical-radius-attaining polynomialsrdquo The Quarterly Jour-nal of Mathematics vol 54 no 1 pp 1ndash10 2003
[13] M D Acosta and S G Kim ldquoDenseness of holomorphicfunctions attaining their numerical radiirdquo Israel Journal ofMathematics vol 161 pp 373ndash386 2007
[14] S G Kim and H J Lee ldquoNumerical peak holomorphicfunctions on Banach spacesrdquo Journal of Mathematical Analysisand Applications vol 364 no 2 pp 437ndash452 2010
[15] M D Acosta R M Aron D Garcıa and M MaestreldquoThe Bishop-Phelps-Bollobas theorem for operatorsrdquo Journal ofFunctional Analysis vol 254 no 11 pp 2780ndash2799 2008
[16] A J Guirao and O Kozhushkina ldquoThe Bishop-Phelps-Bollobasproperty for numerical radius in ℓ
1(119862)rdquo Studia Mathematica
vol 218 no 1 pp 41ndash54 2013[17] J Falco ldquoThe Bishop-Phelps-Bollobas property for numerical
radius on 1198711rdquo Journal of Mathematical Analysis and Applica-
tions vol 414 no 1 pp 125ndash133 2014[18] A Aviles A J Guirao and J Rodriguez ldquoOn the Bishop-
Phelps-Bollobas property for numerical radius in119862(119870)-spacesrdquoPreprint
[19] RM Aron Y S Choi S K Kim H J Lee andMMartin ldquoTheBishop-Phelps-Bollobas version of Lindenstrauss properties Aand Brdquo Preprint
[20] V Kadets M Martın and R Paya ldquoRecent progress and openquestions on the numerical index of Banach spacesrdquo Revista dela Real Academia de Ciencias Exactas Fısicas y Naturales A vol100 no 1-2 pp 155ndash182 2006
[21] M Martın J Merı and M Popov ldquoOn the numerical index ofreal 119871
119901(120583)-spacesrdquo Israel Journal of Mathematics vol 184 pp
183ndash192 2011[22] A Iwanik ldquoNorm attaining operators on Lebesgue spacesrdquo
Pacific Journal of Mathematics vol 83 no 2 pp 381ndash386 1979[23] HH SchaeferBanach Lattices and Positive Operators Springer
New York NY USA 1974[24] J Duncan C M McGregor J D Pryce and A J White ldquoThe
numerical index of a normed spacerdquo Journal of the LondonMathematical Society Second Series vol 2 pp 481ndash488 1970
[25] Y S Choi S K Kim H J Lee and M Martin ldquoThe Bishop-Phelps-Bollobas theorem for operators on 119871
1(120583)rdquo Preprint
[26] H E Lacey The Isometric Theory of Classical Banach SpacesSpringer New York NY USA 1974
[27] G Godefroy ldquoUniversal spaces for strictly convex Banachspacesrdquo Revista de la Real Academia de Ciencias Exactas Fısicasy Naturales A vol 100 no 1-2 pp 137ndash146 2006
[28] C Finet M Martın and R Paya ldquoNumerical index andrenormingrdquo Proceedings of the American Mathematical Societyvol 131 no 3 pp 871ndash877 2003
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical Problems in Engineering
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Differential EquationsInternational Journal of
Volume 2014
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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical PhysicsAdvances in
Complex AnalysisJournal of
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OptimizationJournal of
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CombinatoricsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
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Function Spaces
Abstract and Applied AnalysisHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of Mathematics and Mathematical Sciences
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The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
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Algebra
Discrete Dynamics in Nature and Society
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Decision SciencesAdvances in
Discrete MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom
Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of
4 Abstract and Applied Analysis
⩽
100381610038161003816100381610038161003816100381610038161003816
119909lowast
119899+1119879119899119909119899+1+ 120582
119899+1
120576119899+1
4119899+1119909lowast
119899(119909
119899+1) 119909
lowast
119899+1(119909
119899)
100381610038161003816100381610038161003816100381610038161003816
⩽ V (119879119899) +120576119899+1
4119899+1119909lowast
119899+1(119909
119899)
V (119879119899+1)
⩾1003816100381610038161003816119909
lowast
119899119879119899+1119909119899
1003816100381610038161003816 =
100381610038161003816100381610038161003816100381610038161003816
119909lowast
119899119879119899119909119899+ 120582
119899+1
120576119899+1
4119899+1
100381610038161003816100381610038161003816100381610038161003816
=1003816100381610038161003816119909
lowast
119899119879119899119909119899
1003816100381610038161003816 +120576119899+1
4119899+1⩾ V (119879
119899) minus 120578(
120576119899+1
4119899+1) +120576119899+1
4119899+1
(23)
In summary we have
V (119879119899) +120576119899+1
4119899+1119909lowast
119899+1(119909
119899)
⩾ V (119879119899) minus 120578(
120576119899+1
4119899+1) +120576119899+1
4119899+1minus 120578(
120576119899+2
4119899+2)
(24)
Hence
119909lowast
119899+1(119909
119899) ⩾ 1 minus 2
4119899+1
120576119899+1120578(120576119899+1
4119899+1)
= 1 minus1
2min120575
119883(120576119899+1
4119899+2) 120575
119883lowast (120576119899+1
4119899+2)
1003817100381710038171003817100381710038171003817
119909119899+ 119909
119899+1
2
1003817100381710038171003817100381710038171003817⩾ 119909
lowast
119899+1(119909119899+ 119909
119899+1
2) ⩾ 1 minus 120575
119883(120576119899+1
4119899+2)
100381710038171003817100381710038171003817100381710038171003817
119909lowast
119899+ 119909
lowast
119899+1
2
100381710038171003817100381710038171003817100381710038171003817
⩾119909lowast
119899+ 119909
lowast
119899+1
2(119909
119899) ⩾ 1 minus 120575
119883lowast (120576119899+1
4119899+2)
(25)
This means that
1003817100381710038171003817119909119899 minus 119909119899+11003817100381710038171003817 ⩽ 120576
119899+14
119899+2
1003817100381710038171003817119909lowast
119899minus 119909
lowast
119899+1
1003817100381710038171003817 ⩽ 120576119899+14
119899+2
(26)
for all 119899 So (119909119899) and (119909lowast
119899) are Cauchy Let 119909
infin= lim
119899119909119899and
119909lowast
infin= lim
119899119909lowast
119899 Then 119909
0minus 119909
infin lt 1205764 and 119909lowast
0minus 119909
lowast
infin lt 1205764
Hence |119909lowastinfin(119909
infin)| = lim
119899|119909
lowast
119899(119909
119899)| = 1 and
V (119878) = lim119899V (119879
119899) = lim
119899
1003816100381610038161003816119909lowast
119899119879119899119909119899
1003816100381610038161003816 =1003816100381610038161003816119909
lowast
infin119878119909
infin
1003816100381610038161003816 (27)
Let 120572 = 119909lowastinfin(119909
infin) 119910lowast = 119909lowast
infin and 119910 = 119909
infin Then we have
119910lowast(119910) = 1 V(119878) = |119910lowast119878119910| and 119910 minus 119909
0 lt 120576 Notice that
|120572 minus 1| =1003816100381610038161003816119909
lowast
infin(119909
infin) minus 119909
lowast
0(119909
0)1003816100381610038161003816
⩽1003816100381610038161003816(119909
lowast
infinminus 119909
lowast
0) (119909
infin)1003816100381610038161003816 +1003816100381610038161003816119909
lowast
0(119909
infin) minus 119909
lowast
0(119909
0)1003816100381610038161003816 lt120576
2
(28)
Therefore
1003817100381710038171003817119910lowastminus 119909
lowast1003817100381710038171003817 ⩽1003817100381710038171003817119910
lowastminus 119910
lowast1003817100381710038171003817 +1003817100381710038171003817119910
lowastminus 119909
lowast1003817100381710038171003817 lt120576
2+120576
4lt 120576 (29)
This completes the proof
Let us discuss a little bit about the equivalence betweenthe property in the result above and the BPBp-nu Forconvenience let us introduce the following definition
Definition 5 A Banach space 119883 has the weak Bishop-Phelps-Bollobas property for the numerical radius (in short weak-BPBp-nu) if given 120576 gt 0 there exists 120578(120576) gt 0 such thatwhenever 119879
0isin L(119883) with V(119879
0) = 1 and (119909
0 119909
lowast
0) isin Π(119883)
satisfy |119909lowast011987901199090| gt 1 minus 120578(120576) there exist 119878 isin L(119883) and
(119910 119910lowast) isin Π(119883) such that
V (119878) = 1003816100381610038161003816119910lowast1198781199101003816100381610038161003816
1003817100381710038171003817119909 minus 1199101003817100381710038171003817 lt 120576
1003817100381710038171003817119909lowastminus 119910
lowast1003817100381710038171003817 lt 120576 119878 minus 119879 lt 120576
(30)
Notice that the only difference between this concept andthe BPBp-nu is the normalization of the operator 119878 by thenumerical radius Of course if the numerical radius andthe operator norm are equivalent these two properties arethe same This equivalence is measured by the so-callednumerical index of the Banach space as follows For a Banachspace119883 the numerical index of119883 is defined by
119899 (119883) = inf V (119879) 119879 isinL (119883) 119879 = 1 (31)
It is clear that 0 ⩽ 119899(119883) ⩽ 1 and 119899(119883)119879 ⩽ V(119879) ⩽ 119879for all 119879 isin L(119883) The value 119899(119883) = 1 means that V equalsthe usual operator norm This is the case of 119883 = 119871
1(120583) and
119883 = 119862(119870) among many others On the other hand 119899(119883) gt 0if and only if the numerical radius is equivalent to the normof L(119883) We refer the reader to [20] for more informationand background
The following result is immediate We include a proof forthe sake of completeness
Proposition 6 Let119883 be a Banach space with 119899(119883) gt 0 Then119883 has the BPBp-nu if and only if119883 has the weak-BPBp-nu
Proof Thenecessity is clear For the converse assume that wehave 120578(120576) gt 0 satisfying the conditions of the weak-BPBp-nufor all 0 lt 120576 lt 1 If 119879 isin L(119883) with V(119879) = 1 and (119909
0 119909
lowast
0) isin
Π(119883) satisfy |119909lowast0119879119909
0| gt 1 minus 120578(120576) for 0 lt 120576 lt 1 then there exist
119878 isinL(119883) and (119910 119910lowast) isin Π(119883) such that
V (119878) = 1003816100381610038161003816119910lowast1198781199101003816100381610038161003816 119878 minus 119879 lt 120576
1003817100381710038171003817119909 minus 1199101003817100381710038171003817 lt 120576
1003817100381710038171003817119909lowastminus 119910
lowast1003817100381710038171003817 lt 120576
(32)
As V(119878) gt 0 by the above let 1198781= (1V(119878))119878 Then we have
1 = V (1198781) =1003816100381610038161003816119910
lowast11987811199101003816100381610038161003816
1003817100381710038171003817119909 minus 1199101003817100381710038171003817 lt 120576
1003817100381710038171003817119909lowastminus 119910
lowast1003817100381710038171003817 lt 120576
(33)
Abstract and Applied Analysis 5
Finally we have
10038171003817100381710038171198781 minus 1198791003817100381710038171003817 ⩽
10038171003817100381710038171003817100381710038171003817
1
V (119878)119878 minus 119878
10038171003817100381710038171003817100381710038171003817
+ 119878 minus 119879
=119878
V (119878)|V (119878) minus 1| + 119878 minus 119879
⩽1
119899 (119883)|V (119878) minus V (119879)| + 119878 minus 119879
⩽ (1
119899 (119883)+ 1) 119878 minus 119879 lt
119899 (119883) + 1
119899 (119883)120576
(34)
An obvious change of parameters finishes the proof
We do not know whether the hypothesis of 119899(119883) gt 0 canbe omitted in the above result
Putting together Propositions 4 and 6 we get the follow-ing
Corollary 7 Let 119883 be a uniformly convex and uniformlysmooth Banach space with 119899(119883) gt 0 Then 119883 has the BPBp-nu
Let us comment that every complex Banach space 119883satisfies 119899(119883) ⩾ 1119890 so the above corollary automaticallyapplies in the complex case In the real case this is no longertrue as the numerical index of a Hilbert space of dimensiongreater than or equal to two is 0 On the other hand it isproved in [21] that real 119871
119901(120583) spaces have nonzero numerical
index for every measure 120583when 119901 = 2Therefore we have thefollowing examples
Example 8 (a) Complex Banach spaces which are uniformlysmooth and uniformly convex satisfy the BPBp-nu
(b) In particular for every measure 120583 the complex spaces119871119901(120583) have the BPBp-nu for 1 lt 119901 lt infin(c) For every measure 120583 the real spaces 119871
119901(120583) have the
BPBp-nu for 1 lt 119901 lt infin 119901 = 2
Note Added in Revision Very recently H J Lee M Martınand J Merı have proved that Proposition 6 can be extendedto some Banach spaces with numerical index zero as forinstance real Hilbert spaces Hence they have shown thatHilbert spaces have the BPBp-nu These results will appearelsewhere
4 L1
Spaces
In this section we will show that 1198711(120583) has the BPBp-nu for
every measure 120583 In the proof we are dealing with complexintegrable functions since the real case is followed easilyby applying the same proof Our main result here is thefollowing
Theorem 9 Let 120583 be a measure Then 1198711(120583) has the Bishop-
Phelps-Bollobas property for numerical radius More preciselygiven 120576 gt 0 there exists 120578(120576) gt 0 (which does not depend on120583) such that whenever 119879
0isin L(119871
1(120583)) with V(119879
0) = 1 and
(1198910 119892
0) isin Π(119871
1(120583)) satisfy |⟨119879
01198910 119892
0⟩| gt 1 minus 120578(120576) then there
exist 119879 isinL(1198711(120583)) (119891
1 119892
1) isin Π(119871
1(120583)) such that
1003816100381610038161003816⟨1198791198911 1198921⟩1003816100381610038161003816 = V (119879) = 1 10038171003817100381710038171198910 minus 1198911
1003817100381710038171003817 lt 120576
10038171003817100381710038171198920 minus 11989211003817100381710038171003817 lt 120576
1003817100381710038171003817119879 minus 11987901003817100381710038171003817 lt 120576
(35)
As a first step we have to start dealing with finite regularpositive Borel measures for which a representation theoremfor operators exists
Proposition 10 Let 119898 be a finite regular positive Borelmeasure on a compact Hausdorff spaceΩ Then 119871
1(119898) has the
Bishop-Phelps-Bollobas property for numerical radius Moreprecisely given 120576 gt 0 there is 120578(120576) gt 0 (which is independent ofthemeasure119898) such that if a norm-one element119879 inL(119871
1(119898))
and (1198910 119892
0) isin Π(119871
1(119898)) satisfy |⟨119879119891
0 119892
0⟩| gt 1 minus 120578(120576) then
there exist an operator 119878 isin L(1198711(119898)) (119891
1 119892
1) isin Π(119871
1(119898))
such that1003816100381610038161003816⟨1198781198911 1198921⟩
1003816100381610038161003816 = 119878 = 110038171003817100381710038171198910 minus 1198911
1003817100381710038171003817 ⩽ 120576
10038171003817100381710038171198920 minus 11989211003817100381710038171003817 ⩽ 120576 119879 minus 119878 ⩽ 120576
(36)
To prove this proposition we need some backgroundon representation of operators on Lebesgue spaces on finiteregular positive Borel measures and several preliminarylemmas
Let 119898 be a finite regular positive Borel measure on acompact Hausdorff space Ω If 120583 is a complex-valued Borelmeasure on the product space Ω times Ω then define theirmarginal measures 120583119894 on Ω (119894 = 1 2) as follows 1205831(119860) =120583(119860 times Ω) and 1205832(119861) = 120583(Ω times 119861) where 119860 and 119861 are Borelmeasurable subsets of Ω
Let 119872(119898) be the complex Banach lattice of measuresconsisting of all complex-valued Borel measures 120583 on theproduct spaceΩ timesΩ such that |120583|119894 are absolutely continuouswith respect to119898 for 119894 = 1 2 endowed with the norm
1003817100381710038171003817100381710038171003817100381710038171003817
11988910038161003816100381610038161205831003816100381610038161003816
1
119889119898
1003817100381710038171003817100381710038171003817100381710038171003817infin
(37)
Each 120583 isin 119872(119898) defines a bounded linear operator 119879120583from
1198711(119898) to itself by
⟨119879120583(119891) 119892⟩ = int
ΩtimesΩ
119891 (119909) 119892 (119910) 119889120583 (119909 119910) (38)
where 119891 isin 1198711(119898) and 119892 isin 119871
infin(119898) Iwanik [22] showed that
the mapping 120583 997891rarr 119879120583is a lattice isometric isomorphism from
119872(119898) onto L(1198711(119898)) Even though he showed this for the
real case it can be easily generalized to the complex caseFor details see [22 Theorem 1] and [23 IV Theorem 15(ii)Corollary 2]
We will also use that given an arbitrary measure 120583 every119879 isin L(119871
1(120583)) satisfies V(119879) = 119879 [24] (that is the space
1198711(120583) has numerical index 1)
Lemma 11 (see [15 Lemma 33]) Let 119888119899 be a sequence of
complex numbers with |119888119899| ⩽ 1 for every 119899 and let 120578 gt 0 such
6 Abstract and Applied Analysis
that for a convex series sum120572119899 Resuminfin
119899=1120572119899119888119899gt 1 minus 120578 Then for
every 0 lt 119903 lt 1 the set 119860 = 119894 isin N Re 119888119894gt 119903 satisfies the
estimate
sum
119894isin119860
120572119894⩾ 1 minus
120578
1 minus 119903 (39)
From now on 119898 will be a finite regular positive Borelmeasure on the compact Hausdorff space Ω
Lemma 12 Suppose that there exist a nonnegative simplefunction 119891 isin 119878
1198711(119898)
and a function 119892 isin 119878119871infin(119898)
such that
Re ⟨119891 119892⟩ gt 1 minus 1205763
16 (40)
Then there exist a nonnegative simple function 1198911isin 119878
1198711(119898)
anda function 119892
1isin 119878
119871infin(119898)
such that
1198921(119909) = 120594supp(119891
1)(119909) + 119892 (119909) 120594
Ωsupp(1198911)(119909)
⟨1198911 119892
1⟩ = 1
1003817100381710038171003817119891 minus 119891110038171003817100381710038171lt 120576
1003817100381710038171003817119892 minus 11989211003817100381710038171003817infinlt radic120576 supp (119891
1) sub supp (119891)
(41)
Proof Let119891 = sum119898
119895=1(120573
119895119898(119861
119895))120594
119861119895
for some (120573119895) such that
120573119895⩾ 0 for all 119895 andsum119898
119895=1120573119895= 1 and119861
119895rsquos aremutually disjoint
By the assumption we have
Re ⟨119891 119892⟩ =119899
sum
119895=1
120573119895
1
119898 (119861119895)
int119861119895
Re119892 (119909) 119889119898 (119909) gt 1 minus 1205763
16
(42)
and letting
119869 = 119895 1 ⩽ 119895 ⩽ 1198991
119898 (119861119895)
int119861119895
Re119892 (119909) 119889119898 (119909) gt 1 minus 1205762
4
(43)
we have by Lemma 11
sum
119895isin119869
120573119895gt 1 minus
120576
4 (44)
For each 119895 isin 119869 we have
1 minus1205762
4lt
1
119898 (119861119895)
int119861119895
Re119892 (119909) 119889119898 (119909)
=1
119898 (119861119895)
int119861119895capRe119892⩽1minus120576
Re119892 (119909) 119889119898 (119909)
+ int119861119895capRe119892gt1minus120576
Re119892 (119909) 119889119898 (119909)
⩽1
119898 (119861119895)
((1 minus 120576)119898 (119861119895cap Re119892 ⩽ 1 minus 120576)
+ 119898 (119861119895cap Re119892 gt 1 minus 120576))
= 1 minus 120576
119898 (119861119895cap Re119892 ⩽ 1 minus 120576)
119898 (119861119895)
(45)
This implies that
119898(119861119895cap Re119892 ⩽ 1 minus 120576)
119898 (119861119895)
lt120576
4 (46)
Define 119861119895= 119861
119895cap Re119892 gt 1 minus 120576 for all 119895 isin 119869
1198911= (1sum
119895isin119869
120573119895)sum
119895isin119869
120573119895(120594
119861119895
119898 (119861119895)) (47)
and 1198921(119909) = 1 on supp(119891
1) and 119892
1(119909) = 119892(119909) elsewhereThen
it is clear that supp(1198911) sub supp(119891) 119892 minus 119892
1infinlt radic120576 and
⟨1198911 119892
1⟩ = 1 Finally we will show that 119891 minus 119891
1 lt 120576 Notice
first that10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
⩽
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
= 2sum
119895isin119869
120573119895
119898(119861119895 119861
119895)
119898 (119861119895)
lt120576
2
(48)
Hence1003817100381710038171003817119891 minus 1198911
1003817100381710038171003817
⩽
10038171003817100381710038171003817100381710038171003817100381710038171003817
1
sum119895isin119869120573119895
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus 119891
10038171003817100381710038171003817100381710038171003817100381710038171003817
⩽
1 minus sum119895isin119869120573119895
sum119895isin119869120573119895
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+120576
4
= (1 minussum
119895isin119869
120573119895) +
120576
2+120576
4
⩽120576
4+120576
2+120576
4= 120576
(49)
Lemma 13 (see [25 Lemma 33]) Suppose that 119879120583is a norm-
one element in L(1198711(119898)) for some 120583 isin 119872(119898) and there is
a nonnegative simple function 1198910such that 119891
0is a norm-one
element of 1198711(119898) and 119879
1205831198910 ⩾ 1 minus 120576
32
6 for some 0 lt 120576 lt 1
Abstract and Applied Analysis 7
Then there exist a norm-one bounded linear operator 119879] forsome ] isin 119872(119898119898) and a nonnegative simple function 119891
1
in 1198781198711(119898)
such that 119879120583minus 119879] ⩽ 120576 1198911 minus 1198910 ⩽ 3120576 and
(119889|]|1119889119898)(119909) = 1 for all 119909 isin supp(1198911)
Lemma 14 Suppose that 119879] isin L(1198711(119898)) is a norm-one
operator 119891 = sum119899
119894=1120573119894(120594
119861119894
119898(119861119894)) where 119898(119861
119895) gt 0 for all
1 ⩽ 119895 ⩽ 119899 and 119861119895119899
119895=1are mutually disjoint Borel subsets ofΩ
is a norm-one nonnegative simple function and 119892 is an elementof 119878
119871infin(119898)
such that
Re ⟨119892 119879]119891⟩ ⩾ 1 minus1205766
27(50)
for some 0 lt 120576 lt 1 and
119889|]|1
119889119898(119909) = 1 119892 (119909) = 1 (51)
for all x in the support of 119891Then there exist a nonnegative simple function 119891 isin 119878
1198711(119898)
a function119892 isin 119878
119871infin(119898)
and an operator119879] inL(1198711(119898) 1198711(119898))such that
⟨119892 119879]119891⟩ =1003817100381710038171003817119879]1003817100381710038171003817 = 1
1003817100381710038171003817119879] minus 119879]1003817100381710038171003817 ⩽ 2120576
10038171003817100381710038171003817119891 minus 119891
10038171003817100381710038171003817⩽ 3120576
1003817100381710038171003817119892 minus 1198921003817100381710038171003817 ⩽radic120576 ⟨119891 119892⟩ = 1
(52)
Proof Since
Re ⟨119892 119879]119891⟩ ⩾ 1 minus1205766
27 (53)
we have
1 minus1205766
27lt Re ⟨119892 119879]119891⟩
= intΩtimesΩ
119891 (119909)Re119892 (119910) 119889120583 (119909 119910)
=
119899
sum
119895=1
120573119895intΩtimesΩ
120594119861119895
(119909)
119898 (119861119895)
Re119892 (119910) 119889] (119909 119910)
(54)
Let
119869 = 119895 intΩtimesΩ
(
120594119861119895
(119909)
119898 (119861119895)
)Re119892 (119910) 119889] (119909 119910) gt 1 minus 1205763
26
(55)
Then from Lemma 11 we have sum119895isin119869120573119895gt 1 minus 120576
32 Let 119891
1=
sum119895isin119869120573119895(120594
119861119895
119898(119861119895)) where 120573
119895= 120573
119895(sum
119895isin119869120573119895) for all 119895 isin 119869
Then
10038171003817100381710038171198911 minus 1198911003817100381710038171003817 ⩽
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
(120573119895minus 120573
119895)
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+ sum
119895isin119869
120573119895⩽ 120576
3⩽ 120576 (56)
Note that there is a Borel measurable function ℎ on Ω times Ωsuch that 119889](119909 119910) = ℎ(119909 119910) 119889|]|(119909 119910) and |ℎ(119909 119910)| = 1 forall (119909 119910) isin Ω times Ω Let
119862 = (119909 119910) 1003816100381610038161003816119892 (119910) ℎ (119909 119910) minus 1
1003816100381610038161003816 ltradic120576
232 (57)
Define two measures ]119891and ]
119888as follows
]119891(119860) = ] (119860 119862) ]
119888(119860) = ] (119860 cap 119862) (58)
for every Borel subset 119860 ofΩ times Ω It is clear that
119889] = 119889]119891+ 119889]
119888 119889
10038161003816100381610038161003816]119891
10038161003816100381610038161003816= ℎ 119889]
119891
1198891003816100381610038161003816]1198881003816100381610038161003816 = ℎ 119889]119888 119889 |]| = 119889
10038161003816100381610038161003816]119891
10038161003816100381610038161003816+ 1198891003816100381610038161003816]1198881003816100381610038161003816
(59)
Since (119889|]|11198891198981)(119909) = 1 for all 119909 isin ⋃119899
119895=1119861119895 we have
1 =119889|]|1
1198891198981
(119909) =
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909) +1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909) (60)
for all 119909 isin 119861 = ⋃119899
119895=1119861119895 and we deduce that |]|1(119861
119895) = 119898
1(119861
119895)
for all 1 ⩽ 119895 ⩽ 119899We claim that |]
119891|1(119861
119895)119898
1(119861
119895) ⩽ 120576
22
2 for all 119895 isin 119869Indeed if |119892(119910)ℎ(119909 119910) minus 1| ⩾ radic120576232 then
Re (119892 (119910) ℎ (119909 119910)) ⩽ 1 minus 12057624 (61)
So we have
1 minus1205763
26
⩽1
1198981(119861
119895)
ReintΩtimesΩ
120594119861119895(119909)119892 (119910) 119889] (119909 119910)
=1
1198981(119861
119895)
intΩtimesΩ
120594119861119895(119909)
Re (119892 (119910) ℎ (119909 119910)) 119889 |]| (119909 119910)
=1
1198981(119861
119895)
intΩtimesΩ
120594119861119895(119909)
Re (119892 (119910) ℎ (119909 119910)) 119889 10038161003816100381610038161003816]11989110038161003816100381610038161003816(119909 119910)
+1
1198981(119861
119895)
intΩtimesΩ
120594119861119895(119909)
Re (119892 (119910) ℎ (119909 119910)) 119889 1003816100381610038161003816]1198881003816100381610038161003816 (119909 119910)
⩽1
1198981(119861
119895)
((1 minus120576
24)10038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
(119861119895) +1003816100381610038161003816]1198881003816100381610038161003816
1
(119861119895))
= 1 minus120576
24
10038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
(119861119895)
1198981(119861
119895)
(62)
This proves our claimWe also claim that for each 119895 isin 119869 there exists a Borel
subset 119861119895of 119861
119895such that
(1 minus120576
2)119898
1(119861
119895) ⩽ 119898
1(119861
119895) ⩽ 119898
1(119861
119895)
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909) ⩽120576
2
(63)
8 Abstract and Applied Analysis
for all 119909 isin 119861119895 Indeed set 119861
119895= 119861
119895cap 119909 isin Ω (119889|]
119891|1119889119898
1)
(119909) ⩽ 1205762 Then
int119861119895119861119895
120576
2119889119898
1(119909) ⩽ int
119861119895
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909) 1198891198981(119909)
=10038161003816100381610038161003816]1119891
10038161003816100381610038161003816(119861
119895) ⩽1205762
22119898
1(119861
119895)
(64)
This shows that 1198981(119861
119895 119861
119895) ⩽ (1205762)119898
1(119861
119895) This proves our
second claimNow we define 119892 by 119892(119910) = 119892(119910)|119892(119910)| if |119892(119910)| ⩾ 1 minus
radic120576232 and 119892(119910) = 119892(119910) if |119892(119910)| lt 1 minus radic120576232 and we
write 119891 = sum119895isin119869120573119895(120594
119861119895
1198981(119861
119895)) It is clear that 119892 isin 119878
119871infin(119898)
119892 minus 119892 lt radic120576 and 119892(119910) = 1 for all 119909 isin supp119891
Finally we define the measure
119889] (119909 119910)
= sum
119895isin119869
120594119861119895
(119909) 119892 (119910)ℎ (119909 119910)119889]119888(119909 119910)(
1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+ 1205941198691119861(119909) 119889] (119909 119910)
(65)
where 119861 = ⋃119895isin119869119861119895 It is easy to see that (119889|]|1119889119898
1)(119909) = 1
on 119861 and (119889|]|11198891198981)(119909) ⩽ 1 elsewhere Note that
119889 (] minus ]) (119909 119910)
= sum
119895isin119869
120594119861119895
(119909) [
[
119892 (119910)ℎ (119909 119910)(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1]
]
119889]119888(119909 119910)
minus sum
119895isin119869
120594119861119895
(119909) 119889]119891(119909 119910)
(66)
If (119909 119910) isin 119862 then |119892(119910)| ⩾ 1 minus radic120576232 ⩾ 1 minus 1232 and
100381610038161003816100381610038161003816119892 (119910)ℎ (119909 119910) minus 1
100381610038161003816100381610038161003816
=
100381610038161003816100381610038161003816100381610038161003816
119892 (119910)
1003816100381610038161003816119892 (119910)1003816100381610038161003816
ℎ (119909 119910) minus 1
100381610038161003816100381610038161003816100381610038161003816
⩽
1003816100381610038161003816119892 (119910) ℎ (119909 119910) minus 11003816100381610038161003816
1003816100381610038161003816119892 (119910)1003816100381610038161003816
+
10038161003816100381610038161 minus1003816100381610038161003816119892 (119910)
1003816100381610038161003816
10038161003816100381610038161003816100381610038161003816119892 (119910)
1003816100381610038161003816
⩽ 2
1003816100381610038161003816119892 (119910) ℎ (119909 119910) minus 11003816100381610038161003816
1003816100381610038161003816119892 (119910)1003816100381610038161003816
⩽ 2radic120576
232
232
232 minus 1⩽ 2radic120576
(67)
Hence for all (119909 119910) isin 119862 we have
10038161003816100381610038161003816100381610038161003816100381610038161003816
119892 (119910)ℎ (119909 119910)(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
⩽100381610038161003816100381610038161003816119892 (119910)ℎ (119909 119910) minus 1
100381610038161003816100381610038161003816(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+
10038161003816100381610038161003816100381610038161003816100381610038161003816
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
⩽ 2radic120576(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+
10038161003816100381610038161003816100381610038161003816100381610038161003816
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
(68)
So we have for all 119909 isin 1198691
119889|] minus ]|1
1198891198981
(119909)
⩽ sum
119895isin119869
120594119861119895
(119909) [
[
2radic120576(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+
10038161003816100381610038161003816100381610038161003816100381610038161003816
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
]
]
1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909)
+ sum
119895isin119869
120594119861119895
(119909)
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909)
⩽ sum
119895isin119869
120594119861119895
(119909)(2radic120576 + (1 minus1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909)))
+sum
119895isin119869
120594119861119895
(119909)(
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909))
⩽ 2radic120576 + 120576 lt 3radic120576
(69)
This gives that 119879] minus 119879] lt 3radic120576 Note also that for all 119895 isin 119869
⟨119879]
120594119861119895
1198981(119861
119895)
119892⟩
= intΩtimesΩ
120594119861119895
(119909)
1198981(119861
119895)
119892 (119910) 119889] (119909 119910)
= intΩtimesΩ
120594119861119895
(119909)
1198981(119861
119895)
ℎ (119909 119910)(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
119889]119888(119909 119910)
Abstract and Applied Analysis 9
= intΩ
120594119861119895
(119909)
1198981(119861
119895)
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
1198891003816100381610038161003816]1198881003816100381610038161003816
1
(119909)
= intΩ
120594119861119895
(119909)
1198981(119861
119895)
1198891198981(119909) = 1
(70)
Hence we get ⟨119879]119891 119892⟩ = 1 which implies that 119879]119891 =119879] = 1 Finally10038171003817100381710038171003817119891 minus 119891
10038171003817100381710038171003817
⩽10038171003817100381710038171003817119891 minus 119891
1
10038171003817100381710038171003817+10038171003817100381710038171198911 minus 119891
1003817100381710038171003817
=
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
1198981(119861
119895)
minus sum
119895isin119869
120573119895
120594119861119895
1198981(119861
119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+ 120576
⩽ sum
119895isin119869
120573119895(
10038171003817100381710038171003817100381710038171003817100381710038171003817
120594119861119895
1198981(119861
119895)
minus
120594119861119895
1198981(119861
119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+
10038171003817100381710038171003817100381710038171003817100381710038171003817
120594119861119895
1198981(119861
119895)
minus
120594119861119895
1198981(119861
119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
) + 120576
= 2sum
119895isin119869
120573119895
1198981(119861
119895 119861
119895)
1198981(119861
119895)
+ 120576
⩽ 2sum
119895isin119869
120573119895
(1205762)1198981(119861
119895)
1198981(119861
119895)
+ 120576 ⩽120576
1 minus 1205762+ 120576 lt 3120576
(71)
We are now ready to present the proof of the main resultin the case of finite regular positive Borel measures
Proof of Proposition 10 Let 1205751= 120575
3
2(5sdot2
4) 120575
2= 120575
12
3(3
2sdot2
14)
and 1205753= (12057610)
2 for some 0 lt 120576 lt 1 Suppose that 119879 isinL(119871
1(119898)) with 119879 = 1 and that there is an 119891
0isin 119878
1198711(119898)
and1198920isin 119878
119871infin(119898)
such that ⟨1198910 119892
0⟩ = 1 and |⟨119879119891
0 119892
0⟩| gt 1minus120575
3
12
6Then there is an isometric isomorphism Ψ from 119871
1(119898) onto
itself such thatΨ(1198910) = |119891
0| and there is a scalar number 120572 in
119878R such that |⟨1198791198910 119892
0⟩| = ⟨120572119879119891
0 119892
0⟩ Then letting 119891
1= Ψ119891
0
1198921= (Ψ
minus1)lowast1198920 and 119879
1= 120572Ψ119879Ψ
minus1 we have
⟨1198781198911 119892
1⟩ = ⟨120572Ψ119879
0Ψ
minus1Ψ119891
0 (Ψ
minus1)lowast
1198920⟩
= ⟨1205721198791198910 119892
0⟩ gt 1 minus
1205753
1
26
⟨1198911 119892
1⟩ = ⟨Ψ119891
0 (Ψ
minus1)lowast
1198920⟩ = 1
(72)
Since 11987911198911 gt 1 minus 120575(120575
3
12
6) by Lemma 13 there exists a
norm-one bounded operator 119879] and a nonnegative simplefunction119891
2isin 119878
1198711(119898)
such that 1198791minus119879] ⩽ 1205751 1198912minus1198911 ⩽ 31205751
and (119889|]|11198891198981)(119909) = 1 for all 119909 isin supp(119891
2) Then
⟨119879]1198912 1198921⟩
= ⟨11987911198911 119892
1⟩ minus ⟨119879
11198911minus 119879
11198912 119892
1⟩ minus ⟨119879
11198912minus 119879]1198912 1198921⟩
⩾ ⟨11987911198911 119892
1⟩ minus10038171003817100381710038171198911 minus 1198912
1003817100381710038171003817 minus10038171003817100381710038171198791 minus 119879]
1003817100381710038171003817
⩾ 1 minus1205753
1
26minus 3120575
1minus 120575
1⩾ 1 minus 5120575
1= 1 minus
1205753
2
16
(73)
Notice also that
⟨1198912 119892
1⟩ = ⟨119891
1 119892
1⟩ minus ⟨119891
1minus 119891
2 119892
1⟩ ⩾ 1 minus
10038171003817100381710038171198911 minus 11989121003817100381710038171003817
⩾ 1 minus 31205751⩾ 1 minus 5120575
1= 1 minus
1205753
2
16
(74)
By Lemma 12 there are a nonnegative simple function 1198913isin
1198781198711(119898)
and a function 1198923isin 119878
119871infin(119898)
such that
1198923(119909) = 120594supp119891
3
(119909) + 1198922(119909) 120594
Ωsupp1198913
(119909)
10038171003817100381710038171198912 minus 11989131003817100381710038171003817 ⩽ 1205752
10038171003817100381710038171198923 minus 11989211003817100381710038171003817 ⩽radic120575
2 ⟨119891
3 119892
3⟩ = 1
(75)
So we have
⟨119879]1198913 1198923⟩ = ⟨119879]1198912 1198921⟩ minus ⟨119879]1198912 minus 119879]1198913 1198921⟩
minus ⟨119879]1198913 1198921 minus 1198923⟩
⩾ 1 minus1205753
2
16minus 2radic120575
2⩾ 1 minus 3radic120575
2= 1 minus
1205756
3
27
(76)
By Lemma 14 there exist 1198914isin 119878
1198711(119898)
and 1198924isin 119878
119871infin(119898)
and anoperator 119879
4such that
⟨1198924 119879
41198914⟩ = 1 =
100381710038171003817100381711987941003817100381710038171003817
10038171003817100381710038171198794 minus 119879]1003817100381710038171003817 ⩽ 21205753
10038171003817100381710038171198914 minus 11989131003817100381710038171003817 ⩽ 31205753
10038171003817100381710038171198924 minus 11989231003817100381710038171003817 ⩽radic120575
3 ⟨119891
4 119892
4⟩ = 1
(77)
So we have
10038171003817100381710038171198794 minus 11987911003817100381710038171003817 ⩽10038171003817100381710038171198794 minus 119879]
1003817100381710038171003817 +1003817100381710038171003817119879] minus 1198791
1003817100381710038171003817
⩽ 1205751+ 2120575
3⩽ 3120575
3
10038171003817100381710038171198911 minus 11989141003817100381710038171003817 ⩽10038171003817100381710038171198911 minus 1198912
1003817100381710038171003817 +10038171003817100381710038171198912 minus 1198913
1003817100381710038171003817 +10038171003817100381710038171198913 minus 1198914
1003817100381710038171003817
⩽ 31205751+ 120575
2+ 3120575
3⩽ 10120575
3
10038171003817100381710038171198921 minus 11989241003817100381710038171003817 ⩽10038171003817100381710038171198921 minus 1198923
1003817100381710038171003817 + 1198923 minus 1198924
⩽ 1205752+ radic120575
3⩽ 2radic120575
3
(78)
10 Abstract and Applied Analysis
Let 119878 = 120572Ψminus11198794Ψ 119891 = Ψminus1
1198914 and 119892 = Ψlowast
1198924 then we have
119879 minus 119878 =10038171003817100381710038171003817119879 minus 120572Ψ
minus11198794Ψ10038171003817100381710038171003817=10038171003817100381710038171003817120572Ψ119879Ψ
minus1minus 119879
4
10038171003817100381710038171003817
=10038171003817100381710038171198791 minus 1198794
1003817100381710038171003817 ⩽ 31205753
100381710038171003817100381710038171198910minus 11989110038171003817100381710038171003817=100381710038171003817100381710038171198910minus Ψ
minus11198914
10038171003817100381710038171003817=10038171003817100381710038171198911 minus 1198914
1003817100381710038171003817 ⩽ 101205753
100381710038171003817100381710038171198920minus 11989110038171003817100381710038171003817=10038171003817100381710038171198920 minus Ψ
lowast1198924
1003817100381710038171003817 =100381710038171003817100381710038171003817(Ψ
minus1)lowast
1198920minus 119892
4
100381710038171003817100381710038171003817
=10038171003817100381710038171198921 minus 1198924
1003817100381710038171003817 ⩽ 2radic120575
3
⟨119891 119892⟩ = ⟨Ψminus11198914 Ψ
lowast1198924⟩ = ⟨119891
4 119892
4⟩ = 1
10038161003816100381610038161003816⟨119878119891 119892⟩
10038161003816100381610038161003816=10038161003816100381610038161003816⟨120572Ψ
minus11198794ΨΨ
minus11198914 Ψ
lowast1198924⟩10038161003816100381610038161003816= |120572| = 1
(79)
This completes the proof
Finally we may give the proof of the main result in fullgenerality
Proof of Theorem 9 Notice that the Kakutani representationtheorem (see [26] for a reference) says that for every 120590-finitemeasure ] the space 119871
1(]) is isometrically isomorphic to
1198711(119898) for some positive Borel regular measure on a compact
Hausdorff space Then by Proposition 10 there is a universalfunction 120576 997891rarr 120578(120576) gt 0 which gives the BPBp-nu for 119871
1(]) for
every 120590-finite measure ]Fix 120576 gt 0 Suppose that 119879
0isin L(119871
1(120583)) with V(119879
0) = 1
and (1198910 119891
lowast
0) isin Π(119871
1(120583)) satisfy
1003816100381610038161003816⟨119891lowast
0 119879
01198910⟩1003816100381610038161003816 gt 1 minus 120578 (120576) (80)
Choose a sequence 119891119899 in 119871
1(120583) such that sup
119899119879
0119891119899 = 1
and let 119866 be the closed linear span of
119879119899
0119891119898 119899 119898 isin N cup 0 (81)
As 119866 is separable there is a dense subset 119892119899 119899 isin N of 119866
and let 119864 = ⋃infin
119899=1supp119892
119899 where supp119892
119899is the support of 119892
119899
Then the measure 120583|119864is 120590-finite Let
119884 = 119891 isin 1198711(120583) supp (119891) sub 119864 (82)
be a closed subspace of 1198711(120583) It is clear that 119871
1(120583) = 119884oplus
1119885
and 119884 is isometrically isomorphic to 1198711(120583|
119864) So 119884 has the
BPBp-nu with 120578(120576)Now write 119878
0= 119879
0|119884 119884 rarr 119884 consider 119910
0= 119891
0isin
119878119884 119910lowast
0= 119891
lowast
0|119884isin 119878
119884lowast and observe that 119910lowast
0(119910
0) = 1 and
|119910lowast
0(119878
01199100)| = |119891
lowast
0(119879
01198910)| gt 1 minus 120578(120576) Hence there exist 119878 isin
L(119884) and (1199100 119910
lowast
0) isin Π(119884) such that
1003816100381610038161003816119910lowast
0(119878119910
0)1003816100381610038161003816 = 1 = V (119878) 1003817100381710038171003817119878 minus 1198780
1003817100381710038171003817 lt 120576
10038171003817100381710038171199100 minus 11991001003817100381710038171003817 lt 120576
1003817100381710038171003817119910lowast
0minus 119910
lowast
0
1003817100381710038171003817 lt 120576
(83)
Finally consider the operator 119879 isinL(1198711(120583)) given by
119879 (119910 119911)
= (119878119910 0) + 1198790(0 119911) ((119910 119911) isin 119871
1(120583) equiv 119884oplus
1119885)
(84)
We have 119879 = 1 (and so V(119879) = 1) Indeed1003817100381710038171003817119879 (119910 119911)
1003817100381710038171003817 =1003817100381710038171003817(119878119910 0)
1003817100381710038171003817 +10038171003817100381710038171198790 (0 119911)
1003817100381710038171003817 ⩽10038171003817100381710038171199101003817100381710038171003817 + 119911 =
1003817100381710038171003817(119910 119911)1003817100381710038171003817
(85)
for all (119910 119911) isin 1198711(120583) and 119879(119910
0 0) = (119878119910
0 0) = 119878119910
0 = 1
Let 119909 = (1199100 0) and 119909lowast = (119910lowast
0 119891
0|119885) Then (119909 119909lowast) isin Π(119871
1(120583))
Moreover we have1003816100381610038161003816119909
lowast1198791199091003816100381610038161003816 =1003816100381610038161003816119910
lowast
0119878119910
0
1003816100381610038161003816 = 1 = V (119879) 1003817100381710038171003817119909 minus 1198910
1003817100381710038171003817 =1003817100381710038171003817119910 minus 1199100
1003817100381710038171003817 lt 120576
1003817100381710038171003817119909lowast
0minus 119891
lowast
0
1003817100381710038171003817
= max 1003817100381710038171003817119910 minus 119891lowast
0|119884
1003817100381710038171003817 1003817100381710038171003817119891
lowast
0|119885minus 119891
lowast
0|119885
1003817100381710038171003817
=1003817100381710038171003817119910
lowastminus 119910
lowast
0
1003817100381710038171003817 lt 120576
1003817100381710038171003817119879 minus 11987901003817100381710038171003817 = sup119910+119911⩽1
1003817100381710038171003817119879 (119910 119911) minus 1198790 (119910 119911)1003817100381710038171003817
= sup119910⩽1
1003817100381710038171003817119878119910 minus 11987801199101003817100381710038171003817 =1003817100381710038171003817119878 minus 1198780
1003817100381710038171003817 lt 120576
(86)
This completes the proof
5 Examples of Spaces Failing theBishop-Phelps-Bollobaacutes Property forNumerical Radius
Our goal here is to prove that the density of numerical radiusattaining operators does not imply the BPBp-nu Actuallywe will show that among separable spaces there is noisomorphic property implying the BPBp-nu other than finite-dimensionality
We need to relate the BPBp-nu to the Bishop-Phelps-Bollobas property for operators which as mentioned in theintroduction was introduced in [15] A pair (119883 119884) of Banachspaces has the Bishop-Phelps-Bollobas property for operators(in short BPBp) if given 120576 gt 0 there exists 120578(120576) gt 0 suchthat given 119879 isin L(119883 119884) with 119879 = 1 and 119909 isin 119878
119883such that
119879119909 gt 1 minus 120578(120576) then there exist 119911 isin 119878119883and 119878 isin L(119883 119884)
satisfying
119878 = 119878119911 = 1 119909 minus 119911 lt 120576
119879 minus 119878 lt 120576
(87)
We refer the reader to [15 19 25] and references therein formore information and background Among the interestingresults on the BPBp we emphasize that a pair (119883 119884) when119883is finite-dimensional does not necessarily have the BPBp Forinstance if119884 is a strictly convex space which is not uniformlyconvex then the pair (ℓ(2)
1 119884) fails to have the BPBp (this is
contained in [15] see [19 Section 3])
Abstract and Applied Analysis 11
The next result relates the BPBp-nu to the BPBp foroperators in a particular case We will deduce our examplefrom it
Theorem 15 If 1198711(120583)oplus
1119883 has the BPBp-119899119906 then the pair
(1198711(120583) 119883) has the BPBp for operators
Before proving this proposition we will use it to get themain examples of this section The first example shows thatthe density of numerical radius attaining operators does notimply the BPBp-nu
Example 16 There is a reflexive space (and so numerical radiusattaining operators on it are dense) which fails to have theBPBp-nu Indeed let 119884 be a reflexive separable space whichis not superreflexive and we may suppose that 119884 is strictlyconvex Observe that 119884 cannot be uniformly convex since itis not superreflexive Now 119883 = ℓ(2)
1oplus1119884 is reflexive but the
pair (ℓ(2)1 119884) fails the BPBp since 119884 is strictly convex but not
uniformly convex [19 Corollary 33] Therefore Theorem 15gives us that119883 does not have the BPBp-nu
The example above can be extended to get the result thatevery infinite-dimensional separable Banach space can berenormed to fail the BPBp-nu This follows from the factthat every infinite-dimensional separable Banach space canbe renormed to be strictly convex but not uniformly convex(this result can be proved ldquoby handrdquo an alternative categoricalargument for it can be found in [27] and references therein)With a little more of effort we may get the main result of thesection
Theorem 17 Every infinite-dimensional separable Banachspace can be renormed to fail the weak-BPBp-nu (and so inparticular to fail the BPBp-nu)
We need the following result which is surely well knownAs we have not found a reference we include a nice and easyproof kindly given to us by Vladimir Kadets We recall thatgiven a Banach space 119884 the set of all equivalent norms on119884 can be viewed as a metric space using the Banach-Mazurdistance
Lemma 18 Let119884 be an infinite-dimensional separable Banachspace Then the set of equivalent norms on 119884 which are strictlyconvex and are not (locally) uniformly convex is dense in the setof all equivalent norms on119884 (with respect to the Banach-Mazurdistance)
Proof Fix 119890 isin 119878119884and 119890lowast
1isin 119878
119884lowast such that 119890lowast
1(119890) = 1 For a fixed
120576 isin (0 12) denote
119902 (119910) = max (1 minus 120576) 10038171003817100381710038171199101003817100381710038171003817 1003816100381610038161003816119890lowast
1(119910)1003816100381610038161003816 (119910 isin 119884) (88)
Evidently (1 minus 120576)119910 ⩽ 119902(119910) ⩽ 119910 for every 119910 isin 119884 Fix asequence 119890lowast
119896 119896 ⩾ 2 of norm-one functionals separating
the points of 119884 and denote
119901 (119910) = radic
infin
sum
119896=1
1
2119896
1003816100381610038161003816119890lowast
119896(119910)1003816100381610038161003816
2
(119910 isin 119884) (89)
Then 119901 is a strictly convex norm on 119884 119901(119890) ⩾ 1radic2 and119901(119910) ⩽ 119910 for all 119910 isin 119883 Finally write
100381710038171003817100381711991010038171003817100381710038171= (1 minus 120576) 119902 (119910) + 120576
119901 (119910)
119901 (119890)(119910 isin 119884) (90)
Then sdot 1is a strictly convex norm on 119884 and
(1 minus 120576)2 10038171003817100381710038171199101003817100381710038171003817 ⩽100381710038171003817100381711991010038171003817100381710038171⩽ (1 + 120576)
10038171003817100381710038171199101003817100381710038171003817 (119910 isin 119884) (91)
Wewill finish the proof by showing that sdot 1is not uniformly
convex (actually it is not locally uniformly convex) Indeedfor each 119899 isin N we select 119910
119899isin ⋂
119899
119896=1ker 119890lowast
119896with 119910
119899 = 1 and
consider 119890119899= 119890 + (1205764)119910
119899 Then 119902(119890) = 1 119902(119890
119899) = 1 and
119902(119890+119890119899) = 2 At the same time 119901(119910
119899) rarr 0 so 119901(119890
119899) rarr 119901(119890)
and 119901(119890 + 119890119899) rarr 2119901(119890) Consequently
1198901 = 1100381710038171003817100381711989011989910038171003817100381710038171997888rarr 1
1003817100381710038171003817119890 + 11989011989910038171003817100381710038171997888rarr 2 (92)
but 119890 minus 1198901198991= (1205764)119910
1198991⩾ (1 minus 120576)
2(1205764) which means the
absence of local uniform convexity at 119890
Proof of Theorem 17 Let 119883 be an infinite-dimensional sepa-rable Banach space Take a closed subspace 119884 of 119883 of codi-mension two By [28 Proposition 2] the map carrying everyequivalent norm on 119884 to its numerical index is continuousand so the set of values of the numerical index of 119884 upto reforming is a nontrivial interval [28 Theorem 9] ThenLemma 18 allows us to find an equivalent norm | sdot | on 119884 insuch a way that (119884 | sdot |) is strictly convex and is not uniformlyconvex and 119899(119884 | sdot |) gt 0 Now the space 119883 = ℓ(2)
1oplus1(119884 | sdot |)
is an equivalent renorming of 119883 which does not have theBPBp-119899119906 (indeed otherwise the pair (ℓ(2)
1 (119884 | sdot |)) would
have the BPBp for the operator norm and so (119884 | sdot |) wouldbe uniformly convex by [19 Corollary 33] a contradiction)Moreover as
119899 (119883) = min 119899 (ℓ(2)1) 119899 (119884 |sdot|) gt 0 (93)
(see [20 Proposition 2] for instance) 119883 also fails the weak-BPBp-nu by Proposition 6
To finish the section with the promised proof ofTheorem 15 we first see the following stability result
Lemma 19 Let119883 = [⨁infin
119896=1119883
119896]1198880
or [⨁infin
119896=1119883
119896]ℓ1
If119883 has theBishop-Phelps-Bollobas property for numerical radius with afunction 120578 then each Banach space 119883
119894has the Bishop-Phelps-
Bollobas property for numerical radius with 120578nu(119883119894) ⩾ 120578 That
is inf119894120578nu(119883119894
)(120576) ⩾ 120578nu(119883)(120576) for all 0 lt 120576 lt 1
Proof Let 119875119894 119883 rarr 119883
119894and 1198751015840
119894 119883
lowastrarr 119883
lowast
119894be the natural
projections and let 119876119894 119883
119894rarr 119883 and 1198761015840
119894 119883
lowast
119894rarr 119883
lowast be thenatural embeddings
Assume that an operator 119879119894 119883
119894rarr 119883
119894and a pair
(119909119894 119909
lowast
119894) isin Π(119883
119894) satisfy that
V (119879119894) = 1
1003816100381610038161003816119909lowast
119894119879119894119909119894
1003816100381610038161003816 gt 1 minus 120578 (120576) (94)
We define an operator 119879 119883 rarr 119883 and (119909 119909lowast) isin Π(119883) by
119879 = 119876119894∘ 119879
119894∘ 119875
119894 (119909 119909
lowast) = (119876
119894119909119894 119876
1015840
119894119909lowast
119894) (95)
12 Abstract and Applied Analysis
then clearly we see that1003816100381610038161003816119909
lowast1198791199091003816100381610038161003816 =1003816100381610038161003816119909
lowast
119894119879119894119909119894
1003816100381610038161003816 gt 1 minus 120578 (120576) (96)
From the assumption there exist 119878 119883 rarr 119883 and a pair(119910 119910
lowast) isin Π(119883) such that
1003816100381610038161003816119910lowast1198781199101003816100381610038161003816 = 1 = V (119878) 119878 minus 119879 lt 120576
1003817100381710038171003817119910lowastminus 119909
lowast1003817100381710038171003817 lt 1205761003817100381710038171003817119910 minus 119909
1003817100381710038171003817 lt 120576
(97)
Since this clearly shows that1003817100381710038171003817119875119894 ∘ 119878 ∘ 119876119894
minus 119879119894
1003817100381710038171003817 lt 120576100381710038171003817100381710038171198751015840
119894119910lowastminus 119909
lowast
119894
10038171003817100381710038171003817lt 120576
1003817100381710038171003817119875119894119910 minus 1199091198941003817100381710038171003817 lt 120576
(98)
we only need to show that |1198751015840119894119910lowast(119875
119894∘ 119878 ∘ 119876
119894)119875
119894119910| = 1
We first show the case of 1198880sum Since
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817=10038171003817100381710038171003817119875119895119910 minus 119875
11989511990910038171003817100381710038171003817⩽1003817100381710038171003817119910 minus 119909
1003817100381710038171003817 lt 120576 (99)
for every 119895 = 119894 we have
1 = 119910lowast(119910) = sum
119895isinN
1198751015840
119895119910lowast(119875
119895119910) ⩽ sum
119895isinN
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817
⩽100381710038171003817100381710038171198751015840
119894119910lowast10038171003817100381710038171003817+ 120576 sum
119895isinN119895 = 119894
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817⩽1003817100381710038171003817119910
lowast1003817100381710038171003817 = 1
(100)
This shows that 1198751015840119894119910lowast = 1 and 1198751015840
119895119910lowast= 0 for every 119895 = 119894
So 119910lowast = 1198761015840
1198941198751015840
119894119910lowast and 1198751015840
119894119910lowast(119875
119894119910) = 1 This and the fact that
119910 minus 119876119894119875119894119910 lt 120576 imply that
(119876119894119875119894119910 + (
1
120576) (119910 minus 119876
119894119875119894119910) 119876
1015840
1198941198751015840
119894119910lowast) isin Π (119883) (101)
So we get that (1198761015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910 + (1120576)(119910 minus 119876
119894119875119894119910))V(119878) = 1
Hence we have
1 =1003816100381610038161003816119910
lowast1198781199101003816100381610038161003816 =10038161003816100381610038161003816(119876
1015840
1198941198751015840
119894119910lowast) 11987811991010038161003816100381610038161003816
=
1003816100381610038161003816100381610038161003816(1 minus 120576) (119876
1015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910)
+120576 (1198761015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910 +1
120576(119910 minus 119876
119894119875119894119910))
1003816100381610038161003816100381610038161003816⩽ 1
(102)
and so we get |1198751015840119894119910lowast(119875
119894∘119878 ∘119876
119894)119875
119894119910| = |(119876
1015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910)| = 1
We next show the case of ℓ1sum The proof is almost
the same as that of the 1198880case However for the sake of
completeness we provide it hereSince 1198751015840
119895119910lowast = 119875
1015840
119895119910lowastminus 119875
1015840
119895119909lowast ⩽ 119910
lowastminus 119909
lowast lt 120576 for every
119895 = 119894 we have
1 = 119910lowast(119910) = sum
119895isinN
1198751015840
119895119910lowast(119875
119895119910) ⩽ sum
119895isinN
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817
⩽10038171003817100381710038171198751198941199101003817100381710038171003817 + 120576 sum
119895isinN119895 = 119894
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817⩽10038171003817100381710038171199101003817100381710038171003817 = 1
(103)
which shows 119875119894119910 = 1 and 119875
119895119910 = 0 for every 119895 = 119894 Since this
implies (119876119894119875119894119910119876
1015840
1198941198751015840
119894119910lowast+(1120576)(119910
lowastminus119876
1015840
1198941198751015840
119894119910lowast)) isin Π(119883) we get
that |(1198761015840
1198941198751015840
119894119910lowast+ (1120576)(119910
lowastminus 119876
1015840
1198941198751015840
119894119910lowast))119878(119876
119894119875119894119910)| ⩽ V(119878) = 1
Hence we have
1 =1003816100381610038161003816119910
lowast1198781199101003816100381610038161003816 =1003816100381610038161003816119910
lowast119878 (119876
119894119875119894119910)1003816100381610038161003816
=
1003816100381610038161003816100381610038161003816(1 minus 120576) (119876
1015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910)
+120576 (1198761015840
1198941198751015840
119894119910lowast+1
120576(119910
lowastminus 119876
1015840
1198941198751015840
119894119910lowast)) 119878 (119876
119894119875119894119910)
1003816100381610038161003816100381610038161003816⩽ 1
(104)
and so |1198751015840119894119910lowast(119875
119894∘ 119878 ∘ 119876
119894)119875
119894119910| = |(119876
1015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910)| = 1
Proof of Theorem 15 Note that 120578nu(1198711(120583)oplus1119883)(120576) rarr 0 as120576 rarr 0 Fix 0 lt 120576
0lt 1 and choose 0 lt 120576 lt 1 such that
6120576 + 120578nu(1198711(120583)oplus1119883)(120576) lt 1205760 Let 120578(1205760) = 120578nu(1198711(120583)oplus1119883)(120576)Suppose that 119879
0isin L(119871
1(120583) 119883) with 119879
0 = 1 and 119891
0isin
1198781198711(120583)
satisfy
1003817100381710038171003817119879011989101003817100381710038171003817 gt 1 minus 120578 (1205760) (105)
For any measurable subset 119861 let
1198711(120583|
119861) = 119891|
119861 119891 isin 119871
1(120583) (106)
with the norm 119891|119861 = 119891120594
1198611 Then it is easy to see
that 1198711(120583|
119861) is isometrically isomorphic to a complemented
subspace of 1198711(120583) Let 119875
119861 119871
1(120583) rarr 119871
1(120583|
119861) be the
restriction defined by 119875119861(119891) = 119891|
119861for all 119891 isin 119871
1(120583) and
let 119869119861 119871
1(120583|
119861) rarr 119871
1(120583) be the extension defined by
119869119861(119891)(120596) = 119891(120596) if 120596 isin 119861 and 119869
119861(119891)(120596) = 0 otherwise
It is clear that 119875119861119869119861= Id
1198711(120583|119861)and 119869
119861119875119861(119891) = 119891120594
119861for all
119891 isin 1198711(120583) Notice also that 119871
1(120583) is isometrically isomorphic
to 1198711(120583|
119861)oplus
11198711(120583|
119861119888)
Let 119860 = supp1198910and 119892
0= 119875
1198601198910 Then
1003817100381710038171003817119879011986911986011989201003817100381710038171003817 =100381710038171003817100381711987901198910
1003817100381710038171003817 gt 1 minus 120578 (1205760) gt 0 (107)
and define the operator 119879119860 119871
1(120583|
119860) rarr 119883 by 119879
119860119891 =
1198790119869119860119891119879
0119869119860 for every 119891 isin 119871
1(120583|
119860) Then
100381710038171003817100381711987911986011989201003817100381710038171003817 ⩾100381710038171003817100381711987901198910
1003817100381710038171003817 gt 1 minus 120578 (1205760) (108)
Since 120583|119860is 120590-finite 119871
1(120583|
119860)lowast= 119871
infin(120583|
119860) Let 119892lowast
0isin 119878
119871infin(120583|119860)
be a function such that ⟨119892lowast0 119892
0⟩ = 1 and choose 119909lowast
0isin 119878
119883lowast
such that 119909lowast0(119879
1198601198920) = 119879
1198601198920 Define the operator 119878
0isin
L(1198711(120583|
119860)oplus
1119883) by
1198780(119891 119909) = (0 119879
119860119891) ((119891 119909) isin 119871
1(120583|
119860) oplus
1119883) (109)
and observe that 1198780 = V(119878
0) = 1 Indeed
100381710038171003817100381711987801003817100381710038171003817 ⩽ 1 =
10038171003817100381710038171198791198601003817100381710038171003817
= sup 1003816100381610038161003816119909lowast1198791198601198911003816100381610038161003816 119909
lowastisin 119878
119883lowast 119891 isin 119878
1198711(120583|119860)
= sup 1003816100381610038161003816(119891lowast 119909
lowast) 119878
0(119891 119909)
1003816100381610038161003816
((119891lowast 119909
lowast) (119891 119909)) isin Π (119871
1(120583|
119860) oplus
1119883)
= V (1198780) ⩽100381710038171003817100381711987801003817100381710038171003817
(110)
Abstract and Applied Analysis 13
It is immediate that
(119892lowast
0 119909
lowast
0) 119878
0(119892
0 0) = 119909
lowast
0(119879
1198601198920) =10038171003817100381710038171198791198601198920
1003817100381710038171003817 gt 1 minus 120578 (1205760)
(111)
By Lemma 19 1198711(120583|
119860)oplus
1119883 has the BPBp-nu with the func-
tion 120578 Therefore there exist 1198781isin L(119871
1(120583|
119860)oplus
1119883) (119892
1 119909
1) isin
1198781198711(120583|119860)oplus1119883 and (119892lowast
1 119909
lowast
1) isin 119878
119871infin(120583|119860)oplusinfin119883lowast such that
1003817100381710038171003817(1198921 1199091) minus (1198920 0)1003817100381710038171003817 lt 120576
1003817100381710038171003817(119892lowast
1 119909
lowast
1) minus (119891
lowast
0 119909
lowast
0)1003817100381710038171003817 lt 120576
10038171003817100381710038171198781 minus 11987801003817100381710038171003817 lt 120576 ⟨(119892
lowast
1 119909
lowast
1) (119892
1 119909
1)⟩ = 1
1003816100381610038161003816(119892lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)1003816100381610038161003816 = V (119878
1) = 1
(112)
Claim 1 We claim that 1199091= 0
Otherwise
1 = Re ⟨(119892lowast1 119909
lowast
1) (119892
1 119909
1)⟩
=100381710038171003817100381711989211003817100381710038171003817Re⟨(119892
lowast
1 119909
lowast
1) (119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
⟩
+100381710038171003817100381711990911003817100381710038171003817Re⟨(119892
lowast
1 119909
lowast
1) (0 119909
1)
100381710038171003817100381711990911003817100381710038171003817
⟩ ⩽ 1
(113)
We deduce that
((119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
(119892lowast
1 119909
lowast
1)) (
(0 1199091)
100381710038171003817100381711990911003817100381710038171003817
(119892lowast
1 119909
lowast
1))
isin Π (1198711(120583|
119860) oplus
1119883)
(114)
Since 1198781((0 119909
1)119909
1) = (119878
1minus 119878
0)((0 119909
1)119909
1) lt 120576 we
get that
1 =1003816100381610038161003816⟨(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)⟩1003816100381610038161003816
=
100381610038161003816100381610038161003816100381610038161003816
100381710038171003817100381711989211003817100381710038171003817⟨(119892
lowast
1 119909
lowast
1) 119878
1((119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
)⟩
+100381710038171003817100381711990911003817100381710038171003817⟨(119892
lowast
1 119909
lowast
1) 119878
1((0 119909
1)
100381710038171003817100381711990911003817100381710038171003817
)⟩
100381610038161003816100381610038161003816100381610038161003816
⩽100381710038171003817100381711989211003817100381710038171003817 V (1198781) + 120576
100381710038171003817100381711990911003817100381710038171003817 lt100381710038171003817100381711989211003817100381710038171003817 +100381710038171003817100381711990911003817100381710038171003817 = 1
(115)
a contradiction This proves the claimWe define the operator 119878
2 119871
1(120583|
119860)oplus
1119883 rarr 119871
1(120583|
119860)oplus
1119883
by 1198782(119891 119909) = 119878
1(119891 0) for every 119891 isin 119871
1(120583|
119860) and for every
119909 isin 119883 Then we have
V (1198782) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816 = 1
10038171003817100381710038171198781 minus 11987821003817100381710038171003817 ⩽ 120576 (116)
Indeed from Claim 1 we have
V (1198781) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)1003816100381610038161003816 =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816 ⩽ V (119878
2)
(117)
On the other hand we see that1003816100381610038161003816(119891
lowast 119909
lowast) 119878
2(119891 119909)
1003816100381610038161003816
=1003816100381610038161003816(119891
lowast 119909
lowast) 119878
1(119891 0)
1003816100381610038161003816 ⩽10038171003817100381710038171198911003817100381710038171003817 V (1198781) ⩽ V (119878
1)
(118)
for every ((119891lowast 119909lowast) (119891 119909)) isin Π(1198711(120583|
119860)oplus
1119883) Therefore
V(1198782) ⩽ V(119878
1) Also
10038171003817100381710038171198781 minus 11987821003817100381710038171003817 ⩽ sup
119909isin119878119883
10038171003817100381710038171198781 (0 119909)1003817100381710038171003817
= sup119909isin119878119883
10038171003817100381710038171198781 (0 119909) minus 1198780 (0 119909)1003817100381710038171003817 ⩽ 120576
(119)
Claim 2 There exists 1198783 119871
1(120583|
119860)oplus
1119883 rarr 119871
1(120583|
119860)oplus
1119883 such
that 1198783(119892
1 0) = 119878
3 = 1 119878
3(0 119909) = 0 119878
3(119891 119909) isin 0oplus
1119883
for every (119891 119909) isin 1198711(120583|
119860)oplus
1119883 and 119878
3minus 119878
2 lt 4120576
Indeed write 1198781= (119863
1 119863
2) where 119863
1 119871
1(120583|
119860)oplus
1119883 rarr
1198711(120583|
119860) and119863
2 119871
1(120583|
119860)oplus
1119883 rarr 119883 We have that
sup 1003816100381610038161003816119892lowast119863
1(119892
1 0) + 119909
lowast119863
2(119892
1 0)1003816100381610038161003816
119909lowastisin 119878
119883lowast ⟨119892
lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860)
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816 +10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
⟨119892lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860)
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816
⟨119892lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860) +10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
⩽ V (1198782) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816119892
lowast
1119863
1(119892
1 0) + 119909
lowast
1119863
2(119892
1 0)1003816100381610038161003816
(120)
This implies that
1003816100381610038161003816119909lowast
1119863
2(119892
1 0)1003816100381610038161003816 =10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
1003816100381610038161003816119892lowast
1119863
1(119892
1 0)1003816100381610038161003816
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816 ⟨119892
lowast 119892
1⟩ = 1 119892
lowastisin 119871
infin(120583|
119860)
(121)
Therefore |119892lowast1| equals 1 on the support of 119863
1(119892
1 0) As
|⟨119892lowast
1 119892
1⟩| = 1 we also have that |119892lowast
1| equals 1 on the support
of 1198921 Changing the values of 119892lowast
1by the ones of 119891lowast
0on 119860
(supp(1198631(119892
1 0)) cup supp(g
1)) we may and do suppose that
|119892lowast
1| = 1 on the whole 119860We also have 119863
2(119892
1 0) gt 0 Indeed
10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198920 0)1003817100381710038171003817
⩽10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198921 0)
1003817100381710038171003817 +10038171003817100381710038171198780 (1198921 0) minus 1198780 (1198920 0)
1003817100381710038171003817
lt 2120576 + 120576 = 3120576
(122)
So we have
10038171003817100381710038171198632(119892
1 0) minus 119879
1198601198920
1003817100381710038171003817
⩽10038171003817100381710038171198631(119892
1 0)1003817100381710038171003817 +10038171003817100381710038171198632(119892
1 0) minus 119879
1198601198920
1003817100381710038171003817
14 Abstract and Applied Analysis
=1003817100381710038171003817(1198631
(1198921 0) 119863
2(119892
1 0)) minus (0 119879
1198601198920)1003817100381710038171003817
=10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198920 0)
1003817100381710038171003817 lt 3120576
(123)
and 1198632(119892
1 0) gt 119879
1198601198920 minus 3120576 ⩾ 1 minus 120578(120576
0) minus 3120576 gt 0
Finally define the operator 1198783by
1198783(119891 119909) = (0119863
2(119891 0) + 119892
lowast
1(119863
1(119891 0))
1198632(119892
1 0)
119909lowast
1119863
2(119892
1 0))
(124)
for (119891 119909) isin 1198711(120583|
119860)oplus
1119883 It is clear that
100381710038171003817100381711987831003817100381710038171003817 ⩽ sup
119891isin1198781198711(120583|119860)
(10038171003817100381710038171198632(119891 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119891 0)
1003816100381610038161003816) (125)
Notice also that
10038171003817100381710038171198631(119891 0)
1003817100381710038171003817
⩽10038171003817100381710038171198631(119891 0)
1003817100381710038171003817 +10038171003817100381710038171198632(119891 0) minus 119879
1198601198911003817100381710038171003817
=1003817100381710038171003817(1198631
(119891 0) 1198632(119891 0)) minus (0 119879
119860119891)1003817100381710038171003817
=10038171003817100381710038171198782 (119891 119909) minus 1198780 (119891 119909)
1003817100381710038171003817
(126)
for all (119891 119909) isin 1198711(120583|
119860)oplus
1119883 Hence we have
10038171003817100381710038171198783 minus 11987821003817100381710038171003817
= 2 sup119891isin1198781198711(120583|119860)
10038171003817100381710038171198631(119891 0)
1003817100381710038171003817 ⩽ 210038171003817100381710038171198782 minus 1198780
1003817100381710038171003817 lt 4120576(127)
On the other hand let 119866 1198711(120583|
119860) rarr 119871
1(120583|
119860) be defined by
119866(119891) = 119892lowast
1119891 for every 119891 isin 119871
1(120583|
119860) Then we have
V (1198782)
= sup 1003816100381610038161003816119911lowast11987821199111003816100381610038161003816 (119911 119911
lowast) isin Π (119871
1(120583|
119860) oplus
1119883)
⩾ sup10038161003816100381610038161003816100381610038161003816
119909lowast119863
2(119866(
1
120583 (119862)120594119862) 0)
+119892lowast
1119863
1(119866(
1
120583 (119862)120594119862) 0)
10038161003816100381610038161003816100381610038161003816
119909lowastisin 119878
119883lowast 119862 isin Σ
119860 120583 (119862) gt 0
= sup10038171003817100381710038171003817100381710038171003817
1198632(119866(
1
120583 (119862)120594119862) 0)
10038171003817100381710038171003817100381710038171003817
+
10038161003816100381610038161003816100381610038161003816
119892lowast
1119863
1(119866(
1
120583 (119862)120594119862) 0)
10038161003816100381610038161003816100381610038161003816
119862 isin Σ119860 120583 (119862) gt 0
(128)
where Σ119860is the family of measurable subsets of 119860
Hence for any simple function 119904 = sum119899
119894=1((120572
119894120583(119860
119894))120594
119860119894
) isin
1198781198711(120583|119860) where 119860
119894119894is a family of disjoint measurable subsets
with strictly positive measure we have
V (1198782)
⩾
119899
sum
119894=1
10038161003816100381610038161205721198941003816100381610038161003816 (
100381710038171003817100381710038171003817100381710038171003817
1198632(119866(
1
120583 (119860119894)120594119860119894
) 0)
100381710038171003817100381710038171003817100381710038171003817
+
100381610038161003816100381610038161003816100381610038161003816
119892lowast
1119863
1(119866(
1
120583 (119860119894)120594119860119894
) 0)
100381610038161003816100381610038161003816100381610038161003816
)
⩾10038171003817100381710038171198632(119866 (119904) 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119866 (119904) 0)
1003816100381610038161003816
(129)
Since |119892lowast1| = 1119866 is an isometric isomorphism so for each119891 isin
1198781198711(120583|119860)there exists a sequence of norm-one simple functions
(119904119896) such that 119866(119904
119896) converges to 119891 Therefore
V (1198782) ⩾ sup
119891isin1198781198711(120583|119860)
(10038171003817100381710038171198632(119891 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119891 0)
1003816100381610038161003816) ⩾100381710038171003817100381711987831003817100381710038171003817
(130)
On the other hand we see that100381710038171003817100381711987831003817100381710038171003817 ⩾1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
3(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816119909
lowast
1119863
2(119892
1 0) + 119892
lowast
1119863
1(119892
1 0)1003816100381610038161003816 = V (119878
2) = 1
(131)
Therefore 1 = 1198783 = 119878
3(119892
1 0) which proves Claim 2
Finally set 1198783= (0 ) for a suitable 119871
1(120583|
119860)oplus
1119883 rarr
119883 and define the operator 1198791 119871
1(120583) rarr 119883 by
1198791(119891) = 119879
0(119891120594
119860119888) + (119875
119860119891 0) (132)
for every 119891 isin 1198711(120583) Then we have
10038171003817100381710038171198791 (119891)1003817100381710038171003817 ⩽100381710038171003817100381711987901003817100381710038171003817
10038171003817100381710038171198911205941198601198881003817100381710038171003817 +1003817100381710038171003817100381710038171003817100381710038171003817
10038171003817100381710038171198911205941198601003817100381710038171003817 =10038171003817100381710038171198911003817100381710038171003817 (133)
for every 119891 isin 1198711(120583) so 119879
1 ⩽ 1 Also
10038171003817100381710038171198791 (1198691198601198921)1003817100381710038171003817 =10038171003817100381710038171198783 (1198921 0)
1003817100381710038171003817 =100381710038171003817100381711987831003817100381710038171003817 = 1 (134)
so 1198791attains its norm at 119869
1198601198921isin 119871
1(120583) and
10038171003817100381710038171198691198601198921 minus 11989101003817100381710038171003817 =10038171003817100381710038171198921 minus 1198920
1003817100381710038171003817 lt 120576 (135)
We also have that for any 119891 isin 1198781198711(120583)
10038171003817100381710038171198790 (119891) minus 1198791 (119891)1003817100381710038171003817 =100381710038171003817100381710038171198790(119891120594
119860) minus (119875
119860119891 0)10038171003817100381710038171003817
⩽10038171003817100381710038171198790 (119869119860119875119860119891) minus 119879119860 (119875119860119891)
1003817100381710038171003817
+10038171003817100381710038171003817119879119860(119875
119860119891) minus (119875
119860119891 0)10038171003817100381710038171003817
⩽10038171003817100381710038171198790119869119860 minus 119879119860
1003817100381710038171003817 +10038171003817100381710038171198780 minus 1198783
1003817100381710038171003817
lt 120578 (1205760) + 6120576
(136)
Hence 1198790minus 119879
1 ⩽ 120578(120576
0) + 6120576 lt 120576
0
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Abstract and Applied Analysis 15
Acknowledgments
The authors thank Gilles Godefroy and Rafael Paya forfruitful conversations about the content of this paper andVladimir Kadets for providing Lemma 18 They also appre-ciate anonymous referees for careful reading and fruitfulsuggestions about revision The second author was sup-ported by Basic Science Research Program through theNational Research Foundation of Korea (NRF) fundedby the Ministry of Education Science and Technology(NRF-2012R1A1A1006869) The third author was partiallysupported by Spanish MICINN and FEDER project noMTM2012-31755 and by Junta de Andalucıa and FEDERGrants FQM-185 and P09-FQM-4911
References
[1] F F Bonsall and J Duncan Numerical Ranges of Operators onNormed Spaces and of Elements of Normed Algebras vol 2 ofLondon Mathematical Society Lecture Note Series 2 CambridgeUniversity Press London UK 1971
[2] F F Bonsall and J Duncan Numerical Ranges II CambridgeUniversity Press London UK 1973 London MathematicalSociety Lecture Notes Series 10
[3] B Sims On numerical range and its applications to Banachalgebras [PhD thesis] University of Newcastle New SouthWales Australia 1972
[4] I D Berg and B Sims ldquoDenseness of operators which attaintheir numerical radiusrdquoAustralian Mathematical Society A vol36 no 1 pp 130ndash133 1984
[5] C S Cardassi ldquoDensity of numerical radius attaining operatorson some reflexive spacesrdquo Bulletin of the Australian Mathemat-ical Society vol 31 no 1 pp 1ndash3 1985
[6] C S Cardassi ldquoNumerical radius attaining operatorsrdquo inBanach Spaces (Columbia Mo 1984) vol 1166 of Lecture Notesin Mathematics pp 11ndash14 Springer Berlin Germany 1985
[7] C S Cardassi ldquoNumerical radius-attaining operators on119862(119870)rdquoProceedings of the AmericanMathematical Society vol 95 no 4pp 537ndash543 1985
[8] MDAcostaOperadores que alcanzan su radio numerico [PhDthesis] Universidad de Granada Granada Spain 1990
[9] M D Acosta and R Paya ldquoNumerical radius attaining oper-ators and the Radon-Nikodym propertyrdquo The Bulletin of theLondon Mathematical Society vol 25 no 1 pp 67ndash73 1993
[10] R Paya ldquoA counterexample on numerical radius attainingoperatorsrdquo Israel Journal of Mathematics vol 79 no 1 pp 83ndash101 1992
[11] Y S Choi and S G Kim ldquoNorm or numerical radius attainingmultilinear mappings and polynomialsrdquo Journal of the LondonMathematical Society vol 54 no 1 pp 135ndash147 1996
[12] M D Acosta J Becerra Guerrero and M Ruiz GalanldquoNumerical-radius-attaining polynomialsrdquo The Quarterly Jour-nal of Mathematics vol 54 no 1 pp 1ndash10 2003
[13] M D Acosta and S G Kim ldquoDenseness of holomorphicfunctions attaining their numerical radiirdquo Israel Journal ofMathematics vol 161 pp 373ndash386 2007
[14] S G Kim and H J Lee ldquoNumerical peak holomorphicfunctions on Banach spacesrdquo Journal of Mathematical Analysisand Applications vol 364 no 2 pp 437ndash452 2010
[15] M D Acosta R M Aron D Garcıa and M MaestreldquoThe Bishop-Phelps-Bollobas theorem for operatorsrdquo Journal ofFunctional Analysis vol 254 no 11 pp 2780ndash2799 2008
[16] A J Guirao and O Kozhushkina ldquoThe Bishop-Phelps-Bollobasproperty for numerical radius in ℓ
1(119862)rdquo Studia Mathematica
vol 218 no 1 pp 41ndash54 2013[17] J Falco ldquoThe Bishop-Phelps-Bollobas property for numerical
radius on 1198711rdquo Journal of Mathematical Analysis and Applica-
tions vol 414 no 1 pp 125ndash133 2014[18] A Aviles A J Guirao and J Rodriguez ldquoOn the Bishop-
Phelps-Bollobas property for numerical radius in119862(119870)-spacesrdquoPreprint
[19] RM Aron Y S Choi S K Kim H J Lee andMMartin ldquoTheBishop-Phelps-Bollobas version of Lindenstrauss properties Aand Brdquo Preprint
[20] V Kadets M Martın and R Paya ldquoRecent progress and openquestions on the numerical index of Banach spacesrdquo Revista dela Real Academia de Ciencias Exactas Fısicas y Naturales A vol100 no 1-2 pp 155ndash182 2006
[21] M Martın J Merı and M Popov ldquoOn the numerical index ofreal 119871
119901(120583)-spacesrdquo Israel Journal of Mathematics vol 184 pp
183ndash192 2011[22] A Iwanik ldquoNorm attaining operators on Lebesgue spacesrdquo
Pacific Journal of Mathematics vol 83 no 2 pp 381ndash386 1979[23] HH SchaeferBanach Lattices and Positive Operators Springer
New York NY USA 1974[24] J Duncan C M McGregor J D Pryce and A J White ldquoThe
numerical index of a normed spacerdquo Journal of the LondonMathematical Society Second Series vol 2 pp 481ndash488 1970
[25] Y S Choi S K Kim H J Lee and M Martin ldquoThe Bishop-Phelps-Bollobas theorem for operators on 119871
1(120583)rdquo Preprint
[26] H E Lacey The Isometric Theory of Classical Banach SpacesSpringer New York NY USA 1974
[27] G Godefroy ldquoUniversal spaces for strictly convex Banachspacesrdquo Revista de la Real Academia de Ciencias Exactas Fısicasy Naturales A vol 100 no 1-2 pp 137ndash146 2006
[28] C Finet M Martın and R Paya ldquoNumerical index andrenormingrdquo Proceedings of the American Mathematical Societyvol 131 no 3 pp 871ndash877 2003
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical Problems in Engineering
Hindawi Publishing Corporationhttpwwwhindawicom
Differential EquationsInternational Journal of
Volume 2014
Applied MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Probability and StatisticsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical PhysicsAdvances in
Complex AnalysisJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
OptimizationJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CombinatoricsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Operations ResearchAdvances in
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Function Spaces
Abstract and Applied AnalysisHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of Mathematics and Mathematical Sciences
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Algebra
Discrete Dynamics in Nature and Society
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Decision SciencesAdvances in
Discrete MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom
Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of
Abstract and Applied Analysis 5
Finally we have
10038171003817100381710038171198781 minus 1198791003817100381710038171003817 ⩽
10038171003817100381710038171003817100381710038171003817
1
V (119878)119878 minus 119878
10038171003817100381710038171003817100381710038171003817
+ 119878 minus 119879
=119878
V (119878)|V (119878) minus 1| + 119878 minus 119879
⩽1
119899 (119883)|V (119878) minus V (119879)| + 119878 minus 119879
⩽ (1
119899 (119883)+ 1) 119878 minus 119879 lt
119899 (119883) + 1
119899 (119883)120576
(34)
An obvious change of parameters finishes the proof
We do not know whether the hypothesis of 119899(119883) gt 0 canbe omitted in the above result
Putting together Propositions 4 and 6 we get the follow-ing
Corollary 7 Let 119883 be a uniformly convex and uniformlysmooth Banach space with 119899(119883) gt 0 Then 119883 has the BPBp-nu
Let us comment that every complex Banach space 119883satisfies 119899(119883) ⩾ 1119890 so the above corollary automaticallyapplies in the complex case In the real case this is no longertrue as the numerical index of a Hilbert space of dimensiongreater than or equal to two is 0 On the other hand it isproved in [21] that real 119871
119901(120583) spaces have nonzero numerical
index for every measure 120583when 119901 = 2Therefore we have thefollowing examples
Example 8 (a) Complex Banach spaces which are uniformlysmooth and uniformly convex satisfy the BPBp-nu
(b) In particular for every measure 120583 the complex spaces119871119901(120583) have the BPBp-nu for 1 lt 119901 lt infin(c) For every measure 120583 the real spaces 119871
119901(120583) have the
BPBp-nu for 1 lt 119901 lt infin 119901 = 2
Note Added in Revision Very recently H J Lee M Martınand J Merı have proved that Proposition 6 can be extendedto some Banach spaces with numerical index zero as forinstance real Hilbert spaces Hence they have shown thatHilbert spaces have the BPBp-nu These results will appearelsewhere
4 L1
Spaces
In this section we will show that 1198711(120583) has the BPBp-nu for
every measure 120583 In the proof we are dealing with complexintegrable functions since the real case is followed easilyby applying the same proof Our main result here is thefollowing
Theorem 9 Let 120583 be a measure Then 1198711(120583) has the Bishop-
Phelps-Bollobas property for numerical radius More preciselygiven 120576 gt 0 there exists 120578(120576) gt 0 (which does not depend on120583) such that whenever 119879
0isin L(119871
1(120583)) with V(119879
0) = 1 and
(1198910 119892
0) isin Π(119871
1(120583)) satisfy |⟨119879
01198910 119892
0⟩| gt 1 minus 120578(120576) then there
exist 119879 isinL(1198711(120583)) (119891
1 119892
1) isin Π(119871
1(120583)) such that
1003816100381610038161003816⟨1198791198911 1198921⟩1003816100381610038161003816 = V (119879) = 1 10038171003817100381710038171198910 minus 1198911
1003817100381710038171003817 lt 120576
10038171003817100381710038171198920 minus 11989211003817100381710038171003817 lt 120576
1003817100381710038171003817119879 minus 11987901003817100381710038171003817 lt 120576
(35)
As a first step we have to start dealing with finite regularpositive Borel measures for which a representation theoremfor operators exists
Proposition 10 Let 119898 be a finite regular positive Borelmeasure on a compact Hausdorff spaceΩ Then 119871
1(119898) has the
Bishop-Phelps-Bollobas property for numerical radius Moreprecisely given 120576 gt 0 there is 120578(120576) gt 0 (which is independent ofthemeasure119898) such that if a norm-one element119879 inL(119871
1(119898))
and (1198910 119892
0) isin Π(119871
1(119898)) satisfy |⟨119879119891
0 119892
0⟩| gt 1 minus 120578(120576) then
there exist an operator 119878 isin L(1198711(119898)) (119891
1 119892
1) isin Π(119871
1(119898))
such that1003816100381610038161003816⟨1198781198911 1198921⟩
1003816100381610038161003816 = 119878 = 110038171003817100381710038171198910 minus 1198911
1003817100381710038171003817 ⩽ 120576
10038171003817100381710038171198920 minus 11989211003817100381710038171003817 ⩽ 120576 119879 minus 119878 ⩽ 120576
(36)
To prove this proposition we need some backgroundon representation of operators on Lebesgue spaces on finiteregular positive Borel measures and several preliminarylemmas
Let 119898 be a finite regular positive Borel measure on acompact Hausdorff space Ω If 120583 is a complex-valued Borelmeasure on the product space Ω times Ω then define theirmarginal measures 120583119894 on Ω (119894 = 1 2) as follows 1205831(119860) =120583(119860 times Ω) and 1205832(119861) = 120583(Ω times 119861) where 119860 and 119861 are Borelmeasurable subsets of Ω
Let 119872(119898) be the complex Banach lattice of measuresconsisting of all complex-valued Borel measures 120583 on theproduct spaceΩ timesΩ such that |120583|119894 are absolutely continuouswith respect to119898 for 119894 = 1 2 endowed with the norm
1003817100381710038171003817100381710038171003817100381710038171003817
11988910038161003816100381610038161205831003816100381610038161003816
1
119889119898
1003817100381710038171003817100381710038171003817100381710038171003817infin
(37)
Each 120583 isin 119872(119898) defines a bounded linear operator 119879120583from
1198711(119898) to itself by
⟨119879120583(119891) 119892⟩ = int
ΩtimesΩ
119891 (119909) 119892 (119910) 119889120583 (119909 119910) (38)
where 119891 isin 1198711(119898) and 119892 isin 119871
infin(119898) Iwanik [22] showed that
the mapping 120583 997891rarr 119879120583is a lattice isometric isomorphism from
119872(119898) onto L(1198711(119898)) Even though he showed this for the
real case it can be easily generalized to the complex caseFor details see [22 Theorem 1] and [23 IV Theorem 15(ii)Corollary 2]
We will also use that given an arbitrary measure 120583 every119879 isin L(119871
1(120583)) satisfies V(119879) = 119879 [24] (that is the space
1198711(120583) has numerical index 1)
Lemma 11 (see [15 Lemma 33]) Let 119888119899 be a sequence of
complex numbers with |119888119899| ⩽ 1 for every 119899 and let 120578 gt 0 such
6 Abstract and Applied Analysis
that for a convex series sum120572119899 Resuminfin
119899=1120572119899119888119899gt 1 minus 120578 Then for
every 0 lt 119903 lt 1 the set 119860 = 119894 isin N Re 119888119894gt 119903 satisfies the
estimate
sum
119894isin119860
120572119894⩾ 1 minus
120578
1 minus 119903 (39)
From now on 119898 will be a finite regular positive Borelmeasure on the compact Hausdorff space Ω
Lemma 12 Suppose that there exist a nonnegative simplefunction 119891 isin 119878
1198711(119898)
and a function 119892 isin 119878119871infin(119898)
such that
Re ⟨119891 119892⟩ gt 1 minus 1205763
16 (40)
Then there exist a nonnegative simple function 1198911isin 119878
1198711(119898)
anda function 119892
1isin 119878
119871infin(119898)
such that
1198921(119909) = 120594supp(119891
1)(119909) + 119892 (119909) 120594
Ωsupp(1198911)(119909)
⟨1198911 119892
1⟩ = 1
1003817100381710038171003817119891 minus 119891110038171003817100381710038171lt 120576
1003817100381710038171003817119892 minus 11989211003817100381710038171003817infinlt radic120576 supp (119891
1) sub supp (119891)
(41)
Proof Let119891 = sum119898
119895=1(120573
119895119898(119861
119895))120594
119861119895
for some (120573119895) such that
120573119895⩾ 0 for all 119895 andsum119898
119895=1120573119895= 1 and119861
119895rsquos aremutually disjoint
By the assumption we have
Re ⟨119891 119892⟩ =119899
sum
119895=1
120573119895
1
119898 (119861119895)
int119861119895
Re119892 (119909) 119889119898 (119909) gt 1 minus 1205763
16
(42)
and letting
119869 = 119895 1 ⩽ 119895 ⩽ 1198991
119898 (119861119895)
int119861119895
Re119892 (119909) 119889119898 (119909) gt 1 minus 1205762
4
(43)
we have by Lemma 11
sum
119895isin119869
120573119895gt 1 minus
120576
4 (44)
For each 119895 isin 119869 we have
1 minus1205762
4lt
1
119898 (119861119895)
int119861119895
Re119892 (119909) 119889119898 (119909)
=1
119898 (119861119895)
int119861119895capRe119892⩽1minus120576
Re119892 (119909) 119889119898 (119909)
+ int119861119895capRe119892gt1minus120576
Re119892 (119909) 119889119898 (119909)
⩽1
119898 (119861119895)
((1 minus 120576)119898 (119861119895cap Re119892 ⩽ 1 minus 120576)
+ 119898 (119861119895cap Re119892 gt 1 minus 120576))
= 1 minus 120576
119898 (119861119895cap Re119892 ⩽ 1 minus 120576)
119898 (119861119895)
(45)
This implies that
119898(119861119895cap Re119892 ⩽ 1 minus 120576)
119898 (119861119895)
lt120576
4 (46)
Define 119861119895= 119861
119895cap Re119892 gt 1 minus 120576 for all 119895 isin 119869
1198911= (1sum
119895isin119869
120573119895)sum
119895isin119869
120573119895(120594
119861119895
119898 (119861119895)) (47)
and 1198921(119909) = 1 on supp(119891
1) and 119892
1(119909) = 119892(119909) elsewhereThen
it is clear that supp(1198911) sub supp(119891) 119892 minus 119892
1infinlt radic120576 and
⟨1198911 119892
1⟩ = 1 Finally we will show that 119891 minus 119891
1 lt 120576 Notice
first that10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
⩽
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
= 2sum
119895isin119869
120573119895
119898(119861119895 119861
119895)
119898 (119861119895)
lt120576
2
(48)
Hence1003817100381710038171003817119891 minus 1198911
1003817100381710038171003817
⩽
10038171003817100381710038171003817100381710038171003817100381710038171003817
1
sum119895isin119869120573119895
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus 119891
10038171003817100381710038171003817100381710038171003817100381710038171003817
⩽
1 minus sum119895isin119869120573119895
sum119895isin119869120573119895
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+120576
4
= (1 minussum
119895isin119869
120573119895) +
120576
2+120576
4
⩽120576
4+120576
2+120576
4= 120576
(49)
Lemma 13 (see [25 Lemma 33]) Suppose that 119879120583is a norm-
one element in L(1198711(119898)) for some 120583 isin 119872(119898) and there is
a nonnegative simple function 1198910such that 119891
0is a norm-one
element of 1198711(119898) and 119879
1205831198910 ⩾ 1 minus 120576
32
6 for some 0 lt 120576 lt 1
Abstract and Applied Analysis 7
Then there exist a norm-one bounded linear operator 119879] forsome ] isin 119872(119898119898) and a nonnegative simple function 119891
1
in 1198781198711(119898)
such that 119879120583minus 119879] ⩽ 120576 1198911 minus 1198910 ⩽ 3120576 and
(119889|]|1119889119898)(119909) = 1 for all 119909 isin supp(1198911)
Lemma 14 Suppose that 119879] isin L(1198711(119898)) is a norm-one
operator 119891 = sum119899
119894=1120573119894(120594
119861119894
119898(119861119894)) where 119898(119861
119895) gt 0 for all
1 ⩽ 119895 ⩽ 119899 and 119861119895119899
119895=1are mutually disjoint Borel subsets ofΩ
is a norm-one nonnegative simple function and 119892 is an elementof 119878
119871infin(119898)
such that
Re ⟨119892 119879]119891⟩ ⩾ 1 minus1205766
27(50)
for some 0 lt 120576 lt 1 and
119889|]|1
119889119898(119909) = 1 119892 (119909) = 1 (51)
for all x in the support of 119891Then there exist a nonnegative simple function 119891 isin 119878
1198711(119898)
a function119892 isin 119878
119871infin(119898)
and an operator119879] inL(1198711(119898) 1198711(119898))such that
⟨119892 119879]119891⟩ =1003817100381710038171003817119879]1003817100381710038171003817 = 1
1003817100381710038171003817119879] minus 119879]1003817100381710038171003817 ⩽ 2120576
10038171003817100381710038171003817119891 minus 119891
10038171003817100381710038171003817⩽ 3120576
1003817100381710038171003817119892 minus 1198921003817100381710038171003817 ⩽radic120576 ⟨119891 119892⟩ = 1
(52)
Proof Since
Re ⟨119892 119879]119891⟩ ⩾ 1 minus1205766
27 (53)
we have
1 minus1205766
27lt Re ⟨119892 119879]119891⟩
= intΩtimesΩ
119891 (119909)Re119892 (119910) 119889120583 (119909 119910)
=
119899
sum
119895=1
120573119895intΩtimesΩ
120594119861119895
(119909)
119898 (119861119895)
Re119892 (119910) 119889] (119909 119910)
(54)
Let
119869 = 119895 intΩtimesΩ
(
120594119861119895
(119909)
119898 (119861119895)
)Re119892 (119910) 119889] (119909 119910) gt 1 minus 1205763
26
(55)
Then from Lemma 11 we have sum119895isin119869120573119895gt 1 minus 120576
32 Let 119891
1=
sum119895isin119869120573119895(120594
119861119895
119898(119861119895)) where 120573
119895= 120573
119895(sum
119895isin119869120573119895) for all 119895 isin 119869
Then
10038171003817100381710038171198911 minus 1198911003817100381710038171003817 ⩽
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
(120573119895minus 120573
119895)
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+ sum
119895isin119869
120573119895⩽ 120576
3⩽ 120576 (56)
Note that there is a Borel measurable function ℎ on Ω times Ωsuch that 119889](119909 119910) = ℎ(119909 119910) 119889|]|(119909 119910) and |ℎ(119909 119910)| = 1 forall (119909 119910) isin Ω times Ω Let
119862 = (119909 119910) 1003816100381610038161003816119892 (119910) ℎ (119909 119910) minus 1
1003816100381610038161003816 ltradic120576
232 (57)
Define two measures ]119891and ]
119888as follows
]119891(119860) = ] (119860 119862) ]
119888(119860) = ] (119860 cap 119862) (58)
for every Borel subset 119860 ofΩ times Ω It is clear that
119889] = 119889]119891+ 119889]
119888 119889
10038161003816100381610038161003816]119891
10038161003816100381610038161003816= ℎ 119889]
119891
1198891003816100381610038161003816]1198881003816100381610038161003816 = ℎ 119889]119888 119889 |]| = 119889
10038161003816100381610038161003816]119891
10038161003816100381610038161003816+ 1198891003816100381610038161003816]1198881003816100381610038161003816
(59)
Since (119889|]|11198891198981)(119909) = 1 for all 119909 isin ⋃119899
119895=1119861119895 we have
1 =119889|]|1
1198891198981
(119909) =
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909) +1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909) (60)
for all 119909 isin 119861 = ⋃119899
119895=1119861119895 and we deduce that |]|1(119861
119895) = 119898
1(119861
119895)
for all 1 ⩽ 119895 ⩽ 119899We claim that |]
119891|1(119861
119895)119898
1(119861
119895) ⩽ 120576
22
2 for all 119895 isin 119869Indeed if |119892(119910)ℎ(119909 119910) minus 1| ⩾ radic120576232 then
Re (119892 (119910) ℎ (119909 119910)) ⩽ 1 minus 12057624 (61)
So we have
1 minus1205763
26
⩽1
1198981(119861
119895)
ReintΩtimesΩ
120594119861119895(119909)119892 (119910) 119889] (119909 119910)
=1
1198981(119861
119895)
intΩtimesΩ
120594119861119895(119909)
Re (119892 (119910) ℎ (119909 119910)) 119889 |]| (119909 119910)
=1
1198981(119861
119895)
intΩtimesΩ
120594119861119895(119909)
Re (119892 (119910) ℎ (119909 119910)) 119889 10038161003816100381610038161003816]11989110038161003816100381610038161003816(119909 119910)
+1
1198981(119861
119895)
intΩtimesΩ
120594119861119895(119909)
Re (119892 (119910) ℎ (119909 119910)) 119889 1003816100381610038161003816]1198881003816100381610038161003816 (119909 119910)
⩽1
1198981(119861
119895)
((1 minus120576
24)10038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
(119861119895) +1003816100381610038161003816]1198881003816100381610038161003816
1
(119861119895))
= 1 minus120576
24
10038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
(119861119895)
1198981(119861
119895)
(62)
This proves our claimWe also claim that for each 119895 isin 119869 there exists a Borel
subset 119861119895of 119861
119895such that
(1 minus120576
2)119898
1(119861
119895) ⩽ 119898
1(119861
119895) ⩽ 119898
1(119861
119895)
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909) ⩽120576
2
(63)
8 Abstract and Applied Analysis
for all 119909 isin 119861119895 Indeed set 119861
119895= 119861
119895cap 119909 isin Ω (119889|]
119891|1119889119898
1)
(119909) ⩽ 1205762 Then
int119861119895119861119895
120576
2119889119898
1(119909) ⩽ int
119861119895
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909) 1198891198981(119909)
=10038161003816100381610038161003816]1119891
10038161003816100381610038161003816(119861
119895) ⩽1205762
22119898
1(119861
119895)
(64)
This shows that 1198981(119861
119895 119861
119895) ⩽ (1205762)119898
1(119861
119895) This proves our
second claimNow we define 119892 by 119892(119910) = 119892(119910)|119892(119910)| if |119892(119910)| ⩾ 1 minus
radic120576232 and 119892(119910) = 119892(119910) if |119892(119910)| lt 1 minus radic120576232 and we
write 119891 = sum119895isin119869120573119895(120594
119861119895
1198981(119861
119895)) It is clear that 119892 isin 119878
119871infin(119898)
119892 minus 119892 lt radic120576 and 119892(119910) = 1 for all 119909 isin supp119891
Finally we define the measure
119889] (119909 119910)
= sum
119895isin119869
120594119861119895
(119909) 119892 (119910)ℎ (119909 119910)119889]119888(119909 119910)(
1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+ 1205941198691119861(119909) 119889] (119909 119910)
(65)
where 119861 = ⋃119895isin119869119861119895 It is easy to see that (119889|]|1119889119898
1)(119909) = 1
on 119861 and (119889|]|11198891198981)(119909) ⩽ 1 elsewhere Note that
119889 (] minus ]) (119909 119910)
= sum
119895isin119869
120594119861119895
(119909) [
[
119892 (119910)ℎ (119909 119910)(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1]
]
119889]119888(119909 119910)
minus sum
119895isin119869
120594119861119895
(119909) 119889]119891(119909 119910)
(66)
If (119909 119910) isin 119862 then |119892(119910)| ⩾ 1 minus radic120576232 ⩾ 1 minus 1232 and
100381610038161003816100381610038161003816119892 (119910)ℎ (119909 119910) minus 1
100381610038161003816100381610038161003816
=
100381610038161003816100381610038161003816100381610038161003816
119892 (119910)
1003816100381610038161003816119892 (119910)1003816100381610038161003816
ℎ (119909 119910) minus 1
100381610038161003816100381610038161003816100381610038161003816
⩽
1003816100381610038161003816119892 (119910) ℎ (119909 119910) minus 11003816100381610038161003816
1003816100381610038161003816119892 (119910)1003816100381610038161003816
+
10038161003816100381610038161 minus1003816100381610038161003816119892 (119910)
1003816100381610038161003816
10038161003816100381610038161003816100381610038161003816119892 (119910)
1003816100381610038161003816
⩽ 2
1003816100381610038161003816119892 (119910) ℎ (119909 119910) minus 11003816100381610038161003816
1003816100381610038161003816119892 (119910)1003816100381610038161003816
⩽ 2radic120576
232
232
232 minus 1⩽ 2radic120576
(67)
Hence for all (119909 119910) isin 119862 we have
10038161003816100381610038161003816100381610038161003816100381610038161003816
119892 (119910)ℎ (119909 119910)(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
⩽100381610038161003816100381610038161003816119892 (119910)ℎ (119909 119910) minus 1
100381610038161003816100381610038161003816(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+
10038161003816100381610038161003816100381610038161003816100381610038161003816
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
⩽ 2radic120576(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+
10038161003816100381610038161003816100381610038161003816100381610038161003816
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
(68)
So we have for all 119909 isin 1198691
119889|] minus ]|1
1198891198981
(119909)
⩽ sum
119895isin119869
120594119861119895
(119909) [
[
2radic120576(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+
10038161003816100381610038161003816100381610038161003816100381610038161003816
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
]
]
1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909)
+ sum
119895isin119869
120594119861119895
(119909)
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909)
⩽ sum
119895isin119869
120594119861119895
(119909)(2radic120576 + (1 minus1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909)))
+sum
119895isin119869
120594119861119895
(119909)(
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909))
⩽ 2radic120576 + 120576 lt 3radic120576
(69)
This gives that 119879] minus 119879] lt 3radic120576 Note also that for all 119895 isin 119869
⟨119879]
120594119861119895
1198981(119861
119895)
119892⟩
= intΩtimesΩ
120594119861119895
(119909)
1198981(119861
119895)
119892 (119910) 119889] (119909 119910)
= intΩtimesΩ
120594119861119895
(119909)
1198981(119861
119895)
ℎ (119909 119910)(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
119889]119888(119909 119910)
Abstract and Applied Analysis 9
= intΩ
120594119861119895
(119909)
1198981(119861
119895)
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
1198891003816100381610038161003816]1198881003816100381610038161003816
1
(119909)
= intΩ
120594119861119895
(119909)
1198981(119861
119895)
1198891198981(119909) = 1
(70)
Hence we get ⟨119879]119891 119892⟩ = 1 which implies that 119879]119891 =119879] = 1 Finally10038171003817100381710038171003817119891 minus 119891
10038171003817100381710038171003817
⩽10038171003817100381710038171003817119891 minus 119891
1
10038171003817100381710038171003817+10038171003817100381710038171198911 minus 119891
1003817100381710038171003817
=
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
1198981(119861
119895)
minus sum
119895isin119869
120573119895
120594119861119895
1198981(119861
119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+ 120576
⩽ sum
119895isin119869
120573119895(
10038171003817100381710038171003817100381710038171003817100381710038171003817
120594119861119895
1198981(119861
119895)
minus
120594119861119895
1198981(119861
119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+
10038171003817100381710038171003817100381710038171003817100381710038171003817
120594119861119895
1198981(119861
119895)
minus
120594119861119895
1198981(119861
119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
) + 120576
= 2sum
119895isin119869
120573119895
1198981(119861
119895 119861
119895)
1198981(119861
119895)
+ 120576
⩽ 2sum
119895isin119869
120573119895
(1205762)1198981(119861
119895)
1198981(119861
119895)
+ 120576 ⩽120576
1 minus 1205762+ 120576 lt 3120576
(71)
We are now ready to present the proof of the main resultin the case of finite regular positive Borel measures
Proof of Proposition 10 Let 1205751= 120575
3
2(5sdot2
4) 120575
2= 120575
12
3(3
2sdot2
14)
and 1205753= (12057610)
2 for some 0 lt 120576 lt 1 Suppose that 119879 isinL(119871
1(119898)) with 119879 = 1 and that there is an 119891
0isin 119878
1198711(119898)
and1198920isin 119878
119871infin(119898)
such that ⟨1198910 119892
0⟩ = 1 and |⟨119879119891
0 119892
0⟩| gt 1minus120575
3
12
6Then there is an isometric isomorphism Ψ from 119871
1(119898) onto
itself such thatΨ(1198910) = |119891
0| and there is a scalar number 120572 in
119878R such that |⟨1198791198910 119892
0⟩| = ⟨120572119879119891
0 119892
0⟩ Then letting 119891
1= Ψ119891
0
1198921= (Ψ
minus1)lowast1198920 and 119879
1= 120572Ψ119879Ψ
minus1 we have
⟨1198781198911 119892
1⟩ = ⟨120572Ψ119879
0Ψ
minus1Ψ119891
0 (Ψ
minus1)lowast
1198920⟩
= ⟨1205721198791198910 119892
0⟩ gt 1 minus
1205753
1
26
⟨1198911 119892
1⟩ = ⟨Ψ119891
0 (Ψ
minus1)lowast
1198920⟩ = 1
(72)
Since 11987911198911 gt 1 minus 120575(120575
3
12
6) by Lemma 13 there exists a
norm-one bounded operator 119879] and a nonnegative simplefunction119891
2isin 119878
1198711(119898)
such that 1198791minus119879] ⩽ 1205751 1198912minus1198911 ⩽ 31205751
and (119889|]|11198891198981)(119909) = 1 for all 119909 isin supp(119891
2) Then
⟨119879]1198912 1198921⟩
= ⟨11987911198911 119892
1⟩ minus ⟨119879
11198911minus 119879
11198912 119892
1⟩ minus ⟨119879
11198912minus 119879]1198912 1198921⟩
⩾ ⟨11987911198911 119892
1⟩ minus10038171003817100381710038171198911 minus 1198912
1003817100381710038171003817 minus10038171003817100381710038171198791 minus 119879]
1003817100381710038171003817
⩾ 1 minus1205753
1
26minus 3120575
1minus 120575
1⩾ 1 minus 5120575
1= 1 minus
1205753
2
16
(73)
Notice also that
⟨1198912 119892
1⟩ = ⟨119891
1 119892
1⟩ minus ⟨119891
1minus 119891
2 119892
1⟩ ⩾ 1 minus
10038171003817100381710038171198911 minus 11989121003817100381710038171003817
⩾ 1 minus 31205751⩾ 1 minus 5120575
1= 1 minus
1205753
2
16
(74)
By Lemma 12 there are a nonnegative simple function 1198913isin
1198781198711(119898)
and a function 1198923isin 119878
119871infin(119898)
such that
1198923(119909) = 120594supp119891
3
(119909) + 1198922(119909) 120594
Ωsupp1198913
(119909)
10038171003817100381710038171198912 minus 11989131003817100381710038171003817 ⩽ 1205752
10038171003817100381710038171198923 minus 11989211003817100381710038171003817 ⩽radic120575
2 ⟨119891
3 119892
3⟩ = 1
(75)
So we have
⟨119879]1198913 1198923⟩ = ⟨119879]1198912 1198921⟩ minus ⟨119879]1198912 minus 119879]1198913 1198921⟩
minus ⟨119879]1198913 1198921 minus 1198923⟩
⩾ 1 minus1205753
2
16minus 2radic120575
2⩾ 1 minus 3radic120575
2= 1 minus
1205756
3
27
(76)
By Lemma 14 there exist 1198914isin 119878
1198711(119898)
and 1198924isin 119878
119871infin(119898)
and anoperator 119879
4such that
⟨1198924 119879
41198914⟩ = 1 =
100381710038171003817100381711987941003817100381710038171003817
10038171003817100381710038171198794 minus 119879]1003817100381710038171003817 ⩽ 21205753
10038171003817100381710038171198914 minus 11989131003817100381710038171003817 ⩽ 31205753
10038171003817100381710038171198924 minus 11989231003817100381710038171003817 ⩽radic120575
3 ⟨119891
4 119892
4⟩ = 1
(77)
So we have
10038171003817100381710038171198794 minus 11987911003817100381710038171003817 ⩽10038171003817100381710038171198794 minus 119879]
1003817100381710038171003817 +1003817100381710038171003817119879] minus 1198791
1003817100381710038171003817
⩽ 1205751+ 2120575
3⩽ 3120575
3
10038171003817100381710038171198911 minus 11989141003817100381710038171003817 ⩽10038171003817100381710038171198911 minus 1198912
1003817100381710038171003817 +10038171003817100381710038171198912 minus 1198913
1003817100381710038171003817 +10038171003817100381710038171198913 minus 1198914
1003817100381710038171003817
⩽ 31205751+ 120575
2+ 3120575
3⩽ 10120575
3
10038171003817100381710038171198921 minus 11989241003817100381710038171003817 ⩽10038171003817100381710038171198921 minus 1198923
1003817100381710038171003817 + 1198923 minus 1198924
⩽ 1205752+ radic120575
3⩽ 2radic120575
3
(78)
10 Abstract and Applied Analysis
Let 119878 = 120572Ψminus11198794Ψ 119891 = Ψminus1
1198914 and 119892 = Ψlowast
1198924 then we have
119879 minus 119878 =10038171003817100381710038171003817119879 minus 120572Ψ
minus11198794Ψ10038171003817100381710038171003817=10038171003817100381710038171003817120572Ψ119879Ψ
minus1minus 119879
4
10038171003817100381710038171003817
=10038171003817100381710038171198791 minus 1198794
1003817100381710038171003817 ⩽ 31205753
100381710038171003817100381710038171198910minus 11989110038171003817100381710038171003817=100381710038171003817100381710038171198910minus Ψ
minus11198914
10038171003817100381710038171003817=10038171003817100381710038171198911 minus 1198914
1003817100381710038171003817 ⩽ 101205753
100381710038171003817100381710038171198920minus 11989110038171003817100381710038171003817=10038171003817100381710038171198920 minus Ψ
lowast1198924
1003817100381710038171003817 =100381710038171003817100381710038171003817(Ψ
minus1)lowast
1198920minus 119892
4
100381710038171003817100381710038171003817
=10038171003817100381710038171198921 minus 1198924
1003817100381710038171003817 ⩽ 2radic120575
3
⟨119891 119892⟩ = ⟨Ψminus11198914 Ψ
lowast1198924⟩ = ⟨119891
4 119892
4⟩ = 1
10038161003816100381610038161003816⟨119878119891 119892⟩
10038161003816100381610038161003816=10038161003816100381610038161003816⟨120572Ψ
minus11198794ΨΨ
minus11198914 Ψ
lowast1198924⟩10038161003816100381610038161003816= |120572| = 1
(79)
This completes the proof
Finally we may give the proof of the main result in fullgenerality
Proof of Theorem 9 Notice that the Kakutani representationtheorem (see [26] for a reference) says that for every 120590-finitemeasure ] the space 119871
1(]) is isometrically isomorphic to
1198711(119898) for some positive Borel regular measure on a compact
Hausdorff space Then by Proposition 10 there is a universalfunction 120576 997891rarr 120578(120576) gt 0 which gives the BPBp-nu for 119871
1(]) for
every 120590-finite measure ]Fix 120576 gt 0 Suppose that 119879
0isin L(119871
1(120583)) with V(119879
0) = 1
and (1198910 119891
lowast
0) isin Π(119871
1(120583)) satisfy
1003816100381610038161003816⟨119891lowast
0 119879
01198910⟩1003816100381610038161003816 gt 1 minus 120578 (120576) (80)
Choose a sequence 119891119899 in 119871
1(120583) such that sup
119899119879
0119891119899 = 1
and let 119866 be the closed linear span of
119879119899
0119891119898 119899 119898 isin N cup 0 (81)
As 119866 is separable there is a dense subset 119892119899 119899 isin N of 119866
and let 119864 = ⋃infin
119899=1supp119892
119899 where supp119892
119899is the support of 119892
119899
Then the measure 120583|119864is 120590-finite Let
119884 = 119891 isin 1198711(120583) supp (119891) sub 119864 (82)
be a closed subspace of 1198711(120583) It is clear that 119871
1(120583) = 119884oplus
1119885
and 119884 is isometrically isomorphic to 1198711(120583|
119864) So 119884 has the
BPBp-nu with 120578(120576)Now write 119878
0= 119879
0|119884 119884 rarr 119884 consider 119910
0= 119891
0isin
119878119884 119910lowast
0= 119891
lowast
0|119884isin 119878
119884lowast and observe that 119910lowast
0(119910
0) = 1 and
|119910lowast
0(119878
01199100)| = |119891
lowast
0(119879
01198910)| gt 1 minus 120578(120576) Hence there exist 119878 isin
L(119884) and (1199100 119910
lowast
0) isin Π(119884) such that
1003816100381610038161003816119910lowast
0(119878119910
0)1003816100381610038161003816 = 1 = V (119878) 1003817100381710038171003817119878 minus 1198780
1003817100381710038171003817 lt 120576
10038171003817100381710038171199100 minus 11991001003817100381710038171003817 lt 120576
1003817100381710038171003817119910lowast
0minus 119910
lowast
0
1003817100381710038171003817 lt 120576
(83)
Finally consider the operator 119879 isinL(1198711(120583)) given by
119879 (119910 119911)
= (119878119910 0) + 1198790(0 119911) ((119910 119911) isin 119871
1(120583) equiv 119884oplus
1119885)
(84)
We have 119879 = 1 (and so V(119879) = 1) Indeed1003817100381710038171003817119879 (119910 119911)
1003817100381710038171003817 =1003817100381710038171003817(119878119910 0)
1003817100381710038171003817 +10038171003817100381710038171198790 (0 119911)
1003817100381710038171003817 ⩽10038171003817100381710038171199101003817100381710038171003817 + 119911 =
1003817100381710038171003817(119910 119911)1003817100381710038171003817
(85)
for all (119910 119911) isin 1198711(120583) and 119879(119910
0 0) = (119878119910
0 0) = 119878119910
0 = 1
Let 119909 = (1199100 0) and 119909lowast = (119910lowast
0 119891
0|119885) Then (119909 119909lowast) isin Π(119871
1(120583))
Moreover we have1003816100381610038161003816119909
lowast1198791199091003816100381610038161003816 =1003816100381610038161003816119910
lowast
0119878119910
0
1003816100381610038161003816 = 1 = V (119879) 1003817100381710038171003817119909 minus 1198910
1003817100381710038171003817 =1003817100381710038171003817119910 minus 1199100
1003817100381710038171003817 lt 120576
1003817100381710038171003817119909lowast
0minus 119891
lowast
0
1003817100381710038171003817
= max 1003817100381710038171003817119910 minus 119891lowast
0|119884
1003817100381710038171003817 1003817100381710038171003817119891
lowast
0|119885minus 119891
lowast
0|119885
1003817100381710038171003817
=1003817100381710038171003817119910
lowastminus 119910
lowast
0
1003817100381710038171003817 lt 120576
1003817100381710038171003817119879 minus 11987901003817100381710038171003817 = sup119910+119911⩽1
1003817100381710038171003817119879 (119910 119911) minus 1198790 (119910 119911)1003817100381710038171003817
= sup119910⩽1
1003817100381710038171003817119878119910 minus 11987801199101003817100381710038171003817 =1003817100381710038171003817119878 minus 1198780
1003817100381710038171003817 lt 120576
(86)
This completes the proof
5 Examples of Spaces Failing theBishop-Phelps-Bollobaacutes Property forNumerical Radius
Our goal here is to prove that the density of numerical radiusattaining operators does not imply the BPBp-nu Actuallywe will show that among separable spaces there is noisomorphic property implying the BPBp-nu other than finite-dimensionality
We need to relate the BPBp-nu to the Bishop-Phelps-Bollobas property for operators which as mentioned in theintroduction was introduced in [15] A pair (119883 119884) of Banachspaces has the Bishop-Phelps-Bollobas property for operators(in short BPBp) if given 120576 gt 0 there exists 120578(120576) gt 0 suchthat given 119879 isin L(119883 119884) with 119879 = 1 and 119909 isin 119878
119883such that
119879119909 gt 1 minus 120578(120576) then there exist 119911 isin 119878119883and 119878 isin L(119883 119884)
satisfying
119878 = 119878119911 = 1 119909 minus 119911 lt 120576
119879 minus 119878 lt 120576
(87)
We refer the reader to [15 19 25] and references therein formore information and background Among the interestingresults on the BPBp we emphasize that a pair (119883 119884) when119883is finite-dimensional does not necessarily have the BPBp Forinstance if119884 is a strictly convex space which is not uniformlyconvex then the pair (ℓ(2)
1 119884) fails to have the BPBp (this is
contained in [15] see [19 Section 3])
Abstract and Applied Analysis 11
The next result relates the BPBp-nu to the BPBp foroperators in a particular case We will deduce our examplefrom it
Theorem 15 If 1198711(120583)oplus
1119883 has the BPBp-119899119906 then the pair
(1198711(120583) 119883) has the BPBp for operators
Before proving this proposition we will use it to get themain examples of this section The first example shows thatthe density of numerical radius attaining operators does notimply the BPBp-nu
Example 16 There is a reflexive space (and so numerical radiusattaining operators on it are dense) which fails to have theBPBp-nu Indeed let 119884 be a reflexive separable space whichis not superreflexive and we may suppose that 119884 is strictlyconvex Observe that 119884 cannot be uniformly convex since itis not superreflexive Now 119883 = ℓ(2)
1oplus1119884 is reflexive but the
pair (ℓ(2)1 119884) fails the BPBp since 119884 is strictly convex but not
uniformly convex [19 Corollary 33] Therefore Theorem 15gives us that119883 does not have the BPBp-nu
The example above can be extended to get the result thatevery infinite-dimensional separable Banach space can berenormed to fail the BPBp-nu This follows from the factthat every infinite-dimensional separable Banach space canbe renormed to be strictly convex but not uniformly convex(this result can be proved ldquoby handrdquo an alternative categoricalargument for it can be found in [27] and references therein)With a little more of effort we may get the main result of thesection
Theorem 17 Every infinite-dimensional separable Banachspace can be renormed to fail the weak-BPBp-nu (and so inparticular to fail the BPBp-nu)
We need the following result which is surely well knownAs we have not found a reference we include a nice and easyproof kindly given to us by Vladimir Kadets We recall thatgiven a Banach space 119884 the set of all equivalent norms on119884 can be viewed as a metric space using the Banach-Mazurdistance
Lemma 18 Let119884 be an infinite-dimensional separable Banachspace Then the set of equivalent norms on 119884 which are strictlyconvex and are not (locally) uniformly convex is dense in the setof all equivalent norms on119884 (with respect to the Banach-Mazurdistance)
Proof Fix 119890 isin 119878119884and 119890lowast
1isin 119878
119884lowast such that 119890lowast
1(119890) = 1 For a fixed
120576 isin (0 12) denote
119902 (119910) = max (1 minus 120576) 10038171003817100381710038171199101003817100381710038171003817 1003816100381610038161003816119890lowast
1(119910)1003816100381610038161003816 (119910 isin 119884) (88)
Evidently (1 minus 120576)119910 ⩽ 119902(119910) ⩽ 119910 for every 119910 isin 119884 Fix asequence 119890lowast
119896 119896 ⩾ 2 of norm-one functionals separating
the points of 119884 and denote
119901 (119910) = radic
infin
sum
119896=1
1
2119896
1003816100381610038161003816119890lowast
119896(119910)1003816100381610038161003816
2
(119910 isin 119884) (89)
Then 119901 is a strictly convex norm on 119884 119901(119890) ⩾ 1radic2 and119901(119910) ⩽ 119910 for all 119910 isin 119883 Finally write
100381710038171003817100381711991010038171003817100381710038171= (1 minus 120576) 119902 (119910) + 120576
119901 (119910)
119901 (119890)(119910 isin 119884) (90)
Then sdot 1is a strictly convex norm on 119884 and
(1 minus 120576)2 10038171003817100381710038171199101003817100381710038171003817 ⩽100381710038171003817100381711991010038171003817100381710038171⩽ (1 + 120576)
10038171003817100381710038171199101003817100381710038171003817 (119910 isin 119884) (91)
Wewill finish the proof by showing that sdot 1is not uniformly
convex (actually it is not locally uniformly convex) Indeedfor each 119899 isin N we select 119910
119899isin ⋂
119899
119896=1ker 119890lowast
119896with 119910
119899 = 1 and
consider 119890119899= 119890 + (1205764)119910
119899 Then 119902(119890) = 1 119902(119890
119899) = 1 and
119902(119890+119890119899) = 2 At the same time 119901(119910
119899) rarr 0 so 119901(119890
119899) rarr 119901(119890)
and 119901(119890 + 119890119899) rarr 2119901(119890) Consequently
1198901 = 1100381710038171003817100381711989011989910038171003817100381710038171997888rarr 1
1003817100381710038171003817119890 + 11989011989910038171003817100381710038171997888rarr 2 (92)
but 119890 minus 1198901198991= (1205764)119910
1198991⩾ (1 minus 120576)
2(1205764) which means the
absence of local uniform convexity at 119890
Proof of Theorem 17 Let 119883 be an infinite-dimensional sepa-rable Banach space Take a closed subspace 119884 of 119883 of codi-mension two By [28 Proposition 2] the map carrying everyequivalent norm on 119884 to its numerical index is continuousand so the set of values of the numerical index of 119884 upto reforming is a nontrivial interval [28 Theorem 9] ThenLemma 18 allows us to find an equivalent norm | sdot | on 119884 insuch a way that (119884 | sdot |) is strictly convex and is not uniformlyconvex and 119899(119884 | sdot |) gt 0 Now the space 119883 = ℓ(2)
1oplus1(119884 | sdot |)
is an equivalent renorming of 119883 which does not have theBPBp-119899119906 (indeed otherwise the pair (ℓ(2)
1 (119884 | sdot |)) would
have the BPBp for the operator norm and so (119884 | sdot |) wouldbe uniformly convex by [19 Corollary 33] a contradiction)Moreover as
119899 (119883) = min 119899 (ℓ(2)1) 119899 (119884 |sdot|) gt 0 (93)
(see [20 Proposition 2] for instance) 119883 also fails the weak-BPBp-nu by Proposition 6
To finish the section with the promised proof ofTheorem 15 we first see the following stability result
Lemma 19 Let119883 = [⨁infin
119896=1119883
119896]1198880
or [⨁infin
119896=1119883
119896]ℓ1
If119883 has theBishop-Phelps-Bollobas property for numerical radius with afunction 120578 then each Banach space 119883
119894has the Bishop-Phelps-
Bollobas property for numerical radius with 120578nu(119883119894) ⩾ 120578 That
is inf119894120578nu(119883119894
)(120576) ⩾ 120578nu(119883)(120576) for all 0 lt 120576 lt 1
Proof Let 119875119894 119883 rarr 119883
119894and 1198751015840
119894 119883
lowastrarr 119883
lowast
119894be the natural
projections and let 119876119894 119883
119894rarr 119883 and 1198761015840
119894 119883
lowast
119894rarr 119883
lowast be thenatural embeddings
Assume that an operator 119879119894 119883
119894rarr 119883
119894and a pair
(119909119894 119909
lowast
119894) isin Π(119883
119894) satisfy that
V (119879119894) = 1
1003816100381610038161003816119909lowast
119894119879119894119909119894
1003816100381610038161003816 gt 1 minus 120578 (120576) (94)
We define an operator 119879 119883 rarr 119883 and (119909 119909lowast) isin Π(119883) by
119879 = 119876119894∘ 119879
119894∘ 119875
119894 (119909 119909
lowast) = (119876
119894119909119894 119876
1015840
119894119909lowast
119894) (95)
12 Abstract and Applied Analysis
then clearly we see that1003816100381610038161003816119909
lowast1198791199091003816100381610038161003816 =1003816100381610038161003816119909
lowast
119894119879119894119909119894
1003816100381610038161003816 gt 1 minus 120578 (120576) (96)
From the assumption there exist 119878 119883 rarr 119883 and a pair(119910 119910
lowast) isin Π(119883) such that
1003816100381610038161003816119910lowast1198781199101003816100381610038161003816 = 1 = V (119878) 119878 minus 119879 lt 120576
1003817100381710038171003817119910lowastminus 119909
lowast1003817100381710038171003817 lt 1205761003817100381710038171003817119910 minus 119909
1003817100381710038171003817 lt 120576
(97)
Since this clearly shows that1003817100381710038171003817119875119894 ∘ 119878 ∘ 119876119894
minus 119879119894
1003817100381710038171003817 lt 120576100381710038171003817100381710038171198751015840
119894119910lowastminus 119909
lowast
119894
10038171003817100381710038171003817lt 120576
1003817100381710038171003817119875119894119910 minus 1199091198941003817100381710038171003817 lt 120576
(98)
we only need to show that |1198751015840119894119910lowast(119875
119894∘ 119878 ∘ 119876
119894)119875
119894119910| = 1
We first show the case of 1198880sum Since
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817=10038171003817100381710038171003817119875119895119910 minus 119875
11989511990910038171003817100381710038171003817⩽1003817100381710038171003817119910 minus 119909
1003817100381710038171003817 lt 120576 (99)
for every 119895 = 119894 we have
1 = 119910lowast(119910) = sum
119895isinN
1198751015840
119895119910lowast(119875
119895119910) ⩽ sum
119895isinN
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817
⩽100381710038171003817100381710038171198751015840
119894119910lowast10038171003817100381710038171003817+ 120576 sum
119895isinN119895 = 119894
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817⩽1003817100381710038171003817119910
lowast1003817100381710038171003817 = 1
(100)
This shows that 1198751015840119894119910lowast = 1 and 1198751015840
119895119910lowast= 0 for every 119895 = 119894
So 119910lowast = 1198761015840
1198941198751015840
119894119910lowast and 1198751015840
119894119910lowast(119875
119894119910) = 1 This and the fact that
119910 minus 119876119894119875119894119910 lt 120576 imply that
(119876119894119875119894119910 + (
1
120576) (119910 minus 119876
119894119875119894119910) 119876
1015840
1198941198751015840
119894119910lowast) isin Π (119883) (101)
So we get that (1198761015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910 + (1120576)(119910 minus 119876
119894119875119894119910))V(119878) = 1
Hence we have
1 =1003816100381610038161003816119910
lowast1198781199101003816100381610038161003816 =10038161003816100381610038161003816(119876
1015840
1198941198751015840
119894119910lowast) 11987811991010038161003816100381610038161003816
=
1003816100381610038161003816100381610038161003816(1 minus 120576) (119876
1015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910)
+120576 (1198761015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910 +1
120576(119910 minus 119876
119894119875119894119910))
1003816100381610038161003816100381610038161003816⩽ 1
(102)
and so we get |1198751015840119894119910lowast(119875
119894∘119878 ∘119876
119894)119875
119894119910| = |(119876
1015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910)| = 1
We next show the case of ℓ1sum The proof is almost
the same as that of the 1198880case However for the sake of
completeness we provide it hereSince 1198751015840
119895119910lowast = 119875
1015840
119895119910lowastminus 119875
1015840
119895119909lowast ⩽ 119910
lowastminus 119909
lowast lt 120576 for every
119895 = 119894 we have
1 = 119910lowast(119910) = sum
119895isinN
1198751015840
119895119910lowast(119875
119895119910) ⩽ sum
119895isinN
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817
⩽10038171003817100381710038171198751198941199101003817100381710038171003817 + 120576 sum
119895isinN119895 = 119894
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817⩽10038171003817100381710038171199101003817100381710038171003817 = 1
(103)
which shows 119875119894119910 = 1 and 119875
119895119910 = 0 for every 119895 = 119894 Since this
implies (119876119894119875119894119910119876
1015840
1198941198751015840
119894119910lowast+(1120576)(119910
lowastminus119876
1015840
1198941198751015840
119894119910lowast)) isin Π(119883) we get
that |(1198761015840
1198941198751015840
119894119910lowast+ (1120576)(119910
lowastminus 119876
1015840
1198941198751015840
119894119910lowast))119878(119876
119894119875119894119910)| ⩽ V(119878) = 1
Hence we have
1 =1003816100381610038161003816119910
lowast1198781199101003816100381610038161003816 =1003816100381610038161003816119910
lowast119878 (119876
119894119875119894119910)1003816100381610038161003816
=
1003816100381610038161003816100381610038161003816(1 minus 120576) (119876
1015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910)
+120576 (1198761015840
1198941198751015840
119894119910lowast+1
120576(119910
lowastminus 119876
1015840
1198941198751015840
119894119910lowast)) 119878 (119876
119894119875119894119910)
1003816100381610038161003816100381610038161003816⩽ 1
(104)
and so |1198751015840119894119910lowast(119875
119894∘ 119878 ∘ 119876
119894)119875
119894119910| = |(119876
1015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910)| = 1
Proof of Theorem 15 Note that 120578nu(1198711(120583)oplus1119883)(120576) rarr 0 as120576 rarr 0 Fix 0 lt 120576
0lt 1 and choose 0 lt 120576 lt 1 such that
6120576 + 120578nu(1198711(120583)oplus1119883)(120576) lt 1205760 Let 120578(1205760) = 120578nu(1198711(120583)oplus1119883)(120576)Suppose that 119879
0isin L(119871
1(120583) 119883) with 119879
0 = 1 and 119891
0isin
1198781198711(120583)
satisfy
1003817100381710038171003817119879011989101003817100381710038171003817 gt 1 minus 120578 (1205760) (105)
For any measurable subset 119861 let
1198711(120583|
119861) = 119891|
119861 119891 isin 119871
1(120583) (106)
with the norm 119891|119861 = 119891120594
1198611 Then it is easy to see
that 1198711(120583|
119861) is isometrically isomorphic to a complemented
subspace of 1198711(120583) Let 119875
119861 119871
1(120583) rarr 119871
1(120583|
119861) be the
restriction defined by 119875119861(119891) = 119891|
119861for all 119891 isin 119871
1(120583) and
let 119869119861 119871
1(120583|
119861) rarr 119871
1(120583) be the extension defined by
119869119861(119891)(120596) = 119891(120596) if 120596 isin 119861 and 119869
119861(119891)(120596) = 0 otherwise
It is clear that 119875119861119869119861= Id
1198711(120583|119861)and 119869
119861119875119861(119891) = 119891120594
119861for all
119891 isin 1198711(120583) Notice also that 119871
1(120583) is isometrically isomorphic
to 1198711(120583|
119861)oplus
11198711(120583|
119861119888)
Let 119860 = supp1198910and 119892
0= 119875
1198601198910 Then
1003817100381710038171003817119879011986911986011989201003817100381710038171003817 =100381710038171003817100381711987901198910
1003817100381710038171003817 gt 1 minus 120578 (1205760) gt 0 (107)
and define the operator 119879119860 119871
1(120583|
119860) rarr 119883 by 119879
119860119891 =
1198790119869119860119891119879
0119869119860 for every 119891 isin 119871
1(120583|
119860) Then
100381710038171003817100381711987911986011989201003817100381710038171003817 ⩾100381710038171003817100381711987901198910
1003817100381710038171003817 gt 1 minus 120578 (1205760) (108)
Since 120583|119860is 120590-finite 119871
1(120583|
119860)lowast= 119871
infin(120583|
119860) Let 119892lowast
0isin 119878
119871infin(120583|119860)
be a function such that ⟨119892lowast0 119892
0⟩ = 1 and choose 119909lowast
0isin 119878
119883lowast
such that 119909lowast0(119879
1198601198920) = 119879
1198601198920 Define the operator 119878
0isin
L(1198711(120583|
119860)oplus
1119883) by
1198780(119891 119909) = (0 119879
119860119891) ((119891 119909) isin 119871
1(120583|
119860) oplus
1119883) (109)
and observe that 1198780 = V(119878
0) = 1 Indeed
100381710038171003817100381711987801003817100381710038171003817 ⩽ 1 =
10038171003817100381710038171198791198601003817100381710038171003817
= sup 1003816100381610038161003816119909lowast1198791198601198911003816100381610038161003816 119909
lowastisin 119878
119883lowast 119891 isin 119878
1198711(120583|119860)
= sup 1003816100381610038161003816(119891lowast 119909
lowast) 119878
0(119891 119909)
1003816100381610038161003816
((119891lowast 119909
lowast) (119891 119909)) isin Π (119871
1(120583|
119860) oplus
1119883)
= V (1198780) ⩽100381710038171003817100381711987801003817100381710038171003817
(110)
Abstract and Applied Analysis 13
It is immediate that
(119892lowast
0 119909
lowast
0) 119878
0(119892
0 0) = 119909
lowast
0(119879
1198601198920) =10038171003817100381710038171198791198601198920
1003817100381710038171003817 gt 1 minus 120578 (1205760)
(111)
By Lemma 19 1198711(120583|
119860)oplus
1119883 has the BPBp-nu with the func-
tion 120578 Therefore there exist 1198781isin L(119871
1(120583|
119860)oplus
1119883) (119892
1 119909
1) isin
1198781198711(120583|119860)oplus1119883 and (119892lowast
1 119909
lowast
1) isin 119878
119871infin(120583|119860)oplusinfin119883lowast such that
1003817100381710038171003817(1198921 1199091) minus (1198920 0)1003817100381710038171003817 lt 120576
1003817100381710038171003817(119892lowast
1 119909
lowast
1) minus (119891
lowast
0 119909
lowast
0)1003817100381710038171003817 lt 120576
10038171003817100381710038171198781 minus 11987801003817100381710038171003817 lt 120576 ⟨(119892
lowast
1 119909
lowast
1) (119892
1 119909
1)⟩ = 1
1003816100381610038161003816(119892lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)1003816100381610038161003816 = V (119878
1) = 1
(112)
Claim 1 We claim that 1199091= 0
Otherwise
1 = Re ⟨(119892lowast1 119909
lowast
1) (119892
1 119909
1)⟩
=100381710038171003817100381711989211003817100381710038171003817Re⟨(119892
lowast
1 119909
lowast
1) (119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
⟩
+100381710038171003817100381711990911003817100381710038171003817Re⟨(119892
lowast
1 119909
lowast
1) (0 119909
1)
100381710038171003817100381711990911003817100381710038171003817
⟩ ⩽ 1
(113)
We deduce that
((119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
(119892lowast
1 119909
lowast
1)) (
(0 1199091)
100381710038171003817100381711990911003817100381710038171003817
(119892lowast
1 119909
lowast
1))
isin Π (1198711(120583|
119860) oplus
1119883)
(114)
Since 1198781((0 119909
1)119909
1) = (119878
1minus 119878
0)((0 119909
1)119909
1) lt 120576 we
get that
1 =1003816100381610038161003816⟨(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)⟩1003816100381610038161003816
=
100381610038161003816100381610038161003816100381610038161003816
100381710038171003817100381711989211003817100381710038171003817⟨(119892
lowast
1 119909
lowast
1) 119878
1((119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
)⟩
+100381710038171003817100381711990911003817100381710038171003817⟨(119892
lowast
1 119909
lowast
1) 119878
1((0 119909
1)
100381710038171003817100381711990911003817100381710038171003817
)⟩
100381610038161003816100381610038161003816100381610038161003816
⩽100381710038171003817100381711989211003817100381710038171003817 V (1198781) + 120576
100381710038171003817100381711990911003817100381710038171003817 lt100381710038171003817100381711989211003817100381710038171003817 +100381710038171003817100381711990911003817100381710038171003817 = 1
(115)
a contradiction This proves the claimWe define the operator 119878
2 119871
1(120583|
119860)oplus
1119883 rarr 119871
1(120583|
119860)oplus
1119883
by 1198782(119891 119909) = 119878
1(119891 0) for every 119891 isin 119871
1(120583|
119860) and for every
119909 isin 119883 Then we have
V (1198782) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816 = 1
10038171003817100381710038171198781 minus 11987821003817100381710038171003817 ⩽ 120576 (116)
Indeed from Claim 1 we have
V (1198781) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)1003816100381610038161003816 =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816 ⩽ V (119878
2)
(117)
On the other hand we see that1003816100381610038161003816(119891
lowast 119909
lowast) 119878
2(119891 119909)
1003816100381610038161003816
=1003816100381610038161003816(119891
lowast 119909
lowast) 119878
1(119891 0)
1003816100381610038161003816 ⩽10038171003817100381710038171198911003817100381710038171003817 V (1198781) ⩽ V (119878
1)
(118)
for every ((119891lowast 119909lowast) (119891 119909)) isin Π(1198711(120583|
119860)oplus
1119883) Therefore
V(1198782) ⩽ V(119878
1) Also
10038171003817100381710038171198781 minus 11987821003817100381710038171003817 ⩽ sup
119909isin119878119883
10038171003817100381710038171198781 (0 119909)1003817100381710038171003817
= sup119909isin119878119883
10038171003817100381710038171198781 (0 119909) minus 1198780 (0 119909)1003817100381710038171003817 ⩽ 120576
(119)
Claim 2 There exists 1198783 119871
1(120583|
119860)oplus
1119883 rarr 119871
1(120583|
119860)oplus
1119883 such
that 1198783(119892
1 0) = 119878
3 = 1 119878
3(0 119909) = 0 119878
3(119891 119909) isin 0oplus
1119883
for every (119891 119909) isin 1198711(120583|
119860)oplus
1119883 and 119878
3minus 119878
2 lt 4120576
Indeed write 1198781= (119863
1 119863
2) where 119863
1 119871
1(120583|
119860)oplus
1119883 rarr
1198711(120583|
119860) and119863
2 119871
1(120583|
119860)oplus
1119883 rarr 119883 We have that
sup 1003816100381610038161003816119892lowast119863
1(119892
1 0) + 119909
lowast119863
2(119892
1 0)1003816100381610038161003816
119909lowastisin 119878
119883lowast ⟨119892
lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860)
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816 +10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
⟨119892lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860)
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816
⟨119892lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860) +10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
⩽ V (1198782) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816119892
lowast
1119863
1(119892
1 0) + 119909
lowast
1119863
2(119892
1 0)1003816100381610038161003816
(120)
This implies that
1003816100381610038161003816119909lowast
1119863
2(119892
1 0)1003816100381610038161003816 =10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
1003816100381610038161003816119892lowast
1119863
1(119892
1 0)1003816100381610038161003816
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816 ⟨119892
lowast 119892
1⟩ = 1 119892
lowastisin 119871
infin(120583|
119860)
(121)
Therefore |119892lowast1| equals 1 on the support of 119863
1(119892
1 0) As
|⟨119892lowast
1 119892
1⟩| = 1 we also have that |119892lowast
1| equals 1 on the support
of 1198921 Changing the values of 119892lowast
1by the ones of 119891lowast
0on 119860
(supp(1198631(119892
1 0)) cup supp(g
1)) we may and do suppose that
|119892lowast
1| = 1 on the whole 119860We also have 119863
2(119892
1 0) gt 0 Indeed
10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198920 0)1003817100381710038171003817
⩽10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198921 0)
1003817100381710038171003817 +10038171003817100381710038171198780 (1198921 0) minus 1198780 (1198920 0)
1003817100381710038171003817
lt 2120576 + 120576 = 3120576
(122)
So we have
10038171003817100381710038171198632(119892
1 0) minus 119879
1198601198920
1003817100381710038171003817
⩽10038171003817100381710038171198631(119892
1 0)1003817100381710038171003817 +10038171003817100381710038171198632(119892
1 0) minus 119879
1198601198920
1003817100381710038171003817
14 Abstract and Applied Analysis
=1003817100381710038171003817(1198631
(1198921 0) 119863
2(119892
1 0)) minus (0 119879
1198601198920)1003817100381710038171003817
=10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198920 0)
1003817100381710038171003817 lt 3120576
(123)
and 1198632(119892
1 0) gt 119879
1198601198920 minus 3120576 ⩾ 1 minus 120578(120576
0) minus 3120576 gt 0
Finally define the operator 1198783by
1198783(119891 119909) = (0119863
2(119891 0) + 119892
lowast
1(119863
1(119891 0))
1198632(119892
1 0)
119909lowast
1119863
2(119892
1 0))
(124)
for (119891 119909) isin 1198711(120583|
119860)oplus
1119883 It is clear that
100381710038171003817100381711987831003817100381710038171003817 ⩽ sup
119891isin1198781198711(120583|119860)
(10038171003817100381710038171198632(119891 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119891 0)
1003816100381610038161003816) (125)
Notice also that
10038171003817100381710038171198631(119891 0)
1003817100381710038171003817
⩽10038171003817100381710038171198631(119891 0)
1003817100381710038171003817 +10038171003817100381710038171198632(119891 0) minus 119879
1198601198911003817100381710038171003817
=1003817100381710038171003817(1198631
(119891 0) 1198632(119891 0)) minus (0 119879
119860119891)1003817100381710038171003817
=10038171003817100381710038171198782 (119891 119909) minus 1198780 (119891 119909)
1003817100381710038171003817
(126)
for all (119891 119909) isin 1198711(120583|
119860)oplus
1119883 Hence we have
10038171003817100381710038171198783 minus 11987821003817100381710038171003817
= 2 sup119891isin1198781198711(120583|119860)
10038171003817100381710038171198631(119891 0)
1003817100381710038171003817 ⩽ 210038171003817100381710038171198782 minus 1198780
1003817100381710038171003817 lt 4120576(127)
On the other hand let 119866 1198711(120583|
119860) rarr 119871
1(120583|
119860) be defined by
119866(119891) = 119892lowast
1119891 for every 119891 isin 119871
1(120583|
119860) Then we have
V (1198782)
= sup 1003816100381610038161003816119911lowast11987821199111003816100381610038161003816 (119911 119911
lowast) isin Π (119871
1(120583|
119860) oplus
1119883)
⩾ sup10038161003816100381610038161003816100381610038161003816
119909lowast119863
2(119866(
1
120583 (119862)120594119862) 0)
+119892lowast
1119863
1(119866(
1
120583 (119862)120594119862) 0)
10038161003816100381610038161003816100381610038161003816
119909lowastisin 119878
119883lowast 119862 isin Σ
119860 120583 (119862) gt 0
= sup10038171003817100381710038171003817100381710038171003817
1198632(119866(
1
120583 (119862)120594119862) 0)
10038171003817100381710038171003817100381710038171003817
+
10038161003816100381610038161003816100381610038161003816
119892lowast
1119863
1(119866(
1
120583 (119862)120594119862) 0)
10038161003816100381610038161003816100381610038161003816
119862 isin Σ119860 120583 (119862) gt 0
(128)
where Σ119860is the family of measurable subsets of 119860
Hence for any simple function 119904 = sum119899
119894=1((120572
119894120583(119860
119894))120594
119860119894
) isin
1198781198711(120583|119860) where 119860
119894119894is a family of disjoint measurable subsets
with strictly positive measure we have
V (1198782)
⩾
119899
sum
119894=1
10038161003816100381610038161205721198941003816100381610038161003816 (
100381710038171003817100381710038171003817100381710038171003817
1198632(119866(
1
120583 (119860119894)120594119860119894
) 0)
100381710038171003817100381710038171003817100381710038171003817
+
100381610038161003816100381610038161003816100381610038161003816
119892lowast
1119863
1(119866(
1
120583 (119860119894)120594119860119894
) 0)
100381610038161003816100381610038161003816100381610038161003816
)
⩾10038171003817100381710038171198632(119866 (119904) 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119866 (119904) 0)
1003816100381610038161003816
(129)
Since |119892lowast1| = 1119866 is an isometric isomorphism so for each119891 isin
1198781198711(120583|119860)there exists a sequence of norm-one simple functions
(119904119896) such that 119866(119904
119896) converges to 119891 Therefore
V (1198782) ⩾ sup
119891isin1198781198711(120583|119860)
(10038171003817100381710038171198632(119891 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119891 0)
1003816100381610038161003816) ⩾100381710038171003817100381711987831003817100381710038171003817
(130)
On the other hand we see that100381710038171003817100381711987831003817100381710038171003817 ⩾1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
3(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816119909
lowast
1119863
2(119892
1 0) + 119892
lowast
1119863
1(119892
1 0)1003816100381610038161003816 = V (119878
2) = 1
(131)
Therefore 1 = 1198783 = 119878
3(119892
1 0) which proves Claim 2
Finally set 1198783= (0 ) for a suitable 119871
1(120583|
119860)oplus
1119883 rarr
119883 and define the operator 1198791 119871
1(120583) rarr 119883 by
1198791(119891) = 119879
0(119891120594
119860119888) + (119875
119860119891 0) (132)
for every 119891 isin 1198711(120583) Then we have
10038171003817100381710038171198791 (119891)1003817100381710038171003817 ⩽100381710038171003817100381711987901003817100381710038171003817
10038171003817100381710038171198911205941198601198881003817100381710038171003817 +1003817100381710038171003817100381710038171003817100381710038171003817
10038171003817100381710038171198911205941198601003817100381710038171003817 =10038171003817100381710038171198911003817100381710038171003817 (133)
for every 119891 isin 1198711(120583) so 119879
1 ⩽ 1 Also
10038171003817100381710038171198791 (1198691198601198921)1003817100381710038171003817 =10038171003817100381710038171198783 (1198921 0)
1003817100381710038171003817 =100381710038171003817100381711987831003817100381710038171003817 = 1 (134)
so 1198791attains its norm at 119869
1198601198921isin 119871
1(120583) and
10038171003817100381710038171198691198601198921 minus 11989101003817100381710038171003817 =10038171003817100381710038171198921 minus 1198920
1003817100381710038171003817 lt 120576 (135)
We also have that for any 119891 isin 1198781198711(120583)
10038171003817100381710038171198790 (119891) minus 1198791 (119891)1003817100381710038171003817 =100381710038171003817100381710038171198790(119891120594
119860) minus (119875
119860119891 0)10038171003817100381710038171003817
⩽10038171003817100381710038171198790 (119869119860119875119860119891) minus 119879119860 (119875119860119891)
1003817100381710038171003817
+10038171003817100381710038171003817119879119860(119875
119860119891) minus (119875
119860119891 0)10038171003817100381710038171003817
⩽10038171003817100381710038171198790119869119860 minus 119879119860
1003817100381710038171003817 +10038171003817100381710038171198780 minus 1198783
1003817100381710038171003817
lt 120578 (1205760) + 6120576
(136)
Hence 1198790minus 119879
1 ⩽ 120578(120576
0) + 6120576 lt 120576
0
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Abstract and Applied Analysis 15
Acknowledgments
The authors thank Gilles Godefroy and Rafael Paya forfruitful conversations about the content of this paper andVladimir Kadets for providing Lemma 18 They also appre-ciate anonymous referees for careful reading and fruitfulsuggestions about revision The second author was sup-ported by Basic Science Research Program through theNational Research Foundation of Korea (NRF) fundedby the Ministry of Education Science and Technology(NRF-2012R1A1A1006869) The third author was partiallysupported by Spanish MICINN and FEDER project noMTM2012-31755 and by Junta de Andalucıa and FEDERGrants FQM-185 and P09-FQM-4911
References
[1] F F Bonsall and J Duncan Numerical Ranges of Operators onNormed Spaces and of Elements of Normed Algebras vol 2 ofLondon Mathematical Society Lecture Note Series 2 CambridgeUniversity Press London UK 1971
[2] F F Bonsall and J Duncan Numerical Ranges II CambridgeUniversity Press London UK 1973 London MathematicalSociety Lecture Notes Series 10
[3] B Sims On numerical range and its applications to Banachalgebras [PhD thesis] University of Newcastle New SouthWales Australia 1972
[4] I D Berg and B Sims ldquoDenseness of operators which attaintheir numerical radiusrdquoAustralian Mathematical Society A vol36 no 1 pp 130ndash133 1984
[5] C S Cardassi ldquoDensity of numerical radius attaining operatorson some reflexive spacesrdquo Bulletin of the Australian Mathemat-ical Society vol 31 no 1 pp 1ndash3 1985
[6] C S Cardassi ldquoNumerical radius attaining operatorsrdquo inBanach Spaces (Columbia Mo 1984) vol 1166 of Lecture Notesin Mathematics pp 11ndash14 Springer Berlin Germany 1985
[7] C S Cardassi ldquoNumerical radius-attaining operators on119862(119870)rdquoProceedings of the AmericanMathematical Society vol 95 no 4pp 537ndash543 1985
[8] MDAcostaOperadores que alcanzan su radio numerico [PhDthesis] Universidad de Granada Granada Spain 1990
[9] M D Acosta and R Paya ldquoNumerical radius attaining oper-ators and the Radon-Nikodym propertyrdquo The Bulletin of theLondon Mathematical Society vol 25 no 1 pp 67ndash73 1993
[10] R Paya ldquoA counterexample on numerical radius attainingoperatorsrdquo Israel Journal of Mathematics vol 79 no 1 pp 83ndash101 1992
[11] Y S Choi and S G Kim ldquoNorm or numerical radius attainingmultilinear mappings and polynomialsrdquo Journal of the LondonMathematical Society vol 54 no 1 pp 135ndash147 1996
[12] M D Acosta J Becerra Guerrero and M Ruiz GalanldquoNumerical-radius-attaining polynomialsrdquo The Quarterly Jour-nal of Mathematics vol 54 no 1 pp 1ndash10 2003
[13] M D Acosta and S G Kim ldquoDenseness of holomorphicfunctions attaining their numerical radiirdquo Israel Journal ofMathematics vol 161 pp 373ndash386 2007
[14] S G Kim and H J Lee ldquoNumerical peak holomorphicfunctions on Banach spacesrdquo Journal of Mathematical Analysisand Applications vol 364 no 2 pp 437ndash452 2010
[15] M D Acosta R M Aron D Garcıa and M MaestreldquoThe Bishop-Phelps-Bollobas theorem for operatorsrdquo Journal ofFunctional Analysis vol 254 no 11 pp 2780ndash2799 2008
[16] A J Guirao and O Kozhushkina ldquoThe Bishop-Phelps-Bollobasproperty for numerical radius in ℓ
1(119862)rdquo Studia Mathematica
vol 218 no 1 pp 41ndash54 2013[17] J Falco ldquoThe Bishop-Phelps-Bollobas property for numerical
radius on 1198711rdquo Journal of Mathematical Analysis and Applica-
tions vol 414 no 1 pp 125ndash133 2014[18] A Aviles A J Guirao and J Rodriguez ldquoOn the Bishop-
Phelps-Bollobas property for numerical radius in119862(119870)-spacesrdquoPreprint
[19] RM Aron Y S Choi S K Kim H J Lee andMMartin ldquoTheBishop-Phelps-Bollobas version of Lindenstrauss properties Aand Brdquo Preprint
[20] V Kadets M Martın and R Paya ldquoRecent progress and openquestions on the numerical index of Banach spacesrdquo Revista dela Real Academia de Ciencias Exactas Fısicas y Naturales A vol100 no 1-2 pp 155ndash182 2006
[21] M Martın J Merı and M Popov ldquoOn the numerical index ofreal 119871
119901(120583)-spacesrdquo Israel Journal of Mathematics vol 184 pp
183ndash192 2011[22] A Iwanik ldquoNorm attaining operators on Lebesgue spacesrdquo
Pacific Journal of Mathematics vol 83 no 2 pp 381ndash386 1979[23] HH SchaeferBanach Lattices and Positive Operators Springer
New York NY USA 1974[24] J Duncan C M McGregor J D Pryce and A J White ldquoThe
numerical index of a normed spacerdquo Journal of the LondonMathematical Society Second Series vol 2 pp 481ndash488 1970
[25] Y S Choi S K Kim H J Lee and M Martin ldquoThe Bishop-Phelps-Bollobas theorem for operators on 119871
1(120583)rdquo Preprint
[26] H E Lacey The Isometric Theory of Classical Banach SpacesSpringer New York NY USA 1974
[27] G Godefroy ldquoUniversal spaces for strictly convex Banachspacesrdquo Revista de la Real Academia de Ciencias Exactas Fısicasy Naturales A vol 100 no 1-2 pp 137ndash146 2006
[28] C Finet M Martın and R Paya ldquoNumerical index andrenormingrdquo Proceedings of the American Mathematical Societyvol 131 no 3 pp 871ndash877 2003
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical Problems in Engineering
Hindawi Publishing Corporationhttpwwwhindawicom
Differential EquationsInternational Journal of
Volume 2014
Applied MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Probability and StatisticsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical PhysicsAdvances in
Complex AnalysisJournal of
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OptimizationJournal of
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CombinatoricsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
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Operations ResearchAdvances in
Journal of
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Function Spaces
Abstract and Applied AnalysisHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of Mathematics and Mathematical Sciences
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
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Algebra
Discrete Dynamics in Nature and Society
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Decision SciencesAdvances in
Discrete MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom
Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of
6 Abstract and Applied Analysis
that for a convex series sum120572119899 Resuminfin
119899=1120572119899119888119899gt 1 minus 120578 Then for
every 0 lt 119903 lt 1 the set 119860 = 119894 isin N Re 119888119894gt 119903 satisfies the
estimate
sum
119894isin119860
120572119894⩾ 1 minus
120578
1 minus 119903 (39)
From now on 119898 will be a finite regular positive Borelmeasure on the compact Hausdorff space Ω
Lemma 12 Suppose that there exist a nonnegative simplefunction 119891 isin 119878
1198711(119898)
and a function 119892 isin 119878119871infin(119898)
such that
Re ⟨119891 119892⟩ gt 1 minus 1205763
16 (40)
Then there exist a nonnegative simple function 1198911isin 119878
1198711(119898)
anda function 119892
1isin 119878
119871infin(119898)
such that
1198921(119909) = 120594supp(119891
1)(119909) + 119892 (119909) 120594
Ωsupp(1198911)(119909)
⟨1198911 119892
1⟩ = 1
1003817100381710038171003817119891 minus 119891110038171003817100381710038171lt 120576
1003817100381710038171003817119892 minus 11989211003817100381710038171003817infinlt radic120576 supp (119891
1) sub supp (119891)
(41)
Proof Let119891 = sum119898
119895=1(120573
119895119898(119861
119895))120594
119861119895
for some (120573119895) such that
120573119895⩾ 0 for all 119895 andsum119898
119895=1120573119895= 1 and119861
119895rsquos aremutually disjoint
By the assumption we have
Re ⟨119891 119892⟩ =119899
sum
119895=1
120573119895
1
119898 (119861119895)
int119861119895
Re119892 (119909) 119889119898 (119909) gt 1 minus 1205763
16
(42)
and letting
119869 = 119895 1 ⩽ 119895 ⩽ 1198991
119898 (119861119895)
int119861119895
Re119892 (119909) 119889119898 (119909) gt 1 minus 1205762
4
(43)
we have by Lemma 11
sum
119895isin119869
120573119895gt 1 minus
120576
4 (44)
For each 119895 isin 119869 we have
1 minus1205762
4lt
1
119898 (119861119895)
int119861119895
Re119892 (119909) 119889119898 (119909)
=1
119898 (119861119895)
int119861119895capRe119892⩽1minus120576
Re119892 (119909) 119889119898 (119909)
+ int119861119895capRe119892gt1minus120576
Re119892 (119909) 119889119898 (119909)
⩽1
119898 (119861119895)
((1 minus 120576)119898 (119861119895cap Re119892 ⩽ 1 minus 120576)
+ 119898 (119861119895cap Re119892 gt 1 minus 120576))
= 1 minus 120576
119898 (119861119895cap Re119892 ⩽ 1 minus 120576)
119898 (119861119895)
(45)
This implies that
119898(119861119895cap Re119892 ⩽ 1 minus 120576)
119898 (119861119895)
lt120576
4 (46)
Define 119861119895= 119861
119895cap Re119892 gt 1 minus 120576 for all 119895 isin 119869
1198911= (1sum
119895isin119869
120573119895)sum
119895isin119869
120573119895(120594
119861119895
119898 (119861119895)) (47)
and 1198921(119909) = 1 on supp(119891
1) and 119892
1(119909) = 119892(119909) elsewhereThen
it is clear that supp(1198911) sub supp(119891) 119892 minus 119892
1infinlt radic120576 and
⟨1198911 119892
1⟩ = 1 Finally we will show that 119891 minus 119891
1 lt 120576 Notice
first that10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
⩽
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
= 2sum
119895isin119869
120573119895
119898(119861119895 119861
119895)
119898 (119861119895)
lt120576
2
(48)
Hence1003817100381710038171003817119891 minus 1198911
1003817100381710038171003817
⩽
10038171003817100381710038171003817100381710038171003817100381710038171003817
1
sum119895isin119869120573119895
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus 119891
10038171003817100381710038171003817100381710038171003817100381710038171003817
⩽
1 minus sum119895isin119869120573119895
sum119895isin119869120573119895
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
minus sum
119895isin119869
120573119895
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+120576
4
= (1 minussum
119895isin119869
120573119895) +
120576
2+120576
4
⩽120576
4+120576
2+120576
4= 120576
(49)
Lemma 13 (see [25 Lemma 33]) Suppose that 119879120583is a norm-
one element in L(1198711(119898)) for some 120583 isin 119872(119898) and there is
a nonnegative simple function 1198910such that 119891
0is a norm-one
element of 1198711(119898) and 119879
1205831198910 ⩾ 1 minus 120576
32
6 for some 0 lt 120576 lt 1
Abstract and Applied Analysis 7
Then there exist a norm-one bounded linear operator 119879] forsome ] isin 119872(119898119898) and a nonnegative simple function 119891
1
in 1198781198711(119898)
such that 119879120583minus 119879] ⩽ 120576 1198911 minus 1198910 ⩽ 3120576 and
(119889|]|1119889119898)(119909) = 1 for all 119909 isin supp(1198911)
Lemma 14 Suppose that 119879] isin L(1198711(119898)) is a norm-one
operator 119891 = sum119899
119894=1120573119894(120594
119861119894
119898(119861119894)) where 119898(119861
119895) gt 0 for all
1 ⩽ 119895 ⩽ 119899 and 119861119895119899
119895=1are mutually disjoint Borel subsets ofΩ
is a norm-one nonnegative simple function and 119892 is an elementof 119878
119871infin(119898)
such that
Re ⟨119892 119879]119891⟩ ⩾ 1 minus1205766
27(50)
for some 0 lt 120576 lt 1 and
119889|]|1
119889119898(119909) = 1 119892 (119909) = 1 (51)
for all x in the support of 119891Then there exist a nonnegative simple function 119891 isin 119878
1198711(119898)
a function119892 isin 119878
119871infin(119898)
and an operator119879] inL(1198711(119898) 1198711(119898))such that
⟨119892 119879]119891⟩ =1003817100381710038171003817119879]1003817100381710038171003817 = 1
1003817100381710038171003817119879] minus 119879]1003817100381710038171003817 ⩽ 2120576
10038171003817100381710038171003817119891 minus 119891
10038171003817100381710038171003817⩽ 3120576
1003817100381710038171003817119892 minus 1198921003817100381710038171003817 ⩽radic120576 ⟨119891 119892⟩ = 1
(52)
Proof Since
Re ⟨119892 119879]119891⟩ ⩾ 1 minus1205766
27 (53)
we have
1 minus1205766
27lt Re ⟨119892 119879]119891⟩
= intΩtimesΩ
119891 (119909)Re119892 (119910) 119889120583 (119909 119910)
=
119899
sum
119895=1
120573119895intΩtimesΩ
120594119861119895
(119909)
119898 (119861119895)
Re119892 (119910) 119889] (119909 119910)
(54)
Let
119869 = 119895 intΩtimesΩ
(
120594119861119895
(119909)
119898 (119861119895)
)Re119892 (119910) 119889] (119909 119910) gt 1 minus 1205763
26
(55)
Then from Lemma 11 we have sum119895isin119869120573119895gt 1 minus 120576
32 Let 119891
1=
sum119895isin119869120573119895(120594
119861119895
119898(119861119895)) where 120573
119895= 120573
119895(sum
119895isin119869120573119895) for all 119895 isin 119869
Then
10038171003817100381710038171198911 minus 1198911003817100381710038171003817 ⩽
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
(120573119895minus 120573
119895)
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+ sum
119895isin119869
120573119895⩽ 120576
3⩽ 120576 (56)
Note that there is a Borel measurable function ℎ on Ω times Ωsuch that 119889](119909 119910) = ℎ(119909 119910) 119889|]|(119909 119910) and |ℎ(119909 119910)| = 1 forall (119909 119910) isin Ω times Ω Let
119862 = (119909 119910) 1003816100381610038161003816119892 (119910) ℎ (119909 119910) minus 1
1003816100381610038161003816 ltradic120576
232 (57)
Define two measures ]119891and ]
119888as follows
]119891(119860) = ] (119860 119862) ]
119888(119860) = ] (119860 cap 119862) (58)
for every Borel subset 119860 ofΩ times Ω It is clear that
119889] = 119889]119891+ 119889]
119888 119889
10038161003816100381610038161003816]119891
10038161003816100381610038161003816= ℎ 119889]
119891
1198891003816100381610038161003816]1198881003816100381610038161003816 = ℎ 119889]119888 119889 |]| = 119889
10038161003816100381610038161003816]119891
10038161003816100381610038161003816+ 1198891003816100381610038161003816]1198881003816100381610038161003816
(59)
Since (119889|]|11198891198981)(119909) = 1 for all 119909 isin ⋃119899
119895=1119861119895 we have
1 =119889|]|1
1198891198981
(119909) =
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909) +1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909) (60)
for all 119909 isin 119861 = ⋃119899
119895=1119861119895 and we deduce that |]|1(119861
119895) = 119898
1(119861
119895)
for all 1 ⩽ 119895 ⩽ 119899We claim that |]
119891|1(119861
119895)119898
1(119861
119895) ⩽ 120576
22
2 for all 119895 isin 119869Indeed if |119892(119910)ℎ(119909 119910) minus 1| ⩾ radic120576232 then
Re (119892 (119910) ℎ (119909 119910)) ⩽ 1 minus 12057624 (61)
So we have
1 minus1205763
26
⩽1
1198981(119861
119895)
ReintΩtimesΩ
120594119861119895(119909)119892 (119910) 119889] (119909 119910)
=1
1198981(119861
119895)
intΩtimesΩ
120594119861119895(119909)
Re (119892 (119910) ℎ (119909 119910)) 119889 |]| (119909 119910)
=1
1198981(119861
119895)
intΩtimesΩ
120594119861119895(119909)
Re (119892 (119910) ℎ (119909 119910)) 119889 10038161003816100381610038161003816]11989110038161003816100381610038161003816(119909 119910)
+1
1198981(119861
119895)
intΩtimesΩ
120594119861119895(119909)
Re (119892 (119910) ℎ (119909 119910)) 119889 1003816100381610038161003816]1198881003816100381610038161003816 (119909 119910)
⩽1
1198981(119861
119895)
((1 minus120576
24)10038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
(119861119895) +1003816100381610038161003816]1198881003816100381610038161003816
1
(119861119895))
= 1 minus120576
24
10038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
(119861119895)
1198981(119861
119895)
(62)
This proves our claimWe also claim that for each 119895 isin 119869 there exists a Borel
subset 119861119895of 119861
119895such that
(1 minus120576
2)119898
1(119861
119895) ⩽ 119898
1(119861
119895) ⩽ 119898
1(119861
119895)
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909) ⩽120576
2
(63)
8 Abstract and Applied Analysis
for all 119909 isin 119861119895 Indeed set 119861
119895= 119861
119895cap 119909 isin Ω (119889|]
119891|1119889119898
1)
(119909) ⩽ 1205762 Then
int119861119895119861119895
120576
2119889119898
1(119909) ⩽ int
119861119895
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909) 1198891198981(119909)
=10038161003816100381610038161003816]1119891
10038161003816100381610038161003816(119861
119895) ⩽1205762
22119898
1(119861
119895)
(64)
This shows that 1198981(119861
119895 119861
119895) ⩽ (1205762)119898
1(119861
119895) This proves our
second claimNow we define 119892 by 119892(119910) = 119892(119910)|119892(119910)| if |119892(119910)| ⩾ 1 minus
radic120576232 and 119892(119910) = 119892(119910) if |119892(119910)| lt 1 minus radic120576232 and we
write 119891 = sum119895isin119869120573119895(120594
119861119895
1198981(119861
119895)) It is clear that 119892 isin 119878
119871infin(119898)
119892 minus 119892 lt radic120576 and 119892(119910) = 1 for all 119909 isin supp119891
Finally we define the measure
119889] (119909 119910)
= sum
119895isin119869
120594119861119895
(119909) 119892 (119910)ℎ (119909 119910)119889]119888(119909 119910)(
1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+ 1205941198691119861(119909) 119889] (119909 119910)
(65)
where 119861 = ⋃119895isin119869119861119895 It is easy to see that (119889|]|1119889119898
1)(119909) = 1
on 119861 and (119889|]|11198891198981)(119909) ⩽ 1 elsewhere Note that
119889 (] minus ]) (119909 119910)
= sum
119895isin119869
120594119861119895
(119909) [
[
119892 (119910)ℎ (119909 119910)(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1]
]
119889]119888(119909 119910)
minus sum
119895isin119869
120594119861119895
(119909) 119889]119891(119909 119910)
(66)
If (119909 119910) isin 119862 then |119892(119910)| ⩾ 1 minus radic120576232 ⩾ 1 minus 1232 and
100381610038161003816100381610038161003816119892 (119910)ℎ (119909 119910) minus 1
100381610038161003816100381610038161003816
=
100381610038161003816100381610038161003816100381610038161003816
119892 (119910)
1003816100381610038161003816119892 (119910)1003816100381610038161003816
ℎ (119909 119910) minus 1
100381610038161003816100381610038161003816100381610038161003816
⩽
1003816100381610038161003816119892 (119910) ℎ (119909 119910) minus 11003816100381610038161003816
1003816100381610038161003816119892 (119910)1003816100381610038161003816
+
10038161003816100381610038161 minus1003816100381610038161003816119892 (119910)
1003816100381610038161003816
10038161003816100381610038161003816100381610038161003816119892 (119910)
1003816100381610038161003816
⩽ 2
1003816100381610038161003816119892 (119910) ℎ (119909 119910) minus 11003816100381610038161003816
1003816100381610038161003816119892 (119910)1003816100381610038161003816
⩽ 2radic120576
232
232
232 minus 1⩽ 2radic120576
(67)
Hence for all (119909 119910) isin 119862 we have
10038161003816100381610038161003816100381610038161003816100381610038161003816
119892 (119910)ℎ (119909 119910)(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
⩽100381610038161003816100381610038161003816119892 (119910)ℎ (119909 119910) minus 1
100381610038161003816100381610038161003816(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+
10038161003816100381610038161003816100381610038161003816100381610038161003816
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
⩽ 2radic120576(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+
10038161003816100381610038161003816100381610038161003816100381610038161003816
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
(68)
So we have for all 119909 isin 1198691
119889|] minus ]|1
1198891198981
(119909)
⩽ sum
119895isin119869
120594119861119895
(119909) [
[
2radic120576(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+
10038161003816100381610038161003816100381610038161003816100381610038161003816
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
]
]
1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909)
+ sum
119895isin119869
120594119861119895
(119909)
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909)
⩽ sum
119895isin119869
120594119861119895
(119909)(2radic120576 + (1 minus1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909)))
+sum
119895isin119869
120594119861119895
(119909)(
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909))
⩽ 2radic120576 + 120576 lt 3radic120576
(69)
This gives that 119879] minus 119879] lt 3radic120576 Note also that for all 119895 isin 119869
⟨119879]
120594119861119895
1198981(119861
119895)
119892⟩
= intΩtimesΩ
120594119861119895
(119909)
1198981(119861
119895)
119892 (119910) 119889] (119909 119910)
= intΩtimesΩ
120594119861119895
(119909)
1198981(119861
119895)
ℎ (119909 119910)(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
119889]119888(119909 119910)
Abstract and Applied Analysis 9
= intΩ
120594119861119895
(119909)
1198981(119861
119895)
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
1198891003816100381610038161003816]1198881003816100381610038161003816
1
(119909)
= intΩ
120594119861119895
(119909)
1198981(119861
119895)
1198891198981(119909) = 1
(70)
Hence we get ⟨119879]119891 119892⟩ = 1 which implies that 119879]119891 =119879] = 1 Finally10038171003817100381710038171003817119891 minus 119891
10038171003817100381710038171003817
⩽10038171003817100381710038171003817119891 minus 119891
1
10038171003817100381710038171003817+10038171003817100381710038171198911 minus 119891
1003817100381710038171003817
=
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
1198981(119861
119895)
minus sum
119895isin119869
120573119895
120594119861119895
1198981(119861
119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+ 120576
⩽ sum
119895isin119869
120573119895(
10038171003817100381710038171003817100381710038171003817100381710038171003817
120594119861119895
1198981(119861
119895)
minus
120594119861119895
1198981(119861
119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+
10038171003817100381710038171003817100381710038171003817100381710038171003817
120594119861119895
1198981(119861
119895)
minus
120594119861119895
1198981(119861
119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
) + 120576
= 2sum
119895isin119869
120573119895
1198981(119861
119895 119861
119895)
1198981(119861
119895)
+ 120576
⩽ 2sum
119895isin119869
120573119895
(1205762)1198981(119861
119895)
1198981(119861
119895)
+ 120576 ⩽120576
1 minus 1205762+ 120576 lt 3120576
(71)
We are now ready to present the proof of the main resultin the case of finite regular positive Borel measures
Proof of Proposition 10 Let 1205751= 120575
3
2(5sdot2
4) 120575
2= 120575
12
3(3
2sdot2
14)
and 1205753= (12057610)
2 for some 0 lt 120576 lt 1 Suppose that 119879 isinL(119871
1(119898)) with 119879 = 1 and that there is an 119891
0isin 119878
1198711(119898)
and1198920isin 119878
119871infin(119898)
such that ⟨1198910 119892
0⟩ = 1 and |⟨119879119891
0 119892
0⟩| gt 1minus120575
3
12
6Then there is an isometric isomorphism Ψ from 119871
1(119898) onto
itself such thatΨ(1198910) = |119891
0| and there is a scalar number 120572 in
119878R such that |⟨1198791198910 119892
0⟩| = ⟨120572119879119891
0 119892
0⟩ Then letting 119891
1= Ψ119891
0
1198921= (Ψ
minus1)lowast1198920 and 119879
1= 120572Ψ119879Ψ
minus1 we have
⟨1198781198911 119892
1⟩ = ⟨120572Ψ119879
0Ψ
minus1Ψ119891
0 (Ψ
minus1)lowast
1198920⟩
= ⟨1205721198791198910 119892
0⟩ gt 1 minus
1205753
1
26
⟨1198911 119892
1⟩ = ⟨Ψ119891
0 (Ψ
minus1)lowast
1198920⟩ = 1
(72)
Since 11987911198911 gt 1 minus 120575(120575
3
12
6) by Lemma 13 there exists a
norm-one bounded operator 119879] and a nonnegative simplefunction119891
2isin 119878
1198711(119898)
such that 1198791minus119879] ⩽ 1205751 1198912minus1198911 ⩽ 31205751
and (119889|]|11198891198981)(119909) = 1 for all 119909 isin supp(119891
2) Then
⟨119879]1198912 1198921⟩
= ⟨11987911198911 119892
1⟩ minus ⟨119879
11198911minus 119879
11198912 119892
1⟩ minus ⟨119879
11198912minus 119879]1198912 1198921⟩
⩾ ⟨11987911198911 119892
1⟩ minus10038171003817100381710038171198911 minus 1198912
1003817100381710038171003817 minus10038171003817100381710038171198791 minus 119879]
1003817100381710038171003817
⩾ 1 minus1205753
1
26minus 3120575
1minus 120575
1⩾ 1 minus 5120575
1= 1 minus
1205753
2
16
(73)
Notice also that
⟨1198912 119892
1⟩ = ⟨119891
1 119892
1⟩ minus ⟨119891
1minus 119891
2 119892
1⟩ ⩾ 1 minus
10038171003817100381710038171198911 minus 11989121003817100381710038171003817
⩾ 1 minus 31205751⩾ 1 minus 5120575
1= 1 minus
1205753
2
16
(74)
By Lemma 12 there are a nonnegative simple function 1198913isin
1198781198711(119898)
and a function 1198923isin 119878
119871infin(119898)
such that
1198923(119909) = 120594supp119891
3
(119909) + 1198922(119909) 120594
Ωsupp1198913
(119909)
10038171003817100381710038171198912 minus 11989131003817100381710038171003817 ⩽ 1205752
10038171003817100381710038171198923 minus 11989211003817100381710038171003817 ⩽radic120575
2 ⟨119891
3 119892
3⟩ = 1
(75)
So we have
⟨119879]1198913 1198923⟩ = ⟨119879]1198912 1198921⟩ minus ⟨119879]1198912 minus 119879]1198913 1198921⟩
minus ⟨119879]1198913 1198921 minus 1198923⟩
⩾ 1 minus1205753
2
16minus 2radic120575
2⩾ 1 minus 3radic120575
2= 1 minus
1205756
3
27
(76)
By Lemma 14 there exist 1198914isin 119878
1198711(119898)
and 1198924isin 119878
119871infin(119898)
and anoperator 119879
4such that
⟨1198924 119879
41198914⟩ = 1 =
100381710038171003817100381711987941003817100381710038171003817
10038171003817100381710038171198794 minus 119879]1003817100381710038171003817 ⩽ 21205753
10038171003817100381710038171198914 minus 11989131003817100381710038171003817 ⩽ 31205753
10038171003817100381710038171198924 minus 11989231003817100381710038171003817 ⩽radic120575
3 ⟨119891
4 119892
4⟩ = 1
(77)
So we have
10038171003817100381710038171198794 minus 11987911003817100381710038171003817 ⩽10038171003817100381710038171198794 minus 119879]
1003817100381710038171003817 +1003817100381710038171003817119879] minus 1198791
1003817100381710038171003817
⩽ 1205751+ 2120575
3⩽ 3120575
3
10038171003817100381710038171198911 minus 11989141003817100381710038171003817 ⩽10038171003817100381710038171198911 minus 1198912
1003817100381710038171003817 +10038171003817100381710038171198912 minus 1198913
1003817100381710038171003817 +10038171003817100381710038171198913 minus 1198914
1003817100381710038171003817
⩽ 31205751+ 120575
2+ 3120575
3⩽ 10120575
3
10038171003817100381710038171198921 minus 11989241003817100381710038171003817 ⩽10038171003817100381710038171198921 minus 1198923
1003817100381710038171003817 + 1198923 minus 1198924
⩽ 1205752+ radic120575
3⩽ 2radic120575
3
(78)
10 Abstract and Applied Analysis
Let 119878 = 120572Ψminus11198794Ψ 119891 = Ψminus1
1198914 and 119892 = Ψlowast
1198924 then we have
119879 minus 119878 =10038171003817100381710038171003817119879 minus 120572Ψ
minus11198794Ψ10038171003817100381710038171003817=10038171003817100381710038171003817120572Ψ119879Ψ
minus1minus 119879
4
10038171003817100381710038171003817
=10038171003817100381710038171198791 minus 1198794
1003817100381710038171003817 ⩽ 31205753
100381710038171003817100381710038171198910minus 11989110038171003817100381710038171003817=100381710038171003817100381710038171198910minus Ψ
minus11198914
10038171003817100381710038171003817=10038171003817100381710038171198911 minus 1198914
1003817100381710038171003817 ⩽ 101205753
100381710038171003817100381710038171198920minus 11989110038171003817100381710038171003817=10038171003817100381710038171198920 minus Ψ
lowast1198924
1003817100381710038171003817 =100381710038171003817100381710038171003817(Ψ
minus1)lowast
1198920minus 119892
4
100381710038171003817100381710038171003817
=10038171003817100381710038171198921 minus 1198924
1003817100381710038171003817 ⩽ 2radic120575
3
⟨119891 119892⟩ = ⟨Ψminus11198914 Ψ
lowast1198924⟩ = ⟨119891
4 119892
4⟩ = 1
10038161003816100381610038161003816⟨119878119891 119892⟩
10038161003816100381610038161003816=10038161003816100381610038161003816⟨120572Ψ
minus11198794ΨΨ
minus11198914 Ψ
lowast1198924⟩10038161003816100381610038161003816= |120572| = 1
(79)
This completes the proof
Finally we may give the proof of the main result in fullgenerality
Proof of Theorem 9 Notice that the Kakutani representationtheorem (see [26] for a reference) says that for every 120590-finitemeasure ] the space 119871
1(]) is isometrically isomorphic to
1198711(119898) for some positive Borel regular measure on a compact
Hausdorff space Then by Proposition 10 there is a universalfunction 120576 997891rarr 120578(120576) gt 0 which gives the BPBp-nu for 119871
1(]) for
every 120590-finite measure ]Fix 120576 gt 0 Suppose that 119879
0isin L(119871
1(120583)) with V(119879
0) = 1
and (1198910 119891
lowast
0) isin Π(119871
1(120583)) satisfy
1003816100381610038161003816⟨119891lowast
0 119879
01198910⟩1003816100381610038161003816 gt 1 minus 120578 (120576) (80)
Choose a sequence 119891119899 in 119871
1(120583) such that sup
119899119879
0119891119899 = 1
and let 119866 be the closed linear span of
119879119899
0119891119898 119899 119898 isin N cup 0 (81)
As 119866 is separable there is a dense subset 119892119899 119899 isin N of 119866
and let 119864 = ⋃infin
119899=1supp119892
119899 where supp119892
119899is the support of 119892
119899
Then the measure 120583|119864is 120590-finite Let
119884 = 119891 isin 1198711(120583) supp (119891) sub 119864 (82)
be a closed subspace of 1198711(120583) It is clear that 119871
1(120583) = 119884oplus
1119885
and 119884 is isometrically isomorphic to 1198711(120583|
119864) So 119884 has the
BPBp-nu with 120578(120576)Now write 119878
0= 119879
0|119884 119884 rarr 119884 consider 119910
0= 119891
0isin
119878119884 119910lowast
0= 119891
lowast
0|119884isin 119878
119884lowast and observe that 119910lowast
0(119910
0) = 1 and
|119910lowast
0(119878
01199100)| = |119891
lowast
0(119879
01198910)| gt 1 minus 120578(120576) Hence there exist 119878 isin
L(119884) and (1199100 119910
lowast
0) isin Π(119884) such that
1003816100381610038161003816119910lowast
0(119878119910
0)1003816100381610038161003816 = 1 = V (119878) 1003817100381710038171003817119878 minus 1198780
1003817100381710038171003817 lt 120576
10038171003817100381710038171199100 minus 11991001003817100381710038171003817 lt 120576
1003817100381710038171003817119910lowast
0minus 119910
lowast
0
1003817100381710038171003817 lt 120576
(83)
Finally consider the operator 119879 isinL(1198711(120583)) given by
119879 (119910 119911)
= (119878119910 0) + 1198790(0 119911) ((119910 119911) isin 119871
1(120583) equiv 119884oplus
1119885)
(84)
We have 119879 = 1 (and so V(119879) = 1) Indeed1003817100381710038171003817119879 (119910 119911)
1003817100381710038171003817 =1003817100381710038171003817(119878119910 0)
1003817100381710038171003817 +10038171003817100381710038171198790 (0 119911)
1003817100381710038171003817 ⩽10038171003817100381710038171199101003817100381710038171003817 + 119911 =
1003817100381710038171003817(119910 119911)1003817100381710038171003817
(85)
for all (119910 119911) isin 1198711(120583) and 119879(119910
0 0) = (119878119910
0 0) = 119878119910
0 = 1
Let 119909 = (1199100 0) and 119909lowast = (119910lowast
0 119891
0|119885) Then (119909 119909lowast) isin Π(119871
1(120583))
Moreover we have1003816100381610038161003816119909
lowast1198791199091003816100381610038161003816 =1003816100381610038161003816119910
lowast
0119878119910
0
1003816100381610038161003816 = 1 = V (119879) 1003817100381710038171003817119909 minus 1198910
1003817100381710038171003817 =1003817100381710038171003817119910 minus 1199100
1003817100381710038171003817 lt 120576
1003817100381710038171003817119909lowast
0minus 119891
lowast
0
1003817100381710038171003817
= max 1003817100381710038171003817119910 minus 119891lowast
0|119884
1003817100381710038171003817 1003817100381710038171003817119891
lowast
0|119885minus 119891
lowast
0|119885
1003817100381710038171003817
=1003817100381710038171003817119910
lowastminus 119910
lowast
0
1003817100381710038171003817 lt 120576
1003817100381710038171003817119879 minus 11987901003817100381710038171003817 = sup119910+119911⩽1
1003817100381710038171003817119879 (119910 119911) minus 1198790 (119910 119911)1003817100381710038171003817
= sup119910⩽1
1003817100381710038171003817119878119910 minus 11987801199101003817100381710038171003817 =1003817100381710038171003817119878 minus 1198780
1003817100381710038171003817 lt 120576
(86)
This completes the proof
5 Examples of Spaces Failing theBishop-Phelps-Bollobaacutes Property forNumerical Radius
Our goal here is to prove that the density of numerical radiusattaining operators does not imply the BPBp-nu Actuallywe will show that among separable spaces there is noisomorphic property implying the BPBp-nu other than finite-dimensionality
We need to relate the BPBp-nu to the Bishop-Phelps-Bollobas property for operators which as mentioned in theintroduction was introduced in [15] A pair (119883 119884) of Banachspaces has the Bishop-Phelps-Bollobas property for operators(in short BPBp) if given 120576 gt 0 there exists 120578(120576) gt 0 suchthat given 119879 isin L(119883 119884) with 119879 = 1 and 119909 isin 119878
119883such that
119879119909 gt 1 minus 120578(120576) then there exist 119911 isin 119878119883and 119878 isin L(119883 119884)
satisfying
119878 = 119878119911 = 1 119909 minus 119911 lt 120576
119879 minus 119878 lt 120576
(87)
We refer the reader to [15 19 25] and references therein formore information and background Among the interestingresults on the BPBp we emphasize that a pair (119883 119884) when119883is finite-dimensional does not necessarily have the BPBp Forinstance if119884 is a strictly convex space which is not uniformlyconvex then the pair (ℓ(2)
1 119884) fails to have the BPBp (this is
contained in [15] see [19 Section 3])
Abstract and Applied Analysis 11
The next result relates the BPBp-nu to the BPBp foroperators in a particular case We will deduce our examplefrom it
Theorem 15 If 1198711(120583)oplus
1119883 has the BPBp-119899119906 then the pair
(1198711(120583) 119883) has the BPBp for operators
Before proving this proposition we will use it to get themain examples of this section The first example shows thatthe density of numerical radius attaining operators does notimply the BPBp-nu
Example 16 There is a reflexive space (and so numerical radiusattaining operators on it are dense) which fails to have theBPBp-nu Indeed let 119884 be a reflexive separable space whichis not superreflexive and we may suppose that 119884 is strictlyconvex Observe that 119884 cannot be uniformly convex since itis not superreflexive Now 119883 = ℓ(2)
1oplus1119884 is reflexive but the
pair (ℓ(2)1 119884) fails the BPBp since 119884 is strictly convex but not
uniformly convex [19 Corollary 33] Therefore Theorem 15gives us that119883 does not have the BPBp-nu
The example above can be extended to get the result thatevery infinite-dimensional separable Banach space can berenormed to fail the BPBp-nu This follows from the factthat every infinite-dimensional separable Banach space canbe renormed to be strictly convex but not uniformly convex(this result can be proved ldquoby handrdquo an alternative categoricalargument for it can be found in [27] and references therein)With a little more of effort we may get the main result of thesection
Theorem 17 Every infinite-dimensional separable Banachspace can be renormed to fail the weak-BPBp-nu (and so inparticular to fail the BPBp-nu)
We need the following result which is surely well knownAs we have not found a reference we include a nice and easyproof kindly given to us by Vladimir Kadets We recall thatgiven a Banach space 119884 the set of all equivalent norms on119884 can be viewed as a metric space using the Banach-Mazurdistance
Lemma 18 Let119884 be an infinite-dimensional separable Banachspace Then the set of equivalent norms on 119884 which are strictlyconvex and are not (locally) uniformly convex is dense in the setof all equivalent norms on119884 (with respect to the Banach-Mazurdistance)
Proof Fix 119890 isin 119878119884and 119890lowast
1isin 119878
119884lowast such that 119890lowast
1(119890) = 1 For a fixed
120576 isin (0 12) denote
119902 (119910) = max (1 minus 120576) 10038171003817100381710038171199101003817100381710038171003817 1003816100381610038161003816119890lowast
1(119910)1003816100381610038161003816 (119910 isin 119884) (88)
Evidently (1 minus 120576)119910 ⩽ 119902(119910) ⩽ 119910 for every 119910 isin 119884 Fix asequence 119890lowast
119896 119896 ⩾ 2 of norm-one functionals separating
the points of 119884 and denote
119901 (119910) = radic
infin
sum
119896=1
1
2119896
1003816100381610038161003816119890lowast
119896(119910)1003816100381610038161003816
2
(119910 isin 119884) (89)
Then 119901 is a strictly convex norm on 119884 119901(119890) ⩾ 1radic2 and119901(119910) ⩽ 119910 for all 119910 isin 119883 Finally write
100381710038171003817100381711991010038171003817100381710038171= (1 minus 120576) 119902 (119910) + 120576
119901 (119910)
119901 (119890)(119910 isin 119884) (90)
Then sdot 1is a strictly convex norm on 119884 and
(1 minus 120576)2 10038171003817100381710038171199101003817100381710038171003817 ⩽100381710038171003817100381711991010038171003817100381710038171⩽ (1 + 120576)
10038171003817100381710038171199101003817100381710038171003817 (119910 isin 119884) (91)
Wewill finish the proof by showing that sdot 1is not uniformly
convex (actually it is not locally uniformly convex) Indeedfor each 119899 isin N we select 119910
119899isin ⋂
119899
119896=1ker 119890lowast
119896with 119910
119899 = 1 and
consider 119890119899= 119890 + (1205764)119910
119899 Then 119902(119890) = 1 119902(119890
119899) = 1 and
119902(119890+119890119899) = 2 At the same time 119901(119910
119899) rarr 0 so 119901(119890
119899) rarr 119901(119890)
and 119901(119890 + 119890119899) rarr 2119901(119890) Consequently
1198901 = 1100381710038171003817100381711989011989910038171003817100381710038171997888rarr 1
1003817100381710038171003817119890 + 11989011989910038171003817100381710038171997888rarr 2 (92)
but 119890 minus 1198901198991= (1205764)119910
1198991⩾ (1 minus 120576)
2(1205764) which means the
absence of local uniform convexity at 119890
Proof of Theorem 17 Let 119883 be an infinite-dimensional sepa-rable Banach space Take a closed subspace 119884 of 119883 of codi-mension two By [28 Proposition 2] the map carrying everyequivalent norm on 119884 to its numerical index is continuousand so the set of values of the numerical index of 119884 upto reforming is a nontrivial interval [28 Theorem 9] ThenLemma 18 allows us to find an equivalent norm | sdot | on 119884 insuch a way that (119884 | sdot |) is strictly convex and is not uniformlyconvex and 119899(119884 | sdot |) gt 0 Now the space 119883 = ℓ(2)
1oplus1(119884 | sdot |)
is an equivalent renorming of 119883 which does not have theBPBp-119899119906 (indeed otherwise the pair (ℓ(2)
1 (119884 | sdot |)) would
have the BPBp for the operator norm and so (119884 | sdot |) wouldbe uniformly convex by [19 Corollary 33] a contradiction)Moreover as
119899 (119883) = min 119899 (ℓ(2)1) 119899 (119884 |sdot|) gt 0 (93)
(see [20 Proposition 2] for instance) 119883 also fails the weak-BPBp-nu by Proposition 6
To finish the section with the promised proof ofTheorem 15 we first see the following stability result
Lemma 19 Let119883 = [⨁infin
119896=1119883
119896]1198880
or [⨁infin
119896=1119883
119896]ℓ1
If119883 has theBishop-Phelps-Bollobas property for numerical radius with afunction 120578 then each Banach space 119883
119894has the Bishop-Phelps-
Bollobas property for numerical radius with 120578nu(119883119894) ⩾ 120578 That
is inf119894120578nu(119883119894
)(120576) ⩾ 120578nu(119883)(120576) for all 0 lt 120576 lt 1
Proof Let 119875119894 119883 rarr 119883
119894and 1198751015840
119894 119883
lowastrarr 119883
lowast
119894be the natural
projections and let 119876119894 119883
119894rarr 119883 and 1198761015840
119894 119883
lowast
119894rarr 119883
lowast be thenatural embeddings
Assume that an operator 119879119894 119883
119894rarr 119883
119894and a pair
(119909119894 119909
lowast
119894) isin Π(119883
119894) satisfy that
V (119879119894) = 1
1003816100381610038161003816119909lowast
119894119879119894119909119894
1003816100381610038161003816 gt 1 minus 120578 (120576) (94)
We define an operator 119879 119883 rarr 119883 and (119909 119909lowast) isin Π(119883) by
119879 = 119876119894∘ 119879
119894∘ 119875
119894 (119909 119909
lowast) = (119876
119894119909119894 119876
1015840
119894119909lowast
119894) (95)
12 Abstract and Applied Analysis
then clearly we see that1003816100381610038161003816119909
lowast1198791199091003816100381610038161003816 =1003816100381610038161003816119909
lowast
119894119879119894119909119894
1003816100381610038161003816 gt 1 minus 120578 (120576) (96)
From the assumption there exist 119878 119883 rarr 119883 and a pair(119910 119910
lowast) isin Π(119883) such that
1003816100381610038161003816119910lowast1198781199101003816100381610038161003816 = 1 = V (119878) 119878 minus 119879 lt 120576
1003817100381710038171003817119910lowastminus 119909
lowast1003817100381710038171003817 lt 1205761003817100381710038171003817119910 minus 119909
1003817100381710038171003817 lt 120576
(97)
Since this clearly shows that1003817100381710038171003817119875119894 ∘ 119878 ∘ 119876119894
minus 119879119894
1003817100381710038171003817 lt 120576100381710038171003817100381710038171198751015840
119894119910lowastminus 119909
lowast
119894
10038171003817100381710038171003817lt 120576
1003817100381710038171003817119875119894119910 minus 1199091198941003817100381710038171003817 lt 120576
(98)
we only need to show that |1198751015840119894119910lowast(119875
119894∘ 119878 ∘ 119876
119894)119875
119894119910| = 1
We first show the case of 1198880sum Since
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817=10038171003817100381710038171003817119875119895119910 minus 119875
11989511990910038171003817100381710038171003817⩽1003817100381710038171003817119910 minus 119909
1003817100381710038171003817 lt 120576 (99)
for every 119895 = 119894 we have
1 = 119910lowast(119910) = sum
119895isinN
1198751015840
119895119910lowast(119875
119895119910) ⩽ sum
119895isinN
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817
⩽100381710038171003817100381710038171198751015840
119894119910lowast10038171003817100381710038171003817+ 120576 sum
119895isinN119895 = 119894
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817⩽1003817100381710038171003817119910
lowast1003817100381710038171003817 = 1
(100)
This shows that 1198751015840119894119910lowast = 1 and 1198751015840
119895119910lowast= 0 for every 119895 = 119894
So 119910lowast = 1198761015840
1198941198751015840
119894119910lowast and 1198751015840
119894119910lowast(119875
119894119910) = 1 This and the fact that
119910 minus 119876119894119875119894119910 lt 120576 imply that
(119876119894119875119894119910 + (
1
120576) (119910 minus 119876
119894119875119894119910) 119876
1015840
1198941198751015840
119894119910lowast) isin Π (119883) (101)
So we get that (1198761015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910 + (1120576)(119910 minus 119876
119894119875119894119910))V(119878) = 1
Hence we have
1 =1003816100381610038161003816119910
lowast1198781199101003816100381610038161003816 =10038161003816100381610038161003816(119876
1015840
1198941198751015840
119894119910lowast) 11987811991010038161003816100381610038161003816
=
1003816100381610038161003816100381610038161003816(1 minus 120576) (119876
1015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910)
+120576 (1198761015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910 +1
120576(119910 minus 119876
119894119875119894119910))
1003816100381610038161003816100381610038161003816⩽ 1
(102)
and so we get |1198751015840119894119910lowast(119875
119894∘119878 ∘119876
119894)119875
119894119910| = |(119876
1015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910)| = 1
We next show the case of ℓ1sum The proof is almost
the same as that of the 1198880case However for the sake of
completeness we provide it hereSince 1198751015840
119895119910lowast = 119875
1015840
119895119910lowastminus 119875
1015840
119895119909lowast ⩽ 119910
lowastminus 119909
lowast lt 120576 for every
119895 = 119894 we have
1 = 119910lowast(119910) = sum
119895isinN
1198751015840
119895119910lowast(119875
119895119910) ⩽ sum
119895isinN
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817
⩽10038171003817100381710038171198751198941199101003817100381710038171003817 + 120576 sum
119895isinN119895 = 119894
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817⩽10038171003817100381710038171199101003817100381710038171003817 = 1
(103)
which shows 119875119894119910 = 1 and 119875
119895119910 = 0 for every 119895 = 119894 Since this
implies (119876119894119875119894119910119876
1015840
1198941198751015840
119894119910lowast+(1120576)(119910
lowastminus119876
1015840
1198941198751015840
119894119910lowast)) isin Π(119883) we get
that |(1198761015840
1198941198751015840
119894119910lowast+ (1120576)(119910
lowastminus 119876
1015840
1198941198751015840
119894119910lowast))119878(119876
119894119875119894119910)| ⩽ V(119878) = 1
Hence we have
1 =1003816100381610038161003816119910
lowast1198781199101003816100381610038161003816 =1003816100381610038161003816119910
lowast119878 (119876
119894119875119894119910)1003816100381610038161003816
=
1003816100381610038161003816100381610038161003816(1 minus 120576) (119876
1015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910)
+120576 (1198761015840
1198941198751015840
119894119910lowast+1
120576(119910
lowastminus 119876
1015840
1198941198751015840
119894119910lowast)) 119878 (119876
119894119875119894119910)
1003816100381610038161003816100381610038161003816⩽ 1
(104)
and so |1198751015840119894119910lowast(119875
119894∘ 119878 ∘ 119876
119894)119875
119894119910| = |(119876
1015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910)| = 1
Proof of Theorem 15 Note that 120578nu(1198711(120583)oplus1119883)(120576) rarr 0 as120576 rarr 0 Fix 0 lt 120576
0lt 1 and choose 0 lt 120576 lt 1 such that
6120576 + 120578nu(1198711(120583)oplus1119883)(120576) lt 1205760 Let 120578(1205760) = 120578nu(1198711(120583)oplus1119883)(120576)Suppose that 119879
0isin L(119871
1(120583) 119883) with 119879
0 = 1 and 119891
0isin
1198781198711(120583)
satisfy
1003817100381710038171003817119879011989101003817100381710038171003817 gt 1 minus 120578 (1205760) (105)
For any measurable subset 119861 let
1198711(120583|
119861) = 119891|
119861 119891 isin 119871
1(120583) (106)
with the norm 119891|119861 = 119891120594
1198611 Then it is easy to see
that 1198711(120583|
119861) is isometrically isomorphic to a complemented
subspace of 1198711(120583) Let 119875
119861 119871
1(120583) rarr 119871
1(120583|
119861) be the
restriction defined by 119875119861(119891) = 119891|
119861for all 119891 isin 119871
1(120583) and
let 119869119861 119871
1(120583|
119861) rarr 119871
1(120583) be the extension defined by
119869119861(119891)(120596) = 119891(120596) if 120596 isin 119861 and 119869
119861(119891)(120596) = 0 otherwise
It is clear that 119875119861119869119861= Id
1198711(120583|119861)and 119869
119861119875119861(119891) = 119891120594
119861for all
119891 isin 1198711(120583) Notice also that 119871
1(120583) is isometrically isomorphic
to 1198711(120583|
119861)oplus
11198711(120583|
119861119888)
Let 119860 = supp1198910and 119892
0= 119875
1198601198910 Then
1003817100381710038171003817119879011986911986011989201003817100381710038171003817 =100381710038171003817100381711987901198910
1003817100381710038171003817 gt 1 minus 120578 (1205760) gt 0 (107)
and define the operator 119879119860 119871
1(120583|
119860) rarr 119883 by 119879
119860119891 =
1198790119869119860119891119879
0119869119860 for every 119891 isin 119871
1(120583|
119860) Then
100381710038171003817100381711987911986011989201003817100381710038171003817 ⩾100381710038171003817100381711987901198910
1003817100381710038171003817 gt 1 minus 120578 (1205760) (108)
Since 120583|119860is 120590-finite 119871
1(120583|
119860)lowast= 119871
infin(120583|
119860) Let 119892lowast
0isin 119878
119871infin(120583|119860)
be a function such that ⟨119892lowast0 119892
0⟩ = 1 and choose 119909lowast
0isin 119878
119883lowast
such that 119909lowast0(119879
1198601198920) = 119879
1198601198920 Define the operator 119878
0isin
L(1198711(120583|
119860)oplus
1119883) by
1198780(119891 119909) = (0 119879
119860119891) ((119891 119909) isin 119871
1(120583|
119860) oplus
1119883) (109)
and observe that 1198780 = V(119878
0) = 1 Indeed
100381710038171003817100381711987801003817100381710038171003817 ⩽ 1 =
10038171003817100381710038171198791198601003817100381710038171003817
= sup 1003816100381610038161003816119909lowast1198791198601198911003816100381610038161003816 119909
lowastisin 119878
119883lowast 119891 isin 119878
1198711(120583|119860)
= sup 1003816100381610038161003816(119891lowast 119909
lowast) 119878
0(119891 119909)
1003816100381610038161003816
((119891lowast 119909
lowast) (119891 119909)) isin Π (119871
1(120583|
119860) oplus
1119883)
= V (1198780) ⩽100381710038171003817100381711987801003817100381710038171003817
(110)
Abstract and Applied Analysis 13
It is immediate that
(119892lowast
0 119909
lowast
0) 119878
0(119892
0 0) = 119909
lowast
0(119879
1198601198920) =10038171003817100381710038171198791198601198920
1003817100381710038171003817 gt 1 minus 120578 (1205760)
(111)
By Lemma 19 1198711(120583|
119860)oplus
1119883 has the BPBp-nu with the func-
tion 120578 Therefore there exist 1198781isin L(119871
1(120583|
119860)oplus
1119883) (119892
1 119909
1) isin
1198781198711(120583|119860)oplus1119883 and (119892lowast
1 119909
lowast
1) isin 119878
119871infin(120583|119860)oplusinfin119883lowast such that
1003817100381710038171003817(1198921 1199091) minus (1198920 0)1003817100381710038171003817 lt 120576
1003817100381710038171003817(119892lowast
1 119909
lowast
1) minus (119891
lowast
0 119909
lowast
0)1003817100381710038171003817 lt 120576
10038171003817100381710038171198781 minus 11987801003817100381710038171003817 lt 120576 ⟨(119892
lowast
1 119909
lowast
1) (119892
1 119909
1)⟩ = 1
1003816100381610038161003816(119892lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)1003816100381610038161003816 = V (119878
1) = 1
(112)
Claim 1 We claim that 1199091= 0
Otherwise
1 = Re ⟨(119892lowast1 119909
lowast
1) (119892
1 119909
1)⟩
=100381710038171003817100381711989211003817100381710038171003817Re⟨(119892
lowast
1 119909
lowast
1) (119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
⟩
+100381710038171003817100381711990911003817100381710038171003817Re⟨(119892
lowast
1 119909
lowast
1) (0 119909
1)
100381710038171003817100381711990911003817100381710038171003817
⟩ ⩽ 1
(113)
We deduce that
((119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
(119892lowast
1 119909
lowast
1)) (
(0 1199091)
100381710038171003817100381711990911003817100381710038171003817
(119892lowast
1 119909
lowast
1))
isin Π (1198711(120583|
119860) oplus
1119883)
(114)
Since 1198781((0 119909
1)119909
1) = (119878
1minus 119878
0)((0 119909
1)119909
1) lt 120576 we
get that
1 =1003816100381610038161003816⟨(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)⟩1003816100381610038161003816
=
100381610038161003816100381610038161003816100381610038161003816
100381710038171003817100381711989211003817100381710038171003817⟨(119892
lowast
1 119909
lowast
1) 119878
1((119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
)⟩
+100381710038171003817100381711990911003817100381710038171003817⟨(119892
lowast
1 119909
lowast
1) 119878
1((0 119909
1)
100381710038171003817100381711990911003817100381710038171003817
)⟩
100381610038161003816100381610038161003816100381610038161003816
⩽100381710038171003817100381711989211003817100381710038171003817 V (1198781) + 120576
100381710038171003817100381711990911003817100381710038171003817 lt100381710038171003817100381711989211003817100381710038171003817 +100381710038171003817100381711990911003817100381710038171003817 = 1
(115)
a contradiction This proves the claimWe define the operator 119878
2 119871
1(120583|
119860)oplus
1119883 rarr 119871
1(120583|
119860)oplus
1119883
by 1198782(119891 119909) = 119878
1(119891 0) for every 119891 isin 119871
1(120583|
119860) and for every
119909 isin 119883 Then we have
V (1198782) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816 = 1
10038171003817100381710038171198781 minus 11987821003817100381710038171003817 ⩽ 120576 (116)
Indeed from Claim 1 we have
V (1198781) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)1003816100381610038161003816 =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816 ⩽ V (119878
2)
(117)
On the other hand we see that1003816100381610038161003816(119891
lowast 119909
lowast) 119878
2(119891 119909)
1003816100381610038161003816
=1003816100381610038161003816(119891
lowast 119909
lowast) 119878
1(119891 0)
1003816100381610038161003816 ⩽10038171003817100381710038171198911003817100381710038171003817 V (1198781) ⩽ V (119878
1)
(118)
for every ((119891lowast 119909lowast) (119891 119909)) isin Π(1198711(120583|
119860)oplus
1119883) Therefore
V(1198782) ⩽ V(119878
1) Also
10038171003817100381710038171198781 minus 11987821003817100381710038171003817 ⩽ sup
119909isin119878119883
10038171003817100381710038171198781 (0 119909)1003817100381710038171003817
= sup119909isin119878119883
10038171003817100381710038171198781 (0 119909) minus 1198780 (0 119909)1003817100381710038171003817 ⩽ 120576
(119)
Claim 2 There exists 1198783 119871
1(120583|
119860)oplus
1119883 rarr 119871
1(120583|
119860)oplus
1119883 such
that 1198783(119892
1 0) = 119878
3 = 1 119878
3(0 119909) = 0 119878
3(119891 119909) isin 0oplus
1119883
for every (119891 119909) isin 1198711(120583|
119860)oplus
1119883 and 119878
3minus 119878
2 lt 4120576
Indeed write 1198781= (119863
1 119863
2) where 119863
1 119871
1(120583|
119860)oplus
1119883 rarr
1198711(120583|
119860) and119863
2 119871
1(120583|
119860)oplus
1119883 rarr 119883 We have that
sup 1003816100381610038161003816119892lowast119863
1(119892
1 0) + 119909
lowast119863
2(119892
1 0)1003816100381610038161003816
119909lowastisin 119878
119883lowast ⟨119892
lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860)
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816 +10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
⟨119892lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860)
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816
⟨119892lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860) +10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
⩽ V (1198782) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816119892
lowast
1119863
1(119892
1 0) + 119909
lowast
1119863
2(119892
1 0)1003816100381610038161003816
(120)
This implies that
1003816100381610038161003816119909lowast
1119863
2(119892
1 0)1003816100381610038161003816 =10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
1003816100381610038161003816119892lowast
1119863
1(119892
1 0)1003816100381610038161003816
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816 ⟨119892
lowast 119892
1⟩ = 1 119892
lowastisin 119871
infin(120583|
119860)
(121)
Therefore |119892lowast1| equals 1 on the support of 119863
1(119892
1 0) As
|⟨119892lowast
1 119892
1⟩| = 1 we also have that |119892lowast
1| equals 1 on the support
of 1198921 Changing the values of 119892lowast
1by the ones of 119891lowast
0on 119860
(supp(1198631(119892
1 0)) cup supp(g
1)) we may and do suppose that
|119892lowast
1| = 1 on the whole 119860We also have 119863
2(119892
1 0) gt 0 Indeed
10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198920 0)1003817100381710038171003817
⩽10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198921 0)
1003817100381710038171003817 +10038171003817100381710038171198780 (1198921 0) minus 1198780 (1198920 0)
1003817100381710038171003817
lt 2120576 + 120576 = 3120576
(122)
So we have
10038171003817100381710038171198632(119892
1 0) minus 119879
1198601198920
1003817100381710038171003817
⩽10038171003817100381710038171198631(119892
1 0)1003817100381710038171003817 +10038171003817100381710038171198632(119892
1 0) minus 119879
1198601198920
1003817100381710038171003817
14 Abstract and Applied Analysis
=1003817100381710038171003817(1198631
(1198921 0) 119863
2(119892
1 0)) minus (0 119879
1198601198920)1003817100381710038171003817
=10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198920 0)
1003817100381710038171003817 lt 3120576
(123)
and 1198632(119892
1 0) gt 119879
1198601198920 minus 3120576 ⩾ 1 minus 120578(120576
0) minus 3120576 gt 0
Finally define the operator 1198783by
1198783(119891 119909) = (0119863
2(119891 0) + 119892
lowast
1(119863
1(119891 0))
1198632(119892
1 0)
119909lowast
1119863
2(119892
1 0))
(124)
for (119891 119909) isin 1198711(120583|
119860)oplus
1119883 It is clear that
100381710038171003817100381711987831003817100381710038171003817 ⩽ sup
119891isin1198781198711(120583|119860)
(10038171003817100381710038171198632(119891 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119891 0)
1003816100381610038161003816) (125)
Notice also that
10038171003817100381710038171198631(119891 0)
1003817100381710038171003817
⩽10038171003817100381710038171198631(119891 0)
1003817100381710038171003817 +10038171003817100381710038171198632(119891 0) minus 119879
1198601198911003817100381710038171003817
=1003817100381710038171003817(1198631
(119891 0) 1198632(119891 0)) minus (0 119879
119860119891)1003817100381710038171003817
=10038171003817100381710038171198782 (119891 119909) minus 1198780 (119891 119909)
1003817100381710038171003817
(126)
for all (119891 119909) isin 1198711(120583|
119860)oplus
1119883 Hence we have
10038171003817100381710038171198783 minus 11987821003817100381710038171003817
= 2 sup119891isin1198781198711(120583|119860)
10038171003817100381710038171198631(119891 0)
1003817100381710038171003817 ⩽ 210038171003817100381710038171198782 minus 1198780
1003817100381710038171003817 lt 4120576(127)
On the other hand let 119866 1198711(120583|
119860) rarr 119871
1(120583|
119860) be defined by
119866(119891) = 119892lowast
1119891 for every 119891 isin 119871
1(120583|
119860) Then we have
V (1198782)
= sup 1003816100381610038161003816119911lowast11987821199111003816100381610038161003816 (119911 119911
lowast) isin Π (119871
1(120583|
119860) oplus
1119883)
⩾ sup10038161003816100381610038161003816100381610038161003816
119909lowast119863
2(119866(
1
120583 (119862)120594119862) 0)
+119892lowast
1119863
1(119866(
1
120583 (119862)120594119862) 0)
10038161003816100381610038161003816100381610038161003816
119909lowastisin 119878
119883lowast 119862 isin Σ
119860 120583 (119862) gt 0
= sup10038171003817100381710038171003817100381710038171003817
1198632(119866(
1
120583 (119862)120594119862) 0)
10038171003817100381710038171003817100381710038171003817
+
10038161003816100381610038161003816100381610038161003816
119892lowast
1119863
1(119866(
1
120583 (119862)120594119862) 0)
10038161003816100381610038161003816100381610038161003816
119862 isin Σ119860 120583 (119862) gt 0
(128)
where Σ119860is the family of measurable subsets of 119860
Hence for any simple function 119904 = sum119899
119894=1((120572
119894120583(119860
119894))120594
119860119894
) isin
1198781198711(120583|119860) where 119860
119894119894is a family of disjoint measurable subsets
with strictly positive measure we have
V (1198782)
⩾
119899
sum
119894=1
10038161003816100381610038161205721198941003816100381610038161003816 (
100381710038171003817100381710038171003817100381710038171003817
1198632(119866(
1
120583 (119860119894)120594119860119894
) 0)
100381710038171003817100381710038171003817100381710038171003817
+
100381610038161003816100381610038161003816100381610038161003816
119892lowast
1119863
1(119866(
1
120583 (119860119894)120594119860119894
) 0)
100381610038161003816100381610038161003816100381610038161003816
)
⩾10038171003817100381710038171198632(119866 (119904) 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119866 (119904) 0)
1003816100381610038161003816
(129)
Since |119892lowast1| = 1119866 is an isometric isomorphism so for each119891 isin
1198781198711(120583|119860)there exists a sequence of norm-one simple functions
(119904119896) such that 119866(119904
119896) converges to 119891 Therefore
V (1198782) ⩾ sup
119891isin1198781198711(120583|119860)
(10038171003817100381710038171198632(119891 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119891 0)
1003816100381610038161003816) ⩾100381710038171003817100381711987831003817100381710038171003817
(130)
On the other hand we see that100381710038171003817100381711987831003817100381710038171003817 ⩾1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
3(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816119909
lowast
1119863
2(119892
1 0) + 119892
lowast
1119863
1(119892
1 0)1003816100381610038161003816 = V (119878
2) = 1
(131)
Therefore 1 = 1198783 = 119878
3(119892
1 0) which proves Claim 2
Finally set 1198783= (0 ) for a suitable 119871
1(120583|
119860)oplus
1119883 rarr
119883 and define the operator 1198791 119871
1(120583) rarr 119883 by
1198791(119891) = 119879
0(119891120594
119860119888) + (119875
119860119891 0) (132)
for every 119891 isin 1198711(120583) Then we have
10038171003817100381710038171198791 (119891)1003817100381710038171003817 ⩽100381710038171003817100381711987901003817100381710038171003817
10038171003817100381710038171198911205941198601198881003817100381710038171003817 +1003817100381710038171003817100381710038171003817100381710038171003817
10038171003817100381710038171198911205941198601003817100381710038171003817 =10038171003817100381710038171198911003817100381710038171003817 (133)
for every 119891 isin 1198711(120583) so 119879
1 ⩽ 1 Also
10038171003817100381710038171198791 (1198691198601198921)1003817100381710038171003817 =10038171003817100381710038171198783 (1198921 0)
1003817100381710038171003817 =100381710038171003817100381711987831003817100381710038171003817 = 1 (134)
so 1198791attains its norm at 119869
1198601198921isin 119871
1(120583) and
10038171003817100381710038171198691198601198921 minus 11989101003817100381710038171003817 =10038171003817100381710038171198921 minus 1198920
1003817100381710038171003817 lt 120576 (135)
We also have that for any 119891 isin 1198781198711(120583)
10038171003817100381710038171198790 (119891) minus 1198791 (119891)1003817100381710038171003817 =100381710038171003817100381710038171198790(119891120594
119860) minus (119875
119860119891 0)10038171003817100381710038171003817
⩽10038171003817100381710038171198790 (119869119860119875119860119891) minus 119879119860 (119875119860119891)
1003817100381710038171003817
+10038171003817100381710038171003817119879119860(119875
119860119891) minus (119875
119860119891 0)10038171003817100381710038171003817
⩽10038171003817100381710038171198790119869119860 minus 119879119860
1003817100381710038171003817 +10038171003817100381710038171198780 minus 1198783
1003817100381710038171003817
lt 120578 (1205760) + 6120576
(136)
Hence 1198790minus 119879
1 ⩽ 120578(120576
0) + 6120576 lt 120576
0
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Abstract and Applied Analysis 15
Acknowledgments
The authors thank Gilles Godefroy and Rafael Paya forfruitful conversations about the content of this paper andVladimir Kadets for providing Lemma 18 They also appre-ciate anonymous referees for careful reading and fruitfulsuggestions about revision The second author was sup-ported by Basic Science Research Program through theNational Research Foundation of Korea (NRF) fundedby the Ministry of Education Science and Technology(NRF-2012R1A1A1006869) The third author was partiallysupported by Spanish MICINN and FEDER project noMTM2012-31755 and by Junta de Andalucıa and FEDERGrants FQM-185 and P09-FQM-4911
References
[1] F F Bonsall and J Duncan Numerical Ranges of Operators onNormed Spaces and of Elements of Normed Algebras vol 2 ofLondon Mathematical Society Lecture Note Series 2 CambridgeUniversity Press London UK 1971
[2] F F Bonsall and J Duncan Numerical Ranges II CambridgeUniversity Press London UK 1973 London MathematicalSociety Lecture Notes Series 10
[3] B Sims On numerical range and its applications to Banachalgebras [PhD thesis] University of Newcastle New SouthWales Australia 1972
[4] I D Berg and B Sims ldquoDenseness of operators which attaintheir numerical radiusrdquoAustralian Mathematical Society A vol36 no 1 pp 130ndash133 1984
[5] C S Cardassi ldquoDensity of numerical radius attaining operatorson some reflexive spacesrdquo Bulletin of the Australian Mathemat-ical Society vol 31 no 1 pp 1ndash3 1985
[6] C S Cardassi ldquoNumerical radius attaining operatorsrdquo inBanach Spaces (Columbia Mo 1984) vol 1166 of Lecture Notesin Mathematics pp 11ndash14 Springer Berlin Germany 1985
[7] C S Cardassi ldquoNumerical radius-attaining operators on119862(119870)rdquoProceedings of the AmericanMathematical Society vol 95 no 4pp 537ndash543 1985
[8] MDAcostaOperadores que alcanzan su radio numerico [PhDthesis] Universidad de Granada Granada Spain 1990
[9] M D Acosta and R Paya ldquoNumerical radius attaining oper-ators and the Radon-Nikodym propertyrdquo The Bulletin of theLondon Mathematical Society vol 25 no 1 pp 67ndash73 1993
[10] R Paya ldquoA counterexample on numerical radius attainingoperatorsrdquo Israel Journal of Mathematics vol 79 no 1 pp 83ndash101 1992
[11] Y S Choi and S G Kim ldquoNorm or numerical radius attainingmultilinear mappings and polynomialsrdquo Journal of the LondonMathematical Society vol 54 no 1 pp 135ndash147 1996
[12] M D Acosta J Becerra Guerrero and M Ruiz GalanldquoNumerical-radius-attaining polynomialsrdquo The Quarterly Jour-nal of Mathematics vol 54 no 1 pp 1ndash10 2003
[13] M D Acosta and S G Kim ldquoDenseness of holomorphicfunctions attaining their numerical radiirdquo Israel Journal ofMathematics vol 161 pp 373ndash386 2007
[14] S G Kim and H J Lee ldquoNumerical peak holomorphicfunctions on Banach spacesrdquo Journal of Mathematical Analysisand Applications vol 364 no 2 pp 437ndash452 2010
[15] M D Acosta R M Aron D Garcıa and M MaestreldquoThe Bishop-Phelps-Bollobas theorem for operatorsrdquo Journal ofFunctional Analysis vol 254 no 11 pp 2780ndash2799 2008
[16] A J Guirao and O Kozhushkina ldquoThe Bishop-Phelps-Bollobasproperty for numerical radius in ℓ
1(119862)rdquo Studia Mathematica
vol 218 no 1 pp 41ndash54 2013[17] J Falco ldquoThe Bishop-Phelps-Bollobas property for numerical
radius on 1198711rdquo Journal of Mathematical Analysis and Applica-
tions vol 414 no 1 pp 125ndash133 2014[18] A Aviles A J Guirao and J Rodriguez ldquoOn the Bishop-
Phelps-Bollobas property for numerical radius in119862(119870)-spacesrdquoPreprint
[19] RM Aron Y S Choi S K Kim H J Lee andMMartin ldquoTheBishop-Phelps-Bollobas version of Lindenstrauss properties Aand Brdquo Preprint
[20] V Kadets M Martın and R Paya ldquoRecent progress and openquestions on the numerical index of Banach spacesrdquo Revista dela Real Academia de Ciencias Exactas Fısicas y Naturales A vol100 no 1-2 pp 155ndash182 2006
[21] M Martın J Merı and M Popov ldquoOn the numerical index ofreal 119871
119901(120583)-spacesrdquo Israel Journal of Mathematics vol 184 pp
183ndash192 2011[22] A Iwanik ldquoNorm attaining operators on Lebesgue spacesrdquo
Pacific Journal of Mathematics vol 83 no 2 pp 381ndash386 1979[23] HH SchaeferBanach Lattices and Positive Operators Springer
New York NY USA 1974[24] J Duncan C M McGregor J D Pryce and A J White ldquoThe
numerical index of a normed spacerdquo Journal of the LondonMathematical Society Second Series vol 2 pp 481ndash488 1970
[25] Y S Choi S K Kim H J Lee and M Martin ldquoThe Bishop-Phelps-Bollobas theorem for operators on 119871
1(120583)rdquo Preprint
[26] H E Lacey The Isometric Theory of Classical Banach SpacesSpringer New York NY USA 1974
[27] G Godefroy ldquoUniversal spaces for strictly convex Banachspacesrdquo Revista de la Real Academia de Ciencias Exactas Fısicasy Naturales A vol 100 no 1-2 pp 137ndash146 2006
[28] C Finet M Martın and R Paya ldquoNumerical index andrenormingrdquo Proceedings of the American Mathematical Societyvol 131 no 3 pp 871ndash877 2003
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Differential EquationsInternational Journal of
Volume 2014
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Journal of
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Mathematical PhysicsAdvances in
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OptimizationJournal of
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International Journal of
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Operations ResearchAdvances in
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Function Spaces
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Abstract and Applied Analysis 7
Then there exist a norm-one bounded linear operator 119879] forsome ] isin 119872(119898119898) and a nonnegative simple function 119891
1
in 1198781198711(119898)
such that 119879120583minus 119879] ⩽ 120576 1198911 minus 1198910 ⩽ 3120576 and
(119889|]|1119889119898)(119909) = 1 for all 119909 isin supp(1198911)
Lemma 14 Suppose that 119879] isin L(1198711(119898)) is a norm-one
operator 119891 = sum119899
119894=1120573119894(120594
119861119894
119898(119861119894)) where 119898(119861
119895) gt 0 for all
1 ⩽ 119895 ⩽ 119899 and 119861119895119899
119895=1are mutually disjoint Borel subsets ofΩ
is a norm-one nonnegative simple function and 119892 is an elementof 119878
119871infin(119898)
such that
Re ⟨119892 119879]119891⟩ ⩾ 1 minus1205766
27(50)
for some 0 lt 120576 lt 1 and
119889|]|1
119889119898(119909) = 1 119892 (119909) = 1 (51)
for all x in the support of 119891Then there exist a nonnegative simple function 119891 isin 119878
1198711(119898)
a function119892 isin 119878
119871infin(119898)
and an operator119879] inL(1198711(119898) 1198711(119898))such that
⟨119892 119879]119891⟩ =1003817100381710038171003817119879]1003817100381710038171003817 = 1
1003817100381710038171003817119879] minus 119879]1003817100381710038171003817 ⩽ 2120576
10038171003817100381710038171003817119891 minus 119891
10038171003817100381710038171003817⩽ 3120576
1003817100381710038171003817119892 minus 1198921003817100381710038171003817 ⩽radic120576 ⟨119891 119892⟩ = 1
(52)
Proof Since
Re ⟨119892 119879]119891⟩ ⩾ 1 minus1205766
27 (53)
we have
1 minus1205766
27lt Re ⟨119892 119879]119891⟩
= intΩtimesΩ
119891 (119909)Re119892 (119910) 119889120583 (119909 119910)
=
119899
sum
119895=1
120573119895intΩtimesΩ
120594119861119895
(119909)
119898 (119861119895)
Re119892 (119910) 119889] (119909 119910)
(54)
Let
119869 = 119895 intΩtimesΩ
(
120594119861119895
(119909)
119898 (119861119895)
)Re119892 (119910) 119889] (119909 119910) gt 1 minus 1205763
26
(55)
Then from Lemma 11 we have sum119895isin119869120573119895gt 1 minus 120576
32 Let 119891
1=
sum119895isin119869120573119895(120594
119861119895
119898(119861119895)) where 120573
119895= 120573
119895(sum
119895isin119869120573119895) for all 119895 isin 119869
Then
10038171003817100381710038171198911 minus 1198911003817100381710038171003817 ⩽
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
(120573119895minus 120573
119895)
120594119861119895
119898(119861119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+ sum
119895isin119869
120573119895⩽ 120576
3⩽ 120576 (56)
Note that there is a Borel measurable function ℎ on Ω times Ωsuch that 119889](119909 119910) = ℎ(119909 119910) 119889|]|(119909 119910) and |ℎ(119909 119910)| = 1 forall (119909 119910) isin Ω times Ω Let
119862 = (119909 119910) 1003816100381610038161003816119892 (119910) ℎ (119909 119910) minus 1
1003816100381610038161003816 ltradic120576
232 (57)
Define two measures ]119891and ]
119888as follows
]119891(119860) = ] (119860 119862) ]
119888(119860) = ] (119860 cap 119862) (58)
for every Borel subset 119860 ofΩ times Ω It is clear that
119889] = 119889]119891+ 119889]
119888 119889
10038161003816100381610038161003816]119891
10038161003816100381610038161003816= ℎ 119889]
119891
1198891003816100381610038161003816]1198881003816100381610038161003816 = ℎ 119889]119888 119889 |]| = 119889
10038161003816100381610038161003816]119891
10038161003816100381610038161003816+ 1198891003816100381610038161003816]1198881003816100381610038161003816
(59)
Since (119889|]|11198891198981)(119909) = 1 for all 119909 isin ⋃119899
119895=1119861119895 we have
1 =119889|]|1
1198891198981
(119909) =
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909) +1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909) (60)
for all 119909 isin 119861 = ⋃119899
119895=1119861119895 and we deduce that |]|1(119861
119895) = 119898
1(119861
119895)
for all 1 ⩽ 119895 ⩽ 119899We claim that |]
119891|1(119861
119895)119898
1(119861
119895) ⩽ 120576
22
2 for all 119895 isin 119869Indeed if |119892(119910)ℎ(119909 119910) minus 1| ⩾ radic120576232 then
Re (119892 (119910) ℎ (119909 119910)) ⩽ 1 minus 12057624 (61)
So we have
1 minus1205763
26
⩽1
1198981(119861
119895)
ReintΩtimesΩ
120594119861119895(119909)119892 (119910) 119889] (119909 119910)
=1
1198981(119861
119895)
intΩtimesΩ
120594119861119895(119909)
Re (119892 (119910) ℎ (119909 119910)) 119889 |]| (119909 119910)
=1
1198981(119861
119895)
intΩtimesΩ
120594119861119895(119909)
Re (119892 (119910) ℎ (119909 119910)) 119889 10038161003816100381610038161003816]11989110038161003816100381610038161003816(119909 119910)
+1
1198981(119861
119895)
intΩtimesΩ
120594119861119895(119909)
Re (119892 (119910) ℎ (119909 119910)) 119889 1003816100381610038161003816]1198881003816100381610038161003816 (119909 119910)
⩽1
1198981(119861
119895)
((1 minus120576
24)10038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
(119861119895) +1003816100381610038161003816]1198881003816100381610038161003816
1
(119861119895))
= 1 minus120576
24
10038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
(119861119895)
1198981(119861
119895)
(62)
This proves our claimWe also claim that for each 119895 isin 119869 there exists a Borel
subset 119861119895of 119861
119895such that
(1 minus120576
2)119898
1(119861
119895) ⩽ 119898
1(119861
119895) ⩽ 119898
1(119861
119895)
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909) ⩽120576
2
(63)
8 Abstract and Applied Analysis
for all 119909 isin 119861119895 Indeed set 119861
119895= 119861
119895cap 119909 isin Ω (119889|]
119891|1119889119898
1)
(119909) ⩽ 1205762 Then
int119861119895119861119895
120576
2119889119898
1(119909) ⩽ int
119861119895
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909) 1198891198981(119909)
=10038161003816100381610038161003816]1119891
10038161003816100381610038161003816(119861
119895) ⩽1205762
22119898
1(119861
119895)
(64)
This shows that 1198981(119861
119895 119861
119895) ⩽ (1205762)119898
1(119861
119895) This proves our
second claimNow we define 119892 by 119892(119910) = 119892(119910)|119892(119910)| if |119892(119910)| ⩾ 1 minus
radic120576232 and 119892(119910) = 119892(119910) if |119892(119910)| lt 1 minus radic120576232 and we
write 119891 = sum119895isin119869120573119895(120594
119861119895
1198981(119861
119895)) It is clear that 119892 isin 119878
119871infin(119898)
119892 minus 119892 lt radic120576 and 119892(119910) = 1 for all 119909 isin supp119891
Finally we define the measure
119889] (119909 119910)
= sum
119895isin119869
120594119861119895
(119909) 119892 (119910)ℎ (119909 119910)119889]119888(119909 119910)(
1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+ 1205941198691119861(119909) 119889] (119909 119910)
(65)
where 119861 = ⋃119895isin119869119861119895 It is easy to see that (119889|]|1119889119898
1)(119909) = 1
on 119861 and (119889|]|11198891198981)(119909) ⩽ 1 elsewhere Note that
119889 (] minus ]) (119909 119910)
= sum
119895isin119869
120594119861119895
(119909) [
[
119892 (119910)ℎ (119909 119910)(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1]
]
119889]119888(119909 119910)
minus sum
119895isin119869
120594119861119895
(119909) 119889]119891(119909 119910)
(66)
If (119909 119910) isin 119862 then |119892(119910)| ⩾ 1 minus radic120576232 ⩾ 1 minus 1232 and
100381610038161003816100381610038161003816119892 (119910)ℎ (119909 119910) minus 1
100381610038161003816100381610038161003816
=
100381610038161003816100381610038161003816100381610038161003816
119892 (119910)
1003816100381610038161003816119892 (119910)1003816100381610038161003816
ℎ (119909 119910) minus 1
100381610038161003816100381610038161003816100381610038161003816
⩽
1003816100381610038161003816119892 (119910) ℎ (119909 119910) minus 11003816100381610038161003816
1003816100381610038161003816119892 (119910)1003816100381610038161003816
+
10038161003816100381610038161 minus1003816100381610038161003816119892 (119910)
1003816100381610038161003816
10038161003816100381610038161003816100381610038161003816119892 (119910)
1003816100381610038161003816
⩽ 2
1003816100381610038161003816119892 (119910) ℎ (119909 119910) minus 11003816100381610038161003816
1003816100381610038161003816119892 (119910)1003816100381610038161003816
⩽ 2radic120576
232
232
232 minus 1⩽ 2radic120576
(67)
Hence for all (119909 119910) isin 119862 we have
10038161003816100381610038161003816100381610038161003816100381610038161003816
119892 (119910)ℎ (119909 119910)(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
⩽100381610038161003816100381610038161003816119892 (119910)ℎ (119909 119910) minus 1
100381610038161003816100381610038161003816(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+
10038161003816100381610038161003816100381610038161003816100381610038161003816
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
⩽ 2radic120576(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+
10038161003816100381610038161003816100381610038161003816100381610038161003816
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
(68)
So we have for all 119909 isin 1198691
119889|] minus ]|1
1198891198981
(119909)
⩽ sum
119895isin119869
120594119861119895
(119909) [
[
2radic120576(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+
10038161003816100381610038161003816100381610038161003816100381610038161003816
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
]
]
1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909)
+ sum
119895isin119869
120594119861119895
(119909)
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909)
⩽ sum
119895isin119869
120594119861119895
(119909)(2radic120576 + (1 minus1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909)))
+sum
119895isin119869
120594119861119895
(119909)(
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909))
⩽ 2radic120576 + 120576 lt 3radic120576
(69)
This gives that 119879] minus 119879] lt 3radic120576 Note also that for all 119895 isin 119869
⟨119879]
120594119861119895
1198981(119861
119895)
119892⟩
= intΩtimesΩ
120594119861119895
(119909)
1198981(119861
119895)
119892 (119910) 119889] (119909 119910)
= intΩtimesΩ
120594119861119895
(119909)
1198981(119861
119895)
ℎ (119909 119910)(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
119889]119888(119909 119910)
Abstract and Applied Analysis 9
= intΩ
120594119861119895
(119909)
1198981(119861
119895)
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
1198891003816100381610038161003816]1198881003816100381610038161003816
1
(119909)
= intΩ
120594119861119895
(119909)
1198981(119861
119895)
1198891198981(119909) = 1
(70)
Hence we get ⟨119879]119891 119892⟩ = 1 which implies that 119879]119891 =119879] = 1 Finally10038171003817100381710038171003817119891 minus 119891
10038171003817100381710038171003817
⩽10038171003817100381710038171003817119891 minus 119891
1
10038171003817100381710038171003817+10038171003817100381710038171198911 minus 119891
1003817100381710038171003817
=
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
1198981(119861
119895)
minus sum
119895isin119869
120573119895
120594119861119895
1198981(119861
119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+ 120576
⩽ sum
119895isin119869
120573119895(
10038171003817100381710038171003817100381710038171003817100381710038171003817
120594119861119895
1198981(119861
119895)
minus
120594119861119895
1198981(119861
119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+
10038171003817100381710038171003817100381710038171003817100381710038171003817
120594119861119895
1198981(119861
119895)
minus
120594119861119895
1198981(119861
119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
) + 120576
= 2sum
119895isin119869
120573119895
1198981(119861
119895 119861
119895)
1198981(119861
119895)
+ 120576
⩽ 2sum
119895isin119869
120573119895
(1205762)1198981(119861
119895)
1198981(119861
119895)
+ 120576 ⩽120576
1 minus 1205762+ 120576 lt 3120576
(71)
We are now ready to present the proof of the main resultin the case of finite regular positive Borel measures
Proof of Proposition 10 Let 1205751= 120575
3
2(5sdot2
4) 120575
2= 120575
12
3(3
2sdot2
14)
and 1205753= (12057610)
2 for some 0 lt 120576 lt 1 Suppose that 119879 isinL(119871
1(119898)) with 119879 = 1 and that there is an 119891
0isin 119878
1198711(119898)
and1198920isin 119878
119871infin(119898)
such that ⟨1198910 119892
0⟩ = 1 and |⟨119879119891
0 119892
0⟩| gt 1minus120575
3
12
6Then there is an isometric isomorphism Ψ from 119871
1(119898) onto
itself such thatΨ(1198910) = |119891
0| and there is a scalar number 120572 in
119878R such that |⟨1198791198910 119892
0⟩| = ⟨120572119879119891
0 119892
0⟩ Then letting 119891
1= Ψ119891
0
1198921= (Ψ
minus1)lowast1198920 and 119879
1= 120572Ψ119879Ψ
minus1 we have
⟨1198781198911 119892
1⟩ = ⟨120572Ψ119879
0Ψ
minus1Ψ119891
0 (Ψ
minus1)lowast
1198920⟩
= ⟨1205721198791198910 119892
0⟩ gt 1 minus
1205753
1
26
⟨1198911 119892
1⟩ = ⟨Ψ119891
0 (Ψ
minus1)lowast
1198920⟩ = 1
(72)
Since 11987911198911 gt 1 minus 120575(120575
3
12
6) by Lemma 13 there exists a
norm-one bounded operator 119879] and a nonnegative simplefunction119891
2isin 119878
1198711(119898)
such that 1198791minus119879] ⩽ 1205751 1198912minus1198911 ⩽ 31205751
and (119889|]|11198891198981)(119909) = 1 for all 119909 isin supp(119891
2) Then
⟨119879]1198912 1198921⟩
= ⟨11987911198911 119892
1⟩ minus ⟨119879
11198911minus 119879
11198912 119892
1⟩ minus ⟨119879
11198912minus 119879]1198912 1198921⟩
⩾ ⟨11987911198911 119892
1⟩ minus10038171003817100381710038171198911 minus 1198912
1003817100381710038171003817 minus10038171003817100381710038171198791 minus 119879]
1003817100381710038171003817
⩾ 1 minus1205753
1
26minus 3120575
1minus 120575
1⩾ 1 minus 5120575
1= 1 minus
1205753
2
16
(73)
Notice also that
⟨1198912 119892
1⟩ = ⟨119891
1 119892
1⟩ minus ⟨119891
1minus 119891
2 119892
1⟩ ⩾ 1 minus
10038171003817100381710038171198911 minus 11989121003817100381710038171003817
⩾ 1 minus 31205751⩾ 1 minus 5120575
1= 1 minus
1205753
2
16
(74)
By Lemma 12 there are a nonnegative simple function 1198913isin
1198781198711(119898)
and a function 1198923isin 119878
119871infin(119898)
such that
1198923(119909) = 120594supp119891
3
(119909) + 1198922(119909) 120594
Ωsupp1198913
(119909)
10038171003817100381710038171198912 minus 11989131003817100381710038171003817 ⩽ 1205752
10038171003817100381710038171198923 minus 11989211003817100381710038171003817 ⩽radic120575
2 ⟨119891
3 119892
3⟩ = 1
(75)
So we have
⟨119879]1198913 1198923⟩ = ⟨119879]1198912 1198921⟩ minus ⟨119879]1198912 minus 119879]1198913 1198921⟩
minus ⟨119879]1198913 1198921 minus 1198923⟩
⩾ 1 minus1205753
2
16minus 2radic120575
2⩾ 1 minus 3radic120575
2= 1 minus
1205756
3
27
(76)
By Lemma 14 there exist 1198914isin 119878
1198711(119898)
and 1198924isin 119878
119871infin(119898)
and anoperator 119879
4such that
⟨1198924 119879
41198914⟩ = 1 =
100381710038171003817100381711987941003817100381710038171003817
10038171003817100381710038171198794 minus 119879]1003817100381710038171003817 ⩽ 21205753
10038171003817100381710038171198914 minus 11989131003817100381710038171003817 ⩽ 31205753
10038171003817100381710038171198924 minus 11989231003817100381710038171003817 ⩽radic120575
3 ⟨119891
4 119892
4⟩ = 1
(77)
So we have
10038171003817100381710038171198794 minus 11987911003817100381710038171003817 ⩽10038171003817100381710038171198794 minus 119879]
1003817100381710038171003817 +1003817100381710038171003817119879] minus 1198791
1003817100381710038171003817
⩽ 1205751+ 2120575
3⩽ 3120575
3
10038171003817100381710038171198911 minus 11989141003817100381710038171003817 ⩽10038171003817100381710038171198911 minus 1198912
1003817100381710038171003817 +10038171003817100381710038171198912 minus 1198913
1003817100381710038171003817 +10038171003817100381710038171198913 minus 1198914
1003817100381710038171003817
⩽ 31205751+ 120575
2+ 3120575
3⩽ 10120575
3
10038171003817100381710038171198921 minus 11989241003817100381710038171003817 ⩽10038171003817100381710038171198921 minus 1198923
1003817100381710038171003817 + 1198923 minus 1198924
⩽ 1205752+ radic120575
3⩽ 2radic120575
3
(78)
10 Abstract and Applied Analysis
Let 119878 = 120572Ψminus11198794Ψ 119891 = Ψminus1
1198914 and 119892 = Ψlowast
1198924 then we have
119879 minus 119878 =10038171003817100381710038171003817119879 minus 120572Ψ
minus11198794Ψ10038171003817100381710038171003817=10038171003817100381710038171003817120572Ψ119879Ψ
minus1minus 119879
4
10038171003817100381710038171003817
=10038171003817100381710038171198791 minus 1198794
1003817100381710038171003817 ⩽ 31205753
100381710038171003817100381710038171198910minus 11989110038171003817100381710038171003817=100381710038171003817100381710038171198910minus Ψ
minus11198914
10038171003817100381710038171003817=10038171003817100381710038171198911 minus 1198914
1003817100381710038171003817 ⩽ 101205753
100381710038171003817100381710038171198920minus 11989110038171003817100381710038171003817=10038171003817100381710038171198920 minus Ψ
lowast1198924
1003817100381710038171003817 =100381710038171003817100381710038171003817(Ψ
minus1)lowast
1198920minus 119892
4
100381710038171003817100381710038171003817
=10038171003817100381710038171198921 minus 1198924
1003817100381710038171003817 ⩽ 2radic120575
3
⟨119891 119892⟩ = ⟨Ψminus11198914 Ψ
lowast1198924⟩ = ⟨119891
4 119892
4⟩ = 1
10038161003816100381610038161003816⟨119878119891 119892⟩
10038161003816100381610038161003816=10038161003816100381610038161003816⟨120572Ψ
minus11198794ΨΨ
minus11198914 Ψ
lowast1198924⟩10038161003816100381610038161003816= |120572| = 1
(79)
This completes the proof
Finally we may give the proof of the main result in fullgenerality
Proof of Theorem 9 Notice that the Kakutani representationtheorem (see [26] for a reference) says that for every 120590-finitemeasure ] the space 119871
1(]) is isometrically isomorphic to
1198711(119898) for some positive Borel regular measure on a compact
Hausdorff space Then by Proposition 10 there is a universalfunction 120576 997891rarr 120578(120576) gt 0 which gives the BPBp-nu for 119871
1(]) for
every 120590-finite measure ]Fix 120576 gt 0 Suppose that 119879
0isin L(119871
1(120583)) with V(119879
0) = 1
and (1198910 119891
lowast
0) isin Π(119871
1(120583)) satisfy
1003816100381610038161003816⟨119891lowast
0 119879
01198910⟩1003816100381610038161003816 gt 1 minus 120578 (120576) (80)
Choose a sequence 119891119899 in 119871
1(120583) such that sup
119899119879
0119891119899 = 1
and let 119866 be the closed linear span of
119879119899
0119891119898 119899 119898 isin N cup 0 (81)
As 119866 is separable there is a dense subset 119892119899 119899 isin N of 119866
and let 119864 = ⋃infin
119899=1supp119892
119899 where supp119892
119899is the support of 119892
119899
Then the measure 120583|119864is 120590-finite Let
119884 = 119891 isin 1198711(120583) supp (119891) sub 119864 (82)
be a closed subspace of 1198711(120583) It is clear that 119871
1(120583) = 119884oplus
1119885
and 119884 is isometrically isomorphic to 1198711(120583|
119864) So 119884 has the
BPBp-nu with 120578(120576)Now write 119878
0= 119879
0|119884 119884 rarr 119884 consider 119910
0= 119891
0isin
119878119884 119910lowast
0= 119891
lowast
0|119884isin 119878
119884lowast and observe that 119910lowast
0(119910
0) = 1 and
|119910lowast
0(119878
01199100)| = |119891
lowast
0(119879
01198910)| gt 1 minus 120578(120576) Hence there exist 119878 isin
L(119884) and (1199100 119910
lowast
0) isin Π(119884) such that
1003816100381610038161003816119910lowast
0(119878119910
0)1003816100381610038161003816 = 1 = V (119878) 1003817100381710038171003817119878 minus 1198780
1003817100381710038171003817 lt 120576
10038171003817100381710038171199100 minus 11991001003817100381710038171003817 lt 120576
1003817100381710038171003817119910lowast
0minus 119910
lowast
0
1003817100381710038171003817 lt 120576
(83)
Finally consider the operator 119879 isinL(1198711(120583)) given by
119879 (119910 119911)
= (119878119910 0) + 1198790(0 119911) ((119910 119911) isin 119871
1(120583) equiv 119884oplus
1119885)
(84)
We have 119879 = 1 (and so V(119879) = 1) Indeed1003817100381710038171003817119879 (119910 119911)
1003817100381710038171003817 =1003817100381710038171003817(119878119910 0)
1003817100381710038171003817 +10038171003817100381710038171198790 (0 119911)
1003817100381710038171003817 ⩽10038171003817100381710038171199101003817100381710038171003817 + 119911 =
1003817100381710038171003817(119910 119911)1003817100381710038171003817
(85)
for all (119910 119911) isin 1198711(120583) and 119879(119910
0 0) = (119878119910
0 0) = 119878119910
0 = 1
Let 119909 = (1199100 0) and 119909lowast = (119910lowast
0 119891
0|119885) Then (119909 119909lowast) isin Π(119871
1(120583))
Moreover we have1003816100381610038161003816119909
lowast1198791199091003816100381610038161003816 =1003816100381610038161003816119910
lowast
0119878119910
0
1003816100381610038161003816 = 1 = V (119879) 1003817100381710038171003817119909 minus 1198910
1003817100381710038171003817 =1003817100381710038171003817119910 minus 1199100
1003817100381710038171003817 lt 120576
1003817100381710038171003817119909lowast
0minus 119891
lowast
0
1003817100381710038171003817
= max 1003817100381710038171003817119910 minus 119891lowast
0|119884
1003817100381710038171003817 1003817100381710038171003817119891
lowast
0|119885minus 119891
lowast
0|119885
1003817100381710038171003817
=1003817100381710038171003817119910
lowastminus 119910
lowast
0
1003817100381710038171003817 lt 120576
1003817100381710038171003817119879 minus 11987901003817100381710038171003817 = sup119910+119911⩽1
1003817100381710038171003817119879 (119910 119911) minus 1198790 (119910 119911)1003817100381710038171003817
= sup119910⩽1
1003817100381710038171003817119878119910 minus 11987801199101003817100381710038171003817 =1003817100381710038171003817119878 minus 1198780
1003817100381710038171003817 lt 120576
(86)
This completes the proof
5 Examples of Spaces Failing theBishop-Phelps-Bollobaacutes Property forNumerical Radius
Our goal here is to prove that the density of numerical radiusattaining operators does not imply the BPBp-nu Actuallywe will show that among separable spaces there is noisomorphic property implying the BPBp-nu other than finite-dimensionality
We need to relate the BPBp-nu to the Bishop-Phelps-Bollobas property for operators which as mentioned in theintroduction was introduced in [15] A pair (119883 119884) of Banachspaces has the Bishop-Phelps-Bollobas property for operators(in short BPBp) if given 120576 gt 0 there exists 120578(120576) gt 0 suchthat given 119879 isin L(119883 119884) with 119879 = 1 and 119909 isin 119878
119883such that
119879119909 gt 1 minus 120578(120576) then there exist 119911 isin 119878119883and 119878 isin L(119883 119884)
satisfying
119878 = 119878119911 = 1 119909 minus 119911 lt 120576
119879 minus 119878 lt 120576
(87)
We refer the reader to [15 19 25] and references therein formore information and background Among the interestingresults on the BPBp we emphasize that a pair (119883 119884) when119883is finite-dimensional does not necessarily have the BPBp Forinstance if119884 is a strictly convex space which is not uniformlyconvex then the pair (ℓ(2)
1 119884) fails to have the BPBp (this is
contained in [15] see [19 Section 3])
Abstract and Applied Analysis 11
The next result relates the BPBp-nu to the BPBp foroperators in a particular case We will deduce our examplefrom it
Theorem 15 If 1198711(120583)oplus
1119883 has the BPBp-119899119906 then the pair
(1198711(120583) 119883) has the BPBp for operators
Before proving this proposition we will use it to get themain examples of this section The first example shows thatthe density of numerical radius attaining operators does notimply the BPBp-nu
Example 16 There is a reflexive space (and so numerical radiusattaining operators on it are dense) which fails to have theBPBp-nu Indeed let 119884 be a reflexive separable space whichis not superreflexive and we may suppose that 119884 is strictlyconvex Observe that 119884 cannot be uniformly convex since itis not superreflexive Now 119883 = ℓ(2)
1oplus1119884 is reflexive but the
pair (ℓ(2)1 119884) fails the BPBp since 119884 is strictly convex but not
uniformly convex [19 Corollary 33] Therefore Theorem 15gives us that119883 does not have the BPBp-nu
The example above can be extended to get the result thatevery infinite-dimensional separable Banach space can berenormed to fail the BPBp-nu This follows from the factthat every infinite-dimensional separable Banach space canbe renormed to be strictly convex but not uniformly convex(this result can be proved ldquoby handrdquo an alternative categoricalargument for it can be found in [27] and references therein)With a little more of effort we may get the main result of thesection
Theorem 17 Every infinite-dimensional separable Banachspace can be renormed to fail the weak-BPBp-nu (and so inparticular to fail the BPBp-nu)
We need the following result which is surely well knownAs we have not found a reference we include a nice and easyproof kindly given to us by Vladimir Kadets We recall thatgiven a Banach space 119884 the set of all equivalent norms on119884 can be viewed as a metric space using the Banach-Mazurdistance
Lemma 18 Let119884 be an infinite-dimensional separable Banachspace Then the set of equivalent norms on 119884 which are strictlyconvex and are not (locally) uniformly convex is dense in the setof all equivalent norms on119884 (with respect to the Banach-Mazurdistance)
Proof Fix 119890 isin 119878119884and 119890lowast
1isin 119878
119884lowast such that 119890lowast
1(119890) = 1 For a fixed
120576 isin (0 12) denote
119902 (119910) = max (1 minus 120576) 10038171003817100381710038171199101003817100381710038171003817 1003816100381610038161003816119890lowast
1(119910)1003816100381610038161003816 (119910 isin 119884) (88)
Evidently (1 minus 120576)119910 ⩽ 119902(119910) ⩽ 119910 for every 119910 isin 119884 Fix asequence 119890lowast
119896 119896 ⩾ 2 of norm-one functionals separating
the points of 119884 and denote
119901 (119910) = radic
infin
sum
119896=1
1
2119896
1003816100381610038161003816119890lowast
119896(119910)1003816100381610038161003816
2
(119910 isin 119884) (89)
Then 119901 is a strictly convex norm on 119884 119901(119890) ⩾ 1radic2 and119901(119910) ⩽ 119910 for all 119910 isin 119883 Finally write
100381710038171003817100381711991010038171003817100381710038171= (1 minus 120576) 119902 (119910) + 120576
119901 (119910)
119901 (119890)(119910 isin 119884) (90)
Then sdot 1is a strictly convex norm on 119884 and
(1 minus 120576)2 10038171003817100381710038171199101003817100381710038171003817 ⩽100381710038171003817100381711991010038171003817100381710038171⩽ (1 + 120576)
10038171003817100381710038171199101003817100381710038171003817 (119910 isin 119884) (91)
Wewill finish the proof by showing that sdot 1is not uniformly
convex (actually it is not locally uniformly convex) Indeedfor each 119899 isin N we select 119910
119899isin ⋂
119899
119896=1ker 119890lowast
119896with 119910
119899 = 1 and
consider 119890119899= 119890 + (1205764)119910
119899 Then 119902(119890) = 1 119902(119890
119899) = 1 and
119902(119890+119890119899) = 2 At the same time 119901(119910
119899) rarr 0 so 119901(119890
119899) rarr 119901(119890)
and 119901(119890 + 119890119899) rarr 2119901(119890) Consequently
1198901 = 1100381710038171003817100381711989011989910038171003817100381710038171997888rarr 1
1003817100381710038171003817119890 + 11989011989910038171003817100381710038171997888rarr 2 (92)
but 119890 minus 1198901198991= (1205764)119910
1198991⩾ (1 minus 120576)
2(1205764) which means the
absence of local uniform convexity at 119890
Proof of Theorem 17 Let 119883 be an infinite-dimensional sepa-rable Banach space Take a closed subspace 119884 of 119883 of codi-mension two By [28 Proposition 2] the map carrying everyequivalent norm on 119884 to its numerical index is continuousand so the set of values of the numerical index of 119884 upto reforming is a nontrivial interval [28 Theorem 9] ThenLemma 18 allows us to find an equivalent norm | sdot | on 119884 insuch a way that (119884 | sdot |) is strictly convex and is not uniformlyconvex and 119899(119884 | sdot |) gt 0 Now the space 119883 = ℓ(2)
1oplus1(119884 | sdot |)
is an equivalent renorming of 119883 which does not have theBPBp-119899119906 (indeed otherwise the pair (ℓ(2)
1 (119884 | sdot |)) would
have the BPBp for the operator norm and so (119884 | sdot |) wouldbe uniformly convex by [19 Corollary 33] a contradiction)Moreover as
119899 (119883) = min 119899 (ℓ(2)1) 119899 (119884 |sdot|) gt 0 (93)
(see [20 Proposition 2] for instance) 119883 also fails the weak-BPBp-nu by Proposition 6
To finish the section with the promised proof ofTheorem 15 we first see the following stability result
Lemma 19 Let119883 = [⨁infin
119896=1119883
119896]1198880
or [⨁infin
119896=1119883
119896]ℓ1
If119883 has theBishop-Phelps-Bollobas property for numerical radius with afunction 120578 then each Banach space 119883
119894has the Bishop-Phelps-
Bollobas property for numerical radius with 120578nu(119883119894) ⩾ 120578 That
is inf119894120578nu(119883119894
)(120576) ⩾ 120578nu(119883)(120576) for all 0 lt 120576 lt 1
Proof Let 119875119894 119883 rarr 119883
119894and 1198751015840
119894 119883
lowastrarr 119883
lowast
119894be the natural
projections and let 119876119894 119883
119894rarr 119883 and 1198761015840
119894 119883
lowast
119894rarr 119883
lowast be thenatural embeddings
Assume that an operator 119879119894 119883
119894rarr 119883
119894and a pair
(119909119894 119909
lowast
119894) isin Π(119883
119894) satisfy that
V (119879119894) = 1
1003816100381610038161003816119909lowast
119894119879119894119909119894
1003816100381610038161003816 gt 1 minus 120578 (120576) (94)
We define an operator 119879 119883 rarr 119883 and (119909 119909lowast) isin Π(119883) by
119879 = 119876119894∘ 119879
119894∘ 119875
119894 (119909 119909
lowast) = (119876
119894119909119894 119876
1015840
119894119909lowast
119894) (95)
12 Abstract and Applied Analysis
then clearly we see that1003816100381610038161003816119909
lowast1198791199091003816100381610038161003816 =1003816100381610038161003816119909
lowast
119894119879119894119909119894
1003816100381610038161003816 gt 1 minus 120578 (120576) (96)
From the assumption there exist 119878 119883 rarr 119883 and a pair(119910 119910
lowast) isin Π(119883) such that
1003816100381610038161003816119910lowast1198781199101003816100381610038161003816 = 1 = V (119878) 119878 minus 119879 lt 120576
1003817100381710038171003817119910lowastminus 119909
lowast1003817100381710038171003817 lt 1205761003817100381710038171003817119910 minus 119909
1003817100381710038171003817 lt 120576
(97)
Since this clearly shows that1003817100381710038171003817119875119894 ∘ 119878 ∘ 119876119894
minus 119879119894
1003817100381710038171003817 lt 120576100381710038171003817100381710038171198751015840
119894119910lowastminus 119909
lowast
119894
10038171003817100381710038171003817lt 120576
1003817100381710038171003817119875119894119910 minus 1199091198941003817100381710038171003817 lt 120576
(98)
we only need to show that |1198751015840119894119910lowast(119875
119894∘ 119878 ∘ 119876
119894)119875
119894119910| = 1
We first show the case of 1198880sum Since
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817=10038171003817100381710038171003817119875119895119910 minus 119875
11989511990910038171003817100381710038171003817⩽1003817100381710038171003817119910 minus 119909
1003817100381710038171003817 lt 120576 (99)
for every 119895 = 119894 we have
1 = 119910lowast(119910) = sum
119895isinN
1198751015840
119895119910lowast(119875
119895119910) ⩽ sum
119895isinN
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817
⩽100381710038171003817100381710038171198751015840
119894119910lowast10038171003817100381710038171003817+ 120576 sum
119895isinN119895 = 119894
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817⩽1003817100381710038171003817119910
lowast1003817100381710038171003817 = 1
(100)
This shows that 1198751015840119894119910lowast = 1 and 1198751015840
119895119910lowast= 0 for every 119895 = 119894
So 119910lowast = 1198761015840
1198941198751015840
119894119910lowast and 1198751015840
119894119910lowast(119875
119894119910) = 1 This and the fact that
119910 minus 119876119894119875119894119910 lt 120576 imply that
(119876119894119875119894119910 + (
1
120576) (119910 minus 119876
119894119875119894119910) 119876
1015840
1198941198751015840
119894119910lowast) isin Π (119883) (101)
So we get that (1198761015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910 + (1120576)(119910 minus 119876
119894119875119894119910))V(119878) = 1
Hence we have
1 =1003816100381610038161003816119910
lowast1198781199101003816100381610038161003816 =10038161003816100381610038161003816(119876
1015840
1198941198751015840
119894119910lowast) 11987811991010038161003816100381610038161003816
=
1003816100381610038161003816100381610038161003816(1 minus 120576) (119876
1015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910)
+120576 (1198761015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910 +1
120576(119910 minus 119876
119894119875119894119910))
1003816100381610038161003816100381610038161003816⩽ 1
(102)
and so we get |1198751015840119894119910lowast(119875
119894∘119878 ∘119876
119894)119875
119894119910| = |(119876
1015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910)| = 1
We next show the case of ℓ1sum The proof is almost
the same as that of the 1198880case However for the sake of
completeness we provide it hereSince 1198751015840
119895119910lowast = 119875
1015840
119895119910lowastminus 119875
1015840
119895119909lowast ⩽ 119910
lowastminus 119909
lowast lt 120576 for every
119895 = 119894 we have
1 = 119910lowast(119910) = sum
119895isinN
1198751015840
119895119910lowast(119875
119895119910) ⩽ sum
119895isinN
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817
⩽10038171003817100381710038171198751198941199101003817100381710038171003817 + 120576 sum
119895isinN119895 = 119894
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817⩽10038171003817100381710038171199101003817100381710038171003817 = 1
(103)
which shows 119875119894119910 = 1 and 119875
119895119910 = 0 for every 119895 = 119894 Since this
implies (119876119894119875119894119910119876
1015840
1198941198751015840
119894119910lowast+(1120576)(119910
lowastminus119876
1015840
1198941198751015840
119894119910lowast)) isin Π(119883) we get
that |(1198761015840
1198941198751015840
119894119910lowast+ (1120576)(119910
lowastminus 119876
1015840
1198941198751015840
119894119910lowast))119878(119876
119894119875119894119910)| ⩽ V(119878) = 1
Hence we have
1 =1003816100381610038161003816119910
lowast1198781199101003816100381610038161003816 =1003816100381610038161003816119910
lowast119878 (119876
119894119875119894119910)1003816100381610038161003816
=
1003816100381610038161003816100381610038161003816(1 minus 120576) (119876
1015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910)
+120576 (1198761015840
1198941198751015840
119894119910lowast+1
120576(119910
lowastminus 119876
1015840
1198941198751015840
119894119910lowast)) 119878 (119876
119894119875119894119910)
1003816100381610038161003816100381610038161003816⩽ 1
(104)
and so |1198751015840119894119910lowast(119875
119894∘ 119878 ∘ 119876
119894)119875
119894119910| = |(119876
1015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910)| = 1
Proof of Theorem 15 Note that 120578nu(1198711(120583)oplus1119883)(120576) rarr 0 as120576 rarr 0 Fix 0 lt 120576
0lt 1 and choose 0 lt 120576 lt 1 such that
6120576 + 120578nu(1198711(120583)oplus1119883)(120576) lt 1205760 Let 120578(1205760) = 120578nu(1198711(120583)oplus1119883)(120576)Suppose that 119879
0isin L(119871
1(120583) 119883) with 119879
0 = 1 and 119891
0isin
1198781198711(120583)
satisfy
1003817100381710038171003817119879011989101003817100381710038171003817 gt 1 minus 120578 (1205760) (105)
For any measurable subset 119861 let
1198711(120583|
119861) = 119891|
119861 119891 isin 119871
1(120583) (106)
with the norm 119891|119861 = 119891120594
1198611 Then it is easy to see
that 1198711(120583|
119861) is isometrically isomorphic to a complemented
subspace of 1198711(120583) Let 119875
119861 119871
1(120583) rarr 119871
1(120583|
119861) be the
restriction defined by 119875119861(119891) = 119891|
119861for all 119891 isin 119871
1(120583) and
let 119869119861 119871
1(120583|
119861) rarr 119871
1(120583) be the extension defined by
119869119861(119891)(120596) = 119891(120596) if 120596 isin 119861 and 119869
119861(119891)(120596) = 0 otherwise
It is clear that 119875119861119869119861= Id
1198711(120583|119861)and 119869
119861119875119861(119891) = 119891120594
119861for all
119891 isin 1198711(120583) Notice also that 119871
1(120583) is isometrically isomorphic
to 1198711(120583|
119861)oplus
11198711(120583|
119861119888)
Let 119860 = supp1198910and 119892
0= 119875
1198601198910 Then
1003817100381710038171003817119879011986911986011989201003817100381710038171003817 =100381710038171003817100381711987901198910
1003817100381710038171003817 gt 1 minus 120578 (1205760) gt 0 (107)
and define the operator 119879119860 119871
1(120583|
119860) rarr 119883 by 119879
119860119891 =
1198790119869119860119891119879
0119869119860 for every 119891 isin 119871
1(120583|
119860) Then
100381710038171003817100381711987911986011989201003817100381710038171003817 ⩾100381710038171003817100381711987901198910
1003817100381710038171003817 gt 1 minus 120578 (1205760) (108)
Since 120583|119860is 120590-finite 119871
1(120583|
119860)lowast= 119871
infin(120583|
119860) Let 119892lowast
0isin 119878
119871infin(120583|119860)
be a function such that ⟨119892lowast0 119892
0⟩ = 1 and choose 119909lowast
0isin 119878
119883lowast
such that 119909lowast0(119879
1198601198920) = 119879
1198601198920 Define the operator 119878
0isin
L(1198711(120583|
119860)oplus
1119883) by
1198780(119891 119909) = (0 119879
119860119891) ((119891 119909) isin 119871
1(120583|
119860) oplus
1119883) (109)
and observe that 1198780 = V(119878
0) = 1 Indeed
100381710038171003817100381711987801003817100381710038171003817 ⩽ 1 =
10038171003817100381710038171198791198601003817100381710038171003817
= sup 1003816100381610038161003816119909lowast1198791198601198911003816100381610038161003816 119909
lowastisin 119878
119883lowast 119891 isin 119878
1198711(120583|119860)
= sup 1003816100381610038161003816(119891lowast 119909
lowast) 119878
0(119891 119909)
1003816100381610038161003816
((119891lowast 119909
lowast) (119891 119909)) isin Π (119871
1(120583|
119860) oplus
1119883)
= V (1198780) ⩽100381710038171003817100381711987801003817100381710038171003817
(110)
Abstract and Applied Analysis 13
It is immediate that
(119892lowast
0 119909
lowast
0) 119878
0(119892
0 0) = 119909
lowast
0(119879
1198601198920) =10038171003817100381710038171198791198601198920
1003817100381710038171003817 gt 1 minus 120578 (1205760)
(111)
By Lemma 19 1198711(120583|
119860)oplus
1119883 has the BPBp-nu with the func-
tion 120578 Therefore there exist 1198781isin L(119871
1(120583|
119860)oplus
1119883) (119892
1 119909
1) isin
1198781198711(120583|119860)oplus1119883 and (119892lowast
1 119909
lowast
1) isin 119878
119871infin(120583|119860)oplusinfin119883lowast such that
1003817100381710038171003817(1198921 1199091) minus (1198920 0)1003817100381710038171003817 lt 120576
1003817100381710038171003817(119892lowast
1 119909
lowast
1) minus (119891
lowast
0 119909
lowast
0)1003817100381710038171003817 lt 120576
10038171003817100381710038171198781 minus 11987801003817100381710038171003817 lt 120576 ⟨(119892
lowast
1 119909
lowast
1) (119892
1 119909
1)⟩ = 1
1003816100381610038161003816(119892lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)1003816100381610038161003816 = V (119878
1) = 1
(112)
Claim 1 We claim that 1199091= 0
Otherwise
1 = Re ⟨(119892lowast1 119909
lowast
1) (119892
1 119909
1)⟩
=100381710038171003817100381711989211003817100381710038171003817Re⟨(119892
lowast
1 119909
lowast
1) (119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
⟩
+100381710038171003817100381711990911003817100381710038171003817Re⟨(119892
lowast
1 119909
lowast
1) (0 119909
1)
100381710038171003817100381711990911003817100381710038171003817
⟩ ⩽ 1
(113)
We deduce that
((119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
(119892lowast
1 119909
lowast
1)) (
(0 1199091)
100381710038171003817100381711990911003817100381710038171003817
(119892lowast
1 119909
lowast
1))
isin Π (1198711(120583|
119860) oplus
1119883)
(114)
Since 1198781((0 119909
1)119909
1) = (119878
1minus 119878
0)((0 119909
1)119909
1) lt 120576 we
get that
1 =1003816100381610038161003816⟨(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)⟩1003816100381610038161003816
=
100381610038161003816100381610038161003816100381610038161003816
100381710038171003817100381711989211003817100381710038171003817⟨(119892
lowast
1 119909
lowast
1) 119878
1((119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
)⟩
+100381710038171003817100381711990911003817100381710038171003817⟨(119892
lowast
1 119909
lowast
1) 119878
1((0 119909
1)
100381710038171003817100381711990911003817100381710038171003817
)⟩
100381610038161003816100381610038161003816100381610038161003816
⩽100381710038171003817100381711989211003817100381710038171003817 V (1198781) + 120576
100381710038171003817100381711990911003817100381710038171003817 lt100381710038171003817100381711989211003817100381710038171003817 +100381710038171003817100381711990911003817100381710038171003817 = 1
(115)
a contradiction This proves the claimWe define the operator 119878
2 119871
1(120583|
119860)oplus
1119883 rarr 119871
1(120583|
119860)oplus
1119883
by 1198782(119891 119909) = 119878
1(119891 0) for every 119891 isin 119871
1(120583|
119860) and for every
119909 isin 119883 Then we have
V (1198782) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816 = 1
10038171003817100381710038171198781 minus 11987821003817100381710038171003817 ⩽ 120576 (116)
Indeed from Claim 1 we have
V (1198781) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)1003816100381610038161003816 =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816 ⩽ V (119878
2)
(117)
On the other hand we see that1003816100381610038161003816(119891
lowast 119909
lowast) 119878
2(119891 119909)
1003816100381610038161003816
=1003816100381610038161003816(119891
lowast 119909
lowast) 119878
1(119891 0)
1003816100381610038161003816 ⩽10038171003817100381710038171198911003817100381710038171003817 V (1198781) ⩽ V (119878
1)
(118)
for every ((119891lowast 119909lowast) (119891 119909)) isin Π(1198711(120583|
119860)oplus
1119883) Therefore
V(1198782) ⩽ V(119878
1) Also
10038171003817100381710038171198781 minus 11987821003817100381710038171003817 ⩽ sup
119909isin119878119883
10038171003817100381710038171198781 (0 119909)1003817100381710038171003817
= sup119909isin119878119883
10038171003817100381710038171198781 (0 119909) minus 1198780 (0 119909)1003817100381710038171003817 ⩽ 120576
(119)
Claim 2 There exists 1198783 119871
1(120583|
119860)oplus
1119883 rarr 119871
1(120583|
119860)oplus
1119883 such
that 1198783(119892
1 0) = 119878
3 = 1 119878
3(0 119909) = 0 119878
3(119891 119909) isin 0oplus
1119883
for every (119891 119909) isin 1198711(120583|
119860)oplus
1119883 and 119878
3minus 119878
2 lt 4120576
Indeed write 1198781= (119863
1 119863
2) where 119863
1 119871
1(120583|
119860)oplus
1119883 rarr
1198711(120583|
119860) and119863
2 119871
1(120583|
119860)oplus
1119883 rarr 119883 We have that
sup 1003816100381610038161003816119892lowast119863
1(119892
1 0) + 119909
lowast119863
2(119892
1 0)1003816100381610038161003816
119909lowastisin 119878
119883lowast ⟨119892
lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860)
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816 +10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
⟨119892lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860)
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816
⟨119892lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860) +10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
⩽ V (1198782) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816119892
lowast
1119863
1(119892
1 0) + 119909
lowast
1119863
2(119892
1 0)1003816100381610038161003816
(120)
This implies that
1003816100381610038161003816119909lowast
1119863
2(119892
1 0)1003816100381610038161003816 =10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
1003816100381610038161003816119892lowast
1119863
1(119892
1 0)1003816100381610038161003816
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816 ⟨119892
lowast 119892
1⟩ = 1 119892
lowastisin 119871
infin(120583|
119860)
(121)
Therefore |119892lowast1| equals 1 on the support of 119863
1(119892
1 0) As
|⟨119892lowast
1 119892
1⟩| = 1 we also have that |119892lowast
1| equals 1 on the support
of 1198921 Changing the values of 119892lowast
1by the ones of 119891lowast
0on 119860
(supp(1198631(119892
1 0)) cup supp(g
1)) we may and do suppose that
|119892lowast
1| = 1 on the whole 119860We also have 119863
2(119892
1 0) gt 0 Indeed
10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198920 0)1003817100381710038171003817
⩽10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198921 0)
1003817100381710038171003817 +10038171003817100381710038171198780 (1198921 0) minus 1198780 (1198920 0)
1003817100381710038171003817
lt 2120576 + 120576 = 3120576
(122)
So we have
10038171003817100381710038171198632(119892
1 0) minus 119879
1198601198920
1003817100381710038171003817
⩽10038171003817100381710038171198631(119892
1 0)1003817100381710038171003817 +10038171003817100381710038171198632(119892
1 0) minus 119879
1198601198920
1003817100381710038171003817
14 Abstract and Applied Analysis
=1003817100381710038171003817(1198631
(1198921 0) 119863
2(119892
1 0)) minus (0 119879
1198601198920)1003817100381710038171003817
=10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198920 0)
1003817100381710038171003817 lt 3120576
(123)
and 1198632(119892
1 0) gt 119879
1198601198920 minus 3120576 ⩾ 1 minus 120578(120576
0) minus 3120576 gt 0
Finally define the operator 1198783by
1198783(119891 119909) = (0119863
2(119891 0) + 119892
lowast
1(119863
1(119891 0))
1198632(119892
1 0)
119909lowast
1119863
2(119892
1 0))
(124)
for (119891 119909) isin 1198711(120583|
119860)oplus
1119883 It is clear that
100381710038171003817100381711987831003817100381710038171003817 ⩽ sup
119891isin1198781198711(120583|119860)
(10038171003817100381710038171198632(119891 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119891 0)
1003816100381610038161003816) (125)
Notice also that
10038171003817100381710038171198631(119891 0)
1003817100381710038171003817
⩽10038171003817100381710038171198631(119891 0)
1003817100381710038171003817 +10038171003817100381710038171198632(119891 0) minus 119879
1198601198911003817100381710038171003817
=1003817100381710038171003817(1198631
(119891 0) 1198632(119891 0)) minus (0 119879
119860119891)1003817100381710038171003817
=10038171003817100381710038171198782 (119891 119909) minus 1198780 (119891 119909)
1003817100381710038171003817
(126)
for all (119891 119909) isin 1198711(120583|
119860)oplus
1119883 Hence we have
10038171003817100381710038171198783 minus 11987821003817100381710038171003817
= 2 sup119891isin1198781198711(120583|119860)
10038171003817100381710038171198631(119891 0)
1003817100381710038171003817 ⩽ 210038171003817100381710038171198782 minus 1198780
1003817100381710038171003817 lt 4120576(127)
On the other hand let 119866 1198711(120583|
119860) rarr 119871
1(120583|
119860) be defined by
119866(119891) = 119892lowast
1119891 for every 119891 isin 119871
1(120583|
119860) Then we have
V (1198782)
= sup 1003816100381610038161003816119911lowast11987821199111003816100381610038161003816 (119911 119911
lowast) isin Π (119871
1(120583|
119860) oplus
1119883)
⩾ sup10038161003816100381610038161003816100381610038161003816
119909lowast119863
2(119866(
1
120583 (119862)120594119862) 0)
+119892lowast
1119863
1(119866(
1
120583 (119862)120594119862) 0)
10038161003816100381610038161003816100381610038161003816
119909lowastisin 119878
119883lowast 119862 isin Σ
119860 120583 (119862) gt 0
= sup10038171003817100381710038171003817100381710038171003817
1198632(119866(
1
120583 (119862)120594119862) 0)
10038171003817100381710038171003817100381710038171003817
+
10038161003816100381610038161003816100381610038161003816
119892lowast
1119863
1(119866(
1
120583 (119862)120594119862) 0)
10038161003816100381610038161003816100381610038161003816
119862 isin Σ119860 120583 (119862) gt 0
(128)
where Σ119860is the family of measurable subsets of 119860
Hence for any simple function 119904 = sum119899
119894=1((120572
119894120583(119860
119894))120594
119860119894
) isin
1198781198711(120583|119860) where 119860
119894119894is a family of disjoint measurable subsets
with strictly positive measure we have
V (1198782)
⩾
119899
sum
119894=1
10038161003816100381610038161205721198941003816100381610038161003816 (
100381710038171003817100381710038171003817100381710038171003817
1198632(119866(
1
120583 (119860119894)120594119860119894
) 0)
100381710038171003817100381710038171003817100381710038171003817
+
100381610038161003816100381610038161003816100381610038161003816
119892lowast
1119863
1(119866(
1
120583 (119860119894)120594119860119894
) 0)
100381610038161003816100381610038161003816100381610038161003816
)
⩾10038171003817100381710038171198632(119866 (119904) 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119866 (119904) 0)
1003816100381610038161003816
(129)
Since |119892lowast1| = 1119866 is an isometric isomorphism so for each119891 isin
1198781198711(120583|119860)there exists a sequence of norm-one simple functions
(119904119896) such that 119866(119904
119896) converges to 119891 Therefore
V (1198782) ⩾ sup
119891isin1198781198711(120583|119860)
(10038171003817100381710038171198632(119891 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119891 0)
1003816100381610038161003816) ⩾100381710038171003817100381711987831003817100381710038171003817
(130)
On the other hand we see that100381710038171003817100381711987831003817100381710038171003817 ⩾1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
3(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816119909
lowast
1119863
2(119892
1 0) + 119892
lowast
1119863
1(119892
1 0)1003816100381610038161003816 = V (119878
2) = 1
(131)
Therefore 1 = 1198783 = 119878
3(119892
1 0) which proves Claim 2
Finally set 1198783= (0 ) for a suitable 119871
1(120583|
119860)oplus
1119883 rarr
119883 and define the operator 1198791 119871
1(120583) rarr 119883 by
1198791(119891) = 119879
0(119891120594
119860119888) + (119875
119860119891 0) (132)
for every 119891 isin 1198711(120583) Then we have
10038171003817100381710038171198791 (119891)1003817100381710038171003817 ⩽100381710038171003817100381711987901003817100381710038171003817
10038171003817100381710038171198911205941198601198881003817100381710038171003817 +1003817100381710038171003817100381710038171003817100381710038171003817
10038171003817100381710038171198911205941198601003817100381710038171003817 =10038171003817100381710038171198911003817100381710038171003817 (133)
for every 119891 isin 1198711(120583) so 119879
1 ⩽ 1 Also
10038171003817100381710038171198791 (1198691198601198921)1003817100381710038171003817 =10038171003817100381710038171198783 (1198921 0)
1003817100381710038171003817 =100381710038171003817100381711987831003817100381710038171003817 = 1 (134)
so 1198791attains its norm at 119869
1198601198921isin 119871
1(120583) and
10038171003817100381710038171198691198601198921 minus 11989101003817100381710038171003817 =10038171003817100381710038171198921 minus 1198920
1003817100381710038171003817 lt 120576 (135)
We also have that for any 119891 isin 1198781198711(120583)
10038171003817100381710038171198790 (119891) minus 1198791 (119891)1003817100381710038171003817 =100381710038171003817100381710038171198790(119891120594
119860) minus (119875
119860119891 0)10038171003817100381710038171003817
⩽10038171003817100381710038171198790 (119869119860119875119860119891) minus 119879119860 (119875119860119891)
1003817100381710038171003817
+10038171003817100381710038171003817119879119860(119875
119860119891) minus (119875
119860119891 0)10038171003817100381710038171003817
⩽10038171003817100381710038171198790119869119860 minus 119879119860
1003817100381710038171003817 +10038171003817100381710038171198780 minus 1198783
1003817100381710038171003817
lt 120578 (1205760) + 6120576
(136)
Hence 1198790minus 119879
1 ⩽ 120578(120576
0) + 6120576 lt 120576
0
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Abstract and Applied Analysis 15
Acknowledgments
The authors thank Gilles Godefroy and Rafael Paya forfruitful conversations about the content of this paper andVladimir Kadets for providing Lemma 18 They also appre-ciate anonymous referees for careful reading and fruitfulsuggestions about revision The second author was sup-ported by Basic Science Research Program through theNational Research Foundation of Korea (NRF) fundedby the Ministry of Education Science and Technology(NRF-2012R1A1A1006869) The third author was partiallysupported by Spanish MICINN and FEDER project noMTM2012-31755 and by Junta de Andalucıa and FEDERGrants FQM-185 and P09-FQM-4911
References
[1] F F Bonsall and J Duncan Numerical Ranges of Operators onNormed Spaces and of Elements of Normed Algebras vol 2 ofLondon Mathematical Society Lecture Note Series 2 CambridgeUniversity Press London UK 1971
[2] F F Bonsall and J Duncan Numerical Ranges II CambridgeUniversity Press London UK 1973 London MathematicalSociety Lecture Notes Series 10
[3] B Sims On numerical range and its applications to Banachalgebras [PhD thesis] University of Newcastle New SouthWales Australia 1972
[4] I D Berg and B Sims ldquoDenseness of operators which attaintheir numerical radiusrdquoAustralian Mathematical Society A vol36 no 1 pp 130ndash133 1984
[5] C S Cardassi ldquoDensity of numerical radius attaining operatorson some reflexive spacesrdquo Bulletin of the Australian Mathemat-ical Society vol 31 no 1 pp 1ndash3 1985
[6] C S Cardassi ldquoNumerical radius attaining operatorsrdquo inBanach Spaces (Columbia Mo 1984) vol 1166 of Lecture Notesin Mathematics pp 11ndash14 Springer Berlin Germany 1985
[7] C S Cardassi ldquoNumerical radius-attaining operators on119862(119870)rdquoProceedings of the AmericanMathematical Society vol 95 no 4pp 537ndash543 1985
[8] MDAcostaOperadores que alcanzan su radio numerico [PhDthesis] Universidad de Granada Granada Spain 1990
[9] M D Acosta and R Paya ldquoNumerical radius attaining oper-ators and the Radon-Nikodym propertyrdquo The Bulletin of theLondon Mathematical Society vol 25 no 1 pp 67ndash73 1993
[10] R Paya ldquoA counterexample on numerical radius attainingoperatorsrdquo Israel Journal of Mathematics vol 79 no 1 pp 83ndash101 1992
[11] Y S Choi and S G Kim ldquoNorm or numerical radius attainingmultilinear mappings and polynomialsrdquo Journal of the LondonMathematical Society vol 54 no 1 pp 135ndash147 1996
[12] M D Acosta J Becerra Guerrero and M Ruiz GalanldquoNumerical-radius-attaining polynomialsrdquo The Quarterly Jour-nal of Mathematics vol 54 no 1 pp 1ndash10 2003
[13] M D Acosta and S G Kim ldquoDenseness of holomorphicfunctions attaining their numerical radiirdquo Israel Journal ofMathematics vol 161 pp 373ndash386 2007
[14] S G Kim and H J Lee ldquoNumerical peak holomorphicfunctions on Banach spacesrdquo Journal of Mathematical Analysisand Applications vol 364 no 2 pp 437ndash452 2010
[15] M D Acosta R M Aron D Garcıa and M MaestreldquoThe Bishop-Phelps-Bollobas theorem for operatorsrdquo Journal ofFunctional Analysis vol 254 no 11 pp 2780ndash2799 2008
[16] A J Guirao and O Kozhushkina ldquoThe Bishop-Phelps-Bollobasproperty for numerical radius in ℓ
1(119862)rdquo Studia Mathematica
vol 218 no 1 pp 41ndash54 2013[17] J Falco ldquoThe Bishop-Phelps-Bollobas property for numerical
radius on 1198711rdquo Journal of Mathematical Analysis and Applica-
tions vol 414 no 1 pp 125ndash133 2014[18] A Aviles A J Guirao and J Rodriguez ldquoOn the Bishop-
Phelps-Bollobas property for numerical radius in119862(119870)-spacesrdquoPreprint
[19] RM Aron Y S Choi S K Kim H J Lee andMMartin ldquoTheBishop-Phelps-Bollobas version of Lindenstrauss properties Aand Brdquo Preprint
[20] V Kadets M Martın and R Paya ldquoRecent progress and openquestions on the numerical index of Banach spacesrdquo Revista dela Real Academia de Ciencias Exactas Fısicas y Naturales A vol100 no 1-2 pp 155ndash182 2006
[21] M Martın J Merı and M Popov ldquoOn the numerical index ofreal 119871
119901(120583)-spacesrdquo Israel Journal of Mathematics vol 184 pp
183ndash192 2011[22] A Iwanik ldquoNorm attaining operators on Lebesgue spacesrdquo
Pacific Journal of Mathematics vol 83 no 2 pp 381ndash386 1979[23] HH SchaeferBanach Lattices and Positive Operators Springer
New York NY USA 1974[24] J Duncan C M McGregor J D Pryce and A J White ldquoThe
numerical index of a normed spacerdquo Journal of the LondonMathematical Society Second Series vol 2 pp 481ndash488 1970
[25] Y S Choi S K Kim H J Lee and M Martin ldquoThe Bishop-Phelps-Bollobas theorem for operators on 119871
1(120583)rdquo Preprint
[26] H E Lacey The Isometric Theory of Classical Banach SpacesSpringer New York NY USA 1974
[27] G Godefroy ldquoUniversal spaces for strictly convex Banachspacesrdquo Revista de la Real Academia de Ciencias Exactas Fısicasy Naturales A vol 100 no 1-2 pp 137ndash146 2006
[28] C Finet M Martın and R Paya ldquoNumerical index andrenormingrdquo Proceedings of the American Mathematical Societyvol 131 no 3 pp 871ndash877 2003
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical Problems in Engineering
Hindawi Publishing Corporationhttpwwwhindawicom
Differential EquationsInternational Journal of
Volume 2014
Applied MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Probability and StatisticsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical PhysicsAdvances in
Complex AnalysisJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
OptimizationJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CombinatoricsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Operations ResearchAdvances in
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Function Spaces
Abstract and Applied AnalysisHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of Mathematics and Mathematical Sciences
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Algebra
Discrete Dynamics in Nature and Society
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Decision SciencesAdvances in
Discrete MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom
Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of
8 Abstract and Applied Analysis
for all 119909 isin 119861119895 Indeed set 119861
119895= 119861
119895cap 119909 isin Ω (119889|]
119891|1119889119898
1)
(119909) ⩽ 1205762 Then
int119861119895119861119895
120576
2119889119898
1(119909) ⩽ int
119861119895
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909) 1198891198981(119909)
=10038161003816100381610038161003816]1119891
10038161003816100381610038161003816(119861
119895) ⩽1205762
22119898
1(119861
119895)
(64)
This shows that 1198981(119861
119895 119861
119895) ⩽ (1205762)119898
1(119861
119895) This proves our
second claimNow we define 119892 by 119892(119910) = 119892(119910)|119892(119910)| if |119892(119910)| ⩾ 1 minus
radic120576232 and 119892(119910) = 119892(119910) if |119892(119910)| lt 1 minus radic120576232 and we
write 119891 = sum119895isin119869120573119895(120594
119861119895
1198981(119861
119895)) It is clear that 119892 isin 119878
119871infin(119898)
119892 minus 119892 lt radic120576 and 119892(119910) = 1 for all 119909 isin supp119891
Finally we define the measure
119889] (119909 119910)
= sum
119895isin119869
120594119861119895
(119909) 119892 (119910)ℎ (119909 119910)119889]119888(119909 119910)(
1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+ 1205941198691119861(119909) 119889] (119909 119910)
(65)
where 119861 = ⋃119895isin119869119861119895 It is easy to see that (119889|]|1119889119898
1)(119909) = 1
on 119861 and (119889|]|11198891198981)(119909) ⩽ 1 elsewhere Note that
119889 (] minus ]) (119909 119910)
= sum
119895isin119869
120594119861119895
(119909) [
[
119892 (119910)ℎ (119909 119910)(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1]
]
119889]119888(119909 119910)
minus sum
119895isin119869
120594119861119895
(119909) 119889]119891(119909 119910)
(66)
If (119909 119910) isin 119862 then |119892(119910)| ⩾ 1 minus radic120576232 ⩾ 1 minus 1232 and
100381610038161003816100381610038161003816119892 (119910)ℎ (119909 119910) minus 1
100381610038161003816100381610038161003816
=
100381610038161003816100381610038161003816100381610038161003816
119892 (119910)
1003816100381610038161003816119892 (119910)1003816100381610038161003816
ℎ (119909 119910) minus 1
100381610038161003816100381610038161003816100381610038161003816
⩽
1003816100381610038161003816119892 (119910) ℎ (119909 119910) minus 11003816100381610038161003816
1003816100381610038161003816119892 (119910)1003816100381610038161003816
+
10038161003816100381610038161 minus1003816100381610038161003816119892 (119910)
1003816100381610038161003816
10038161003816100381610038161003816100381610038161003816119892 (119910)
1003816100381610038161003816
⩽ 2
1003816100381610038161003816119892 (119910) ℎ (119909 119910) minus 11003816100381610038161003816
1003816100381610038161003816119892 (119910)1003816100381610038161003816
⩽ 2radic120576
232
232
232 minus 1⩽ 2radic120576
(67)
Hence for all (119909 119910) isin 119862 we have
10038161003816100381610038161003816100381610038161003816100381610038161003816
119892 (119910)ℎ (119909 119910)(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
⩽100381610038161003816100381610038161003816119892 (119910)ℎ (119909 119910) minus 1
100381610038161003816100381610038161003816(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+
10038161003816100381610038161003816100381610038161003816100381610038161003816
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
⩽ 2radic120576(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+
10038161003816100381610038161003816100381610038161003816100381610038161003816
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
(68)
So we have for all 119909 isin 1198691
119889|] minus ]|1
1198891198981
(119909)
⩽ sum
119895isin119869
120594119861119895
(119909) [
[
2radic120576(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
+
10038161003816100381610038161003816100381610038161003816100381610038161003816
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
minus 1
10038161003816100381610038161003816100381610038161003816100381610038161003816
]
]
1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909)
+ sum
119895isin119869
120594119861119895
(119909)
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909)
⩽ sum
119895isin119869
120594119861119895
(119909)(2radic120576 + (1 minus1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909)))
+sum
119895isin119869
120594119861119895
(119909)(
11988910038161003816100381610038161003816]119891
10038161003816100381610038161003816
1
1198891198981
(119909))
⩽ 2radic120576 + 120576 lt 3radic120576
(69)
This gives that 119879] minus 119879] lt 3radic120576 Note also that for all 119895 isin 119869
⟨119879]
120594119861119895
1198981(119861
119895)
119892⟩
= intΩtimesΩ
120594119861119895
(119909)
1198981(119861
119895)
119892 (119910) 119889] (119909 119910)
= intΩtimesΩ
120594119861119895
(119909)
1198981(119861
119895)
ℎ (119909 119910)(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
119889]119888(119909 119910)
Abstract and Applied Analysis 9
= intΩ
120594119861119895
(119909)
1198981(119861
119895)
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
1198891003816100381610038161003816]1198881003816100381610038161003816
1
(119909)
= intΩ
120594119861119895
(119909)
1198981(119861
119895)
1198891198981(119909) = 1
(70)
Hence we get ⟨119879]119891 119892⟩ = 1 which implies that 119879]119891 =119879] = 1 Finally10038171003817100381710038171003817119891 minus 119891
10038171003817100381710038171003817
⩽10038171003817100381710038171003817119891 minus 119891
1
10038171003817100381710038171003817+10038171003817100381710038171198911 minus 119891
1003817100381710038171003817
=
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
1198981(119861
119895)
minus sum
119895isin119869
120573119895
120594119861119895
1198981(119861
119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+ 120576
⩽ sum
119895isin119869
120573119895(
10038171003817100381710038171003817100381710038171003817100381710038171003817
120594119861119895
1198981(119861
119895)
minus
120594119861119895
1198981(119861
119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+
10038171003817100381710038171003817100381710038171003817100381710038171003817
120594119861119895
1198981(119861
119895)
minus
120594119861119895
1198981(119861
119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
) + 120576
= 2sum
119895isin119869
120573119895
1198981(119861
119895 119861
119895)
1198981(119861
119895)
+ 120576
⩽ 2sum
119895isin119869
120573119895
(1205762)1198981(119861
119895)
1198981(119861
119895)
+ 120576 ⩽120576
1 minus 1205762+ 120576 lt 3120576
(71)
We are now ready to present the proof of the main resultin the case of finite regular positive Borel measures
Proof of Proposition 10 Let 1205751= 120575
3
2(5sdot2
4) 120575
2= 120575
12
3(3
2sdot2
14)
and 1205753= (12057610)
2 for some 0 lt 120576 lt 1 Suppose that 119879 isinL(119871
1(119898)) with 119879 = 1 and that there is an 119891
0isin 119878
1198711(119898)
and1198920isin 119878
119871infin(119898)
such that ⟨1198910 119892
0⟩ = 1 and |⟨119879119891
0 119892
0⟩| gt 1minus120575
3
12
6Then there is an isometric isomorphism Ψ from 119871
1(119898) onto
itself such thatΨ(1198910) = |119891
0| and there is a scalar number 120572 in
119878R such that |⟨1198791198910 119892
0⟩| = ⟨120572119879119891
0 119892
0⟩ Then letting 119891
1= Ψ119891
0
1198921= (Ψ
minus1)lowast1198920 and 119879
1= 120572Ψ119879Ψ
minus1 we have
⟨1198781198911 119892
1⟩ = ⟨120572Ψ119879
0Ψ
minus1Ψ119891
0 (Ψ
minus1)lowast
1198920⟩
= ⟨1205721198791198910 119892
0⟩ gt 1 minus
1205753
1
26
⟨1198911 119892
1⟩ = ⟨Ψ119891
0 (Ψ
minus1)lowast
1198920⟩ = 1
(72)
Since 11987911198911 gt 1 minus 120575(120575
3
12
6) by Lemma 13 there exists a
norm-one bounded operator 119879] and a nonnegative simplefunction119891
2isin 119878
1198711(119898)
such that 1198791minus119879] ⩽ 1205751 1198912minus1198911 ⩽ 31205751
and (119889|]|11198891198981)(119909) = 1 for all 119909 isin supp(119891
2) Then
⟨119879]1198912 1198921⟩
= ⟨11987911198911 119892
1⟩ minus ⟨119879
11198911minus 119879
11198912 119892
1⟩ minus ⟨119879
11198912minus 119879]1198912 1198921⟩
⩾ ⟨11987911198911 119892
1⟩ minus10038171003817100381710038171198911 minus 1198912
1003817100381710038171003817 minus10038171003817100381710038171198791 minus 119879]
1003817100381710038171003817
⩾ 1 minus1205753
1
26minus 3120575
1minus 120575
1⩾ 1 minus 5120575
1= 1 minus
1205753
2
16
(73)
Notice also that
⟨1198912 119892
1⟩ = ⟨119891
1 119892
1⟩ minus ⟨119891
1minus 119891
2 119892
1⟩ ⩾ 1 minus
10038171003817100381710038171198911 minus 11989121003817100381710038171003817
⩾ 1 minus 31205751⩾ 1 minus 5120575
1= 1 minus
1205753
2
16
(74)
By Lemma 12 there are a nonnegative simple function 1198913isin
1198781198711(119898)
and a function 1198923isin 119878
119871infin(119898)
such that
1198923(119909) = 120594supp119891
3
(119909) + 1198922(119909) 120594
Ωsupp1198913
(119909)
10038171003817100381710038171198912 minus 11989131003817100381710038171003817 ⩽ 1205752
10038171003817100381710038171198923 minus 11989211003817100381710038171003817 ⩽radic120575
2 ⟨119891
3 119892
3⟩ = 1
(75)
So we have
⟨119879]1198913 1198923⟩ = ⟨119879]1198912 1198921⟩ minus ⟨119879]1198912 minus 119879]1198913 1198921⟩
minus ⟨119879]1198913 1198921 minus 1198923⟩
⩾ 1 minus1205753
2
16minus 2radic120575
2⩾ 1 minus 3radic120575
2= 1 minus
1205756
3
27
(76)
By Lemma 14 there exist 1198914isin 119878
1198711(119898)
and 1198924isin 119878
119871infin(119898)
and anoperator 119879
4such that
⟨1198924 119879
41198914⟩ = 1 =
100381710038171003817100381711987941003817100381710038171003817
10038171003817100381710038171198794 minus 119879]1003817100381710038171003817 ⩽ 21205753
10038171003817100381710038171198914 minus 11989131003817100381710038171003817 ⩽ 31205753
10038171003817100381710038171198924 minus 11989231003817100381710038171003817 ⩽radic120575
3 ⟨119891
4 119892
4⟩ = 1
(77)
So we have
10038171003817100381710038171198794 minus 11987911003817100381710038171003817 ⩽10038171003817100381710038171198794 minus 119879]
1003817100381710038171003817 +1003817100381710038171003817119879] minus 1198791
1003817100381710038171003817
⩽ 1205751+ 2120575
3⩽ 3120575
3
10038171003817100381710038171198911 minus 11989141003817100381710038171003817 ⩽10038171003817100381710038171198911 minus 1198912
1003817100381710038171003817 +10038171003817100381710038171198912 minus 1198913
1003817100381710038171003817 +10038171003817100381710038171198913 minus 1198914
1003817100381710038171003817
⩽ 31205751+ 120575
2+ 3120575
3⩽ 10120575
3
10038171003817100381710038171198921 minus 11989241003817100381710038171003817 ⩽10038171003817100381710038171198921 minus 1198923
1003817100381710038171003817 + 1198923 minus 1198924
⩽ 1205752+ radic120575
3⩽ 2radic120575
3
(78)
10 Abstract and Applied Analysis
Let 119878 = 120572Ψminus11198794Ψ 119891 = Ψminus1
1198914 and 119892 = Ψlowast
1198924 then we have
119879 minus 119878 =10038171003817100381710038171003817119879 minus 120572Ψ
minus11198794Ψ10038171003817100381710038171003817=10038171003817100381710038171003817120572Ψ119879Ψ
minus1minus 119879
4
10038171003817100381710038171003817
=10038171003817100381710038171198791 minus 1198794
1003817100381710038171003817 ⩽ 31205753
100381710038171003817100381710038171198910minus 11989110038171003817100381710038171003817=100381710038171003817100381710038171198910minus Ψ
minus11198914
10038171003817100381710038171003817=10038171003817100381710038171198911 minus 1198914
1003817100381710038171003817 ⩽ 101205753
100381710038171003817100381710038171198920minus 11989110038171003817100381710038171003817=10038171003817100381710038171198920 minus Ψ
lowast1198924
1003817100381710038171003817 =100381710038171003817100381710038171003817(Ψ
minus1)lowast
1198920minus 119892
4
100381710038171003817100381710038171003817
=10038171003817100381710038171198921 minus 1198924
1003817100381710038171003817 ⩽ 2radic120575
3
⟨119891 119892⟩ = ⟨Ψminus11198914 Ψ
lowast1198924⟩ = ⟨119891
4 119892
4⟩ = 1
10038161003816100381610038161003816⟨119878119891 119892⟩
10038161003816100381610038161003816=10038161003816100381610038161003816⟨120572Ψ
minus11198794ΨΨ
minus11198914 Ψ
lowast1198924⟩10038161003816100381610038161003816= |120572| = 1
(79)
This completes the proof
Finally we may give the proof of the main result in fullgenerality
Proof of Theorem 9 Notice that the Kakutani representationtheorem (see [26] for a reference) says that for every 120590-finitemeasure ] the space 119871
1(]) is isometrically isomorphic to
1198711(119898) for some positive Borel regular measure on a compact
Hausdorff space Then by Proposition 10 there is a universalfunction 120576 997891rarr 120578(120576) gt 0 which gives the BPBp-nu for 119871
1(]) for
every 120590-finite measure ]Fix 120576 gt 0 Suppose that 119879
0isin L(119871
1(120583)) with V(119879
0) = 1
and (1198910 119891
lowast
0) isin Π(119871
1(120583)) satisfy
1003816100381610038161003816⟨119891lowast
0 119879
01198910⟩1003816100381610038161003816 gt 1 minus 120578 (120576) (80)
Choose a sequence 119891119899 in 119871
1(120583) such that sup
119899119879
0119891119899 = 1
and let 119866 be the closed linear span of
119879119899
0119891119898 119899 119898 isin N cup 0 (81)
As 119866 is separable there is a dense subset 119892119899 119899 isin N of 119866
and let 119864 = ⋃infin
119899=1supp119892
119899 where supp119892
119899is the support of 119892
119899
Then the measure 120583|119864is 120590-finite Let
119884 = 119891 isin 1198711(120583) supp (119891) sub 119864 (82)
be a closed subspace of 1198711(120583) It is clear that 119871
1(120583) = 119884oplus
1119885
and 119884 is isometrically isomorphic to 1198711(120583|
119864) So 119884 has the
BPBp-nu with 120578(120576)Now write 119878
0= 119879
0|119884 119884 rarr 119884 consider 119910
0= 119891
0isin
119878119884 119910lowast
0= 119891
lowast
0|119884isin 119878
119884lowast and observe that 119910lowast
0(119910
0) = 1 and
|119910lowast
0(119878
01199100)| = |119891
lowast
0(119879
01198910)| gt 1 minus 120578(120576) Hence there exist 119878 isin
L(119884) and (1199100 119910
lowast
0) isin Π(119884) such that
1003816100381610038161003816119910lowast
0(119878119910
0)1003816100381610038161003816 = 1 = V (119878) 1003817100381710038171003817119878 minus 1198780
1003817100381710038171003817 lt 120576
10038171003817100381710038171199100 minus 11991001003817100381710038171003817 lt 120576
1003817100381710038171003817119910lowast
0minus 119910
lowast
0
1003817100381710038171003817 lt 120576
(83)
Finally consider the operator 119879 isinL(1198711(120583)) given by
119879 (119910 119911)
= (119878119910 0) + 1198790(0 119911) ((119910 119911) isin 119871
1(120583) equiv 119884oplus
1119885)
(84)
We have 119879 = 1 (and so V(119879) = 1) Indeed1003817100381710038171003817119879 (119910 119911)
1003817100381710038171003817 =1003817100381710038171003817(119878119910 0)
1003817100381710038171003817 +10038171003817100381710038171198790 (0 119911)
1003817100381710038171003817 ⩽10038171003817100381710038171199101003817100381710038171003817 + 119911 =
1003817100381710038171003817(119910 119911)1003817100381710038171003817
(85)
for all (119910 119911) isin 1198711(120583) and 119879(119910
0 0) = (119878119910
0 0) = 119878119910
0 = 1
Let 119909 = (1199100 0) and 119909lowast = (119910lowast
0 119891
0|119885) Then (119909 119909lowast) isin Π(119871
1(120583))
Moreover we have1003816100381610038161003816119909
lowast1198791199091003816100381610038161003816 =1003816100381610038161003816119910
lowast
0119878119910
0
1003816100381610038161003816 = 1 = V (119879) 1003817100381710038171003817119909 minus 1198910
1003817100381710038171003817 =1003817100381710038171003817119910 minus 1199100
1003817100381710038171003817 lt 120576
1003817100381710038171003817119909lowast
0minus 119891
lowast
0
1003817100381710038171003817
= max 1003817100381710038171003817119910 minus 119891lowast
0|119884
1003817100381710038171003817 1003817100381710038171003817119891
lowast
0|119885minus 119891
lowast
0|119885
1003817100381710038171003817
=1003817100381710038171003817119910
lowastminus 119910
lowast
0
1003817100381710038171003817 lt 120576
1003817100381710038171003817119879 minus 11987901003817100381710038171003817 = sup119910+119911⩽1
1003817100381710038171003817119879 (119910 119911) minus 1198790 (119910 119911)1003817100381710038171003817
= sup119910⩽1
1003817100381710038171003817119878119910 minus 11987801199101003817100381710038171003817 =1003817100381710038171003817119878 minus 1198780
1003817100381710038171003817 lt 120576
(86)
This completes the proof
5 Examples of Spaces Failing theBishop-Phelps-Bollobaacutes Property forNumerical Radius
Our goal here is to prove that the density of numerical radiusattaining operators does not imply the BPBp-nu Actuallywe will show that among separable spaces there is noisomorphic property implying the BPBp-nu other than finite-dimensionality
We need to relate the BPBp-nu to the Bishop-Phelps-Bollobas property for operators which as mentioned in theintroduction was introduced in [15] A pair (119883 119884) of Banachspaces has the Bishop-Phelps-Bollobas property for operators(in short BPBp) if given 120576 gt 0 there exists 120578(120576) gt 0 suchthat given 119879 isin L(119883 119884) with 119879 = 1 and 119909 isin 119878
119883such that
119879119909 gt 1 minus 120578(120576) then there exist 119911 isin 119878119883and 119878 isin L(119883 119884)
satisfying
119878 = 119878119911 = 1 119909 minus 119911 lt 120576
119879 minus 119878 lt 120576
(87)
We refer the reader to [15 19 25] and references therein formore information and background Among the interestingresults on the BPBp we emphasize that a pair (119883 119884) when119883is finite-dimensional does not necessarily have the BPBp Forinstance if119884 is a strictly convex space which is not uniformlyconvex then the pair (ℓ(2)
1 119884) fails to have the BPBp (this is
contained in [15] see [19 Section 3])
Abstract and Applied Analysis 11
The next result relates the BPBp-nu to the BPBp foroperators in a particular case We will deduce our examplefrom it
Theorem 15 If 1198711(120583)oplus
1119883 has the BPBp-119899119906 then the pair
(1198711(120583) 119883) has the BPBp for operators
Before proving this proposition we will use it to get themain examples of this section The first example shows thatthe density of numerical radius attaining operators does notimply the BPBp-nu
Example 16 There is a reflexive space (and so numerical radiusattaining operators on it are dense) which fails to have theBPBp-nu Indeed let 119884 be a reflexive separable space whichis not superreflexive and we may suppose that 119884 is strictlyconvex Observe that 119884 cannot be uniformly convex since itis not superreflexive Now 119883 = ℓ(2)
1oplus1119884 is reflexive but the
pair (ℓ(2)1 119884) fails the BPBp since 119884 is strictly convex but not
uniformly convex [19 Corollary 33] Therefore Theorem 15gives us that119883 does not have the BPBp-nu
The example above can be extended to get the result thatevery infinite-dimensional separable Banach space can berenormed to fail the BPBp-nu This follows from the factthat every infinite-dimensional separable Banach space canbe renormed to be strictly convex but not uniformly convex(this result can be proved ldquoby handrdquo an alternative categoricalargument for it can be found in [27] and references therein)With a little more of effort we may get the main result of thesection
Theorem 17 Every infinite-dimensional separable Banachspace can be renormed to fail the weak-BPBp-nu (and so inparticular to fail the BPBp-nu)
We need the following result which is surely well knownAs we have not found a reference we include a nice and easyproof kindly given to us by Vladimir Kadets We recall thatgiven a Banach space 119884 the set of all equivalent norms on119884 can be viewed as a metric space using the Banach-Mazurdistance
Lemma 18 Let119884 be an infinite-dimensional separable Banachspace Then the set of equivalent norms on 119884 which are strictlyconvex and are not (locally) uniformly convex is dense in the setof all equivalent norms on119884 (with respect to the Banach-Mazurdistance)
Proof Fix 119890 isin 119878119884and 119890lowast
1isin 119878
119884lowast such that 119890lowast
1(119890) = 1 For a fixed
120576 isin (0 12) denote
119902 (119910) = max (1 minus 120576) 10038171003817100381710038171199101003817100381710038171003817 1003816100381610038161003816119890lowast
1(119910)1003816100381610038161003816 (119910 isin 119884) (88)
Evidently (1 minus 120576)119910 ⩽ 119902(119910) ⩽ 119910 for every 119910 isin 119884 Fix asequence 119890lowast
119896 119896 ⩾ 2 of norm-one functionals separating
the points of 119884 and denote
119901 (119910) = radic
infin
sum
119896=1
1
2119896
1003816100381610038161003816119890lowast
119896(119910)1003816100381610038161003816
2
(119910 isin 119884) (89)
Then 119901 is a strictly convex norm on 119884 119901(119890) ⩾ 1radic2 and119901(119910) ⩽ 119910 for all 119910 isin 119883 Finally write
100381710038171003817100381711991010038171003817100381710038171= (1 minus 120576) 119902 (119910) + 120576
119901 (119910)
119901 (119890)(119910 isin 119884) (90)
Then sdot 1is a strictly convex norm on 119884 and
(1 minus 120576)2 10038171003817100381710038171199101003817100381710038171003817 ⩽100381710038171003817100381711991010038171003817100381710038171⩽ (1 + 120576)
10038171003817100381710038171199101003817100381710038171003817 (119910 isin 119884) (91)
Wewill finish the proof by showing that sdot 1is not uniformly
convex (actually it is not locally uniformly convex) Indeedfor each 119899 isin N we select 119910
119899isin ⋂
119899
119896=1ker 119890lowast
119896with 119910
119899 = 1 and
consider 119890119899= 119890 + (1205764)119910
119899 Then 119902(119890) = 1 119902(119890
119899) = 1 and
119902(119890+119890119899) = 2 At the same time 119901(119910
119899) rarr 0 so 119901(119890
119899) rarr 119901(119890)
and 119901(119890 + 119890119899) rarr 2119901(119890) Consequently
1198901 = 1100381710038171003817100381711989011989910038171003817100381710038171997888rarr 1
1003817100381710038171003817119890 + 11989011989910038171003817100381710038171997888rarr 2 (92)
but 119890 minus 1198901198991= (1205764)119910
1198991⩾ (1 minus 120576)
2(1205764) which means the
absence of local uniform convexity at 119890
Proof of Theorem 17 Let 119883 be an infinite-dimensional sepa-rable Banach space Take a closed subspace 119884 of 119883 of codi-mension two By [28 Proposition 2] the map carrying everyequivalent norm on 119884 to its numerical index is continuousand so the set of values of the numerical index of 119884 upto reforming is a nontrivial interval [28 Theorem 9] ThenLemma 18 allows us to find an equivalent norm | sdot | on 119884 insuch a way that (119884 | sdot |) is strictly convex and is not uniformlyconvex and 119899(119884 | sdot |) gt 0 Now the space 119883 = ℓ(2)
1oplus1(119884 | sdot |)
is an equivalent renorming of 119883 which does not have theBPBp-119899119906 (indeed otherwise the pair (ℓ(2)
1 (119884 | sdot |)) would
have the BPBp for the operator norm and so (119884 | sdot |) wouldbe uniformly convex by [19 Corollary 33] a contradiction)Moreover as
119899 (119883) = min 119899 (ℓ(2)1) 119899 (119884 |sdot|) gt 0 (93)
(see [20 Proposition 2] for instance) 119883 also fails the weak-BPBp-nu by Proposition 6
To finish the section with the promised proof ofTheorem 15 we first see the following stability result
Lemma 19 Let119883 = [⨁infin
119896=1119883
119896]1198880
or [⨁infin
119896=1119883
119896]ℓ1
If119883 has theBishop-Phelps-Bollobas property for numerical radius with afunction 120578 then each Banach space 119883
119894has the Bishop-Phelps-
Bollobas property for numerical radius with 120578nu(119883119894) ⩾ 120578 That
is inf119894120578nu(119883119894
)(120576) ⩾ 120578nu(119883)(120576) for all 0 lt 120576 lt 1
Proof Let 119875119894 119883 rarr 119883
119894and 1198751015840
119894 119883
lowastrarr 119883
lowast
119894be the natural
projections and let 119876119894 119883
119894rarr 119883 and 1198761015840
119894 119883
lowast
119894rarr 119883
lowast be thenatural embeddings
Assume that an operator 119879119894 119883
119894rarr 119883
119894and a pair
(119909119894 119909
lowast
119894) isin Π(119883
119894) satisfy that
V (119879119894) = 1
1003816100381610038161003816119909lowast
119894119879119894119909119894
1003816100381610038161003816 gt 1 minus 120578 (120576) (94)
We define an operator 119879 119883 rarr 119883 and (119909 119909lowast) isin Π(119883) by
119879 = 119876119894∘ 119879
119894∘ 119875
119894 (119909 119909
lowast) = (119876
119894119909119894 119876
1015840
119894119909lowast
119894) (95)
12 Abstract and Applied Analysis
then clearly we see that1003816100381610038161003816119909
lowast1198791199091003816100381610038161003816 =1003816100381610038161003816119909
lowast
119894119879119894119909119894
1003816100381610038161003816 gt 1 minus 120578 (120576) (96)
From the assumption there exist 119878 119883 rarr 119883 and a pair(119910 119910
lowast) isin Π(119883) such that
1003816100381610038161003816119910lowast1198781199101003816100381610038161003816 = 1 = V (119878) 119878 minus 119879 lt 120576
1003817100381710038171003817119910lowastminus 119909
lowast1003817100381710038171003817 lt 1205761003817100381710038171003817119910 minus 119909
1003817100381710038171003817 lt 120576
(97)
Since this clearly shows that1003817100381710038171003817119875119894 ∘ 119878 ∘ 119876119894
minus 119879119894
1003817100381710038171003817 lt 120576100381710038171003817100381710038171198751015840
119894119910lowastminus 119909
lowast
119894
10038171003817100381710038171003817lt 120576
1003817100381710038171003817119875119894119910 minus 1199091198941003817100381710038171003817 lt 120576
(98)
we only need to show that |1198751015840119894119910lowast(119875
119894∘ 119878 ∘ 119876
119894)119875
119894119910| = 1
We first show the case of 1198880sum Since
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817=10038171003817100381710038171003817119875119895119910 minus 119875
11989511990910038171003817100381710038171003817⩽1003817100381710038171003817119910 minus 119909
1003817100381710038171003817 lt 120576 (99)
for every 119895 = 119894 we have
1 = 119910lowast(119910) = sum
119895isinN
1198751015840
119895119910lowast(119875
119895119910) ⩽ sum
119895isinN
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817
⩽100381710038171003817100381710038171198751015840
119894119910lowast10038171003817100381710038171003817+ 120576 sum
119895isinN119895 = 119894
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817⩽1003817100381710038171003817119910
lowast1003817100381710038171003817 = 1
(100)
This shows that 1198751015840119894119910lowast = 1 and 1198751015840
119895119910lowast= 0 for every 119895 = 119894
So 119910lowast = 1198761015840
1198941198751015840
119894119910lowast and 1198751015840
119894119910lowast(119875
119894119910) = 1 This and the fact that
119910 minus 119876119894119875119894119910 lt 120576 imply that
(119876119894119875119894119910 + (
1
120576) (119910 minus 119876
119894119875119894119910) 119876
1015840
1198941198751015840
119894119910lowast) isin Π (119883) (101)
So we get that (1198761015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910 + (1120576)(119910 minus 119876
119894119875119894119910))V(119878) = 1
Hence we have
1 =1003816100381610038161003816119910
lowast1198781199101003816100381610038161003816 =10038161003816100381610038161003816(119876
1015840
1198941198751015840
119894119910lowast) 11987811991010038161003816100381610038161003816
=
1003816100381610038161003816100381610038161003816(1 minus 120576) (119876
1015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910)
+120576 (1198761015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910 +1
120576(119910 minus 119876
119894119875119894119910))
1003816100381610038161003816100381610038161003816⩽ 1
(102)
and so we get |1198751015840119894119910lowast(119875
119894∘119878 ∘119876
119894)119875
119894119910| = |(119876
1015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910)| = 1
We next show the case of ℓ1sum The proof is almost
the same as that of the 1198880case However for the sake of
completeness we provide it hereSince 1198751015840
119895119910lowast = 119875
1015840
119895119910lowastminus 119875
1015840
119895119909lowast ⩽ 119910
lowastminus 119909
lowast lt 120576 for every
119895 = 119894 we have
1 = 119910lowast(119910) = sum
119895isinN
1198751015840
119895119910lowast(119875
119895119910) ⩽ sum
119895isinN
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817
⩽10038171003817100381710038171198751198941199101003817100381710038171003817 + 120576 sum
119895isinN119895 = 119894
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817⩽10038171003817100381710038171199101003817100381710038171003817 = 1
(103)
which shows 119875119894119910 = 1 and 119875
119895119910 = 0 for every 119895 = 119894 Since this
implies (119876119894119875119894119910119876
1015840
1198941198751015840
119894119910lowast+(1120576)(119910
lowastminus119876
1015840
1198941198751015840
119894119910lowast)) isin Π(119883) we get
that |(1198761015840
1198941198751015840
119894119910lowast+ (1120576)(119910
lowastminus 119876
1015840
1198941198751015840
119894119910lowast))119878(119876
119894119875119894119910)| ⩽ V(119878) = 1
Hence we have
1 =1003816100381610038161003816119910
lowast1198781199101003816100381610038161003816 =1003816100381610038161003816119910
lowast119878 (119876
119894119875119894119910)1003816100381610038161003816
=
1003816100381610038161003816100381610038161003816(1 minus 120576) (119876
1015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910)
+120576 (1198761015840
1198941198751015840
119894119910lowast+1
120576(119910
lowastminus 119876
1015840
1198941198751015840
119894119910lowast)) 119878 (119876
119894119875119894119910)
1003816100381610038161003816100381610038161003816⩽ 1
(104)
and so |1198751015840119894119910lowast(119875
119894∘ 119878 ∘ 119876
119894)119875
119894119910| = |(119876
1015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910)| = 1
Proof of Theorem 15 Note that 120578nu(1198711(120583)oplus1119883)(120576) rarr 0 as120576 rarr 0 Fix 0 lt 120576
0lt 1 and choose 0 lt 120576 lt 1 such that
6120576 + 120578nu(1198711(120583)oplus1119883)(120576) lt 1205760 Let 120578(1205760) = 120578nu(1198711(120583)oplus1119883)(120576)Suppose that 119879
0isin L(119871
1(120583) 119883) with 119879
0 = 1 and 119891
0isin
1198781198711(120583)
satisfy
1003817100381710038171003817119879011989101003817100381710038171003817 gt 1 minus 120578 (1205760) (105)
For any measurable subset 119861 let
1198711(120583|
119861) = 119891|
119861 119891 isin 119871
1(120583) (106)
with the norm 119891|119861 = 119891120594
1198611 Then it is easy to see
that 1198711(120583|
119861) is isometrically isomorphic to a complemented
subspace of 1198711(120583) Let 119875
119861 119871
1(120583) rarr 119871
1(120583|
119861) be the
restriction defined by 119875119861(119891) = 119891|
119861for all 119891 isin 119871
1(120583) and
let 119869119861 119871
1(120583|
119861) rarr 119871
1(120583) be the extension defined by
119869119861(119891)(120596) = 119891(120596) if 120596 isin 119861 and 119869
119861(119891)(120596) = 0 otherwise
It is clear that 119875119861119869119861= Id
1198711(120583|119861)and 119869
119861119875119861(119891) = 119891120594
119861for all
119891 isin 1198711(120583) Notice also that 119871
1(120583) is isometrically isomorphic
to 1198711(120583|
119861)oplus
11198711(120583|
119861119888)
Let 119860 = supp1198910and 119892
0= 119875
1198601198910 Then
1003817100381710038171003817119879011986911986011989201003817100381710038171003817 =100381710038171003817100381711987901198910
1003817100381710038171003817 gt 1 minus 120578 (1205760) gt 0 (107)
and define the operator 119879119860 119871
1(120583|
119860) rarr 119883 by 119879
119860119891 =
1198790119869119860119891119879
0119869119860 for every 119891 isin 119871
1(120583|
119860) Then
100381710038171003817100381711987911986011989201003817100381710038171003817 ⩾100381710038171003817100381711987901198910
1003817100381710038171003817 gt 1 minus 120578 (1205760) (108)
Since 120583|119860is 120590-finite 119871
1(120583|
119860)lowast= 119871
infin(120583|
119860) Let 119892lowast
0isin 119878
119871infin(120583|119860)
be a function such that ⟨119892lowast0 119892
0⟩ = 1 and choose 119909lowast
0isin 119878
119883lowast
such that 119909lowast0(119879
1198601198920) = 119879
1198601198920 Define the operator 119878
0isin
L(1198711(120583|
119860)oplus
1119883) by
1198780(119891 119909) = (0 119879
119860119891) ((119891 119909) isin 119871
1(120583|
119860) oplus
1119883) (109)
and observe that 1198780 = V(119878
0) = 1 Indeed
100381710038171003817100381711987801003817100381710038171003817 ⩽ 1 =
10038171003817100381710038171198791198601003817100381710038171003817
= sup 1003816100381610038161003816119909lowast1198791198601198911003816100381610038161003816 119909
lowastisin 119878
119883lowast 119891 isin 119878
1198711(120583|119860)
= sup 1003816100381610038161003816(119891lowast 119909
lowast) 119878
0(119891 119909)
1003816100381610038161003816
((119891lowast 119909
lowast) (119891 119909)) isin Π (119871
1(120583|
119860) oplus
1119883)
= V (1198780) ⩽100381710038171003817100381711987801003817100381710038171003817
(110)
Abstract and Applied Analysis 13
It is immediate that
(119892lowast
0 119909
lowast
0) 119878
0(119892
0 0) = 119909
lowast
0(119879
1198601198920) =10038171003817100381710038171198791198601198920
1003817100381710038171003817 gt 1 minus 120578 (1205760)
(111)
By Lemma 19 1198711(120583|
119860)oplus
1119883 has the BPBp-nu with the func-
tion 120578 Therefore there exist 1198781isin L(119871
1(120583|
119860)oplus
1119883) (119892
1 119909
1) isin
1198781198711(120583|119860)oplus1119883 and (119892lowast
1 119909
lowast
1) isin 119878
119871infin(120583|119860)oplusinfin119883lowast such that
1003817100381710038171003817(1198921 1199091) minus (1198920 0)1003817100381710038171003817 lt 120576
1003817100381710038171003817(119892lowast
1 119909
lowast
1) minus (119891
lowast
0 119909
lowast
0)1003817100381710038171003817 lt 120576
10038171003817100381710038171198781 minus 11987801003817100381710038171003817 lt 120576 ⟨(119892
lowast
1 119909
lowast
1) (119892
1 119909
1)⟩ = 1
1003816100381610038161003816(119892lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)1003816100381610038161003816 = V (119878
1) = 1
(112)
Claim 1 We claim that 1199091= 0
Otherwise
1 = Re ⟨(119892lowast1 119909
lowast
1) (119892
1 119909
1)⟩
=100381710038171003817100381711989211003817100381710038171003817Re⟨(119892
lowast
1 119909
lowast
1) (119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
⟩
+100381710038171003817100381711990911003817100381710038171003817Re⟨(119892
lowast
1 119909
lowast
1) (0 119909
1)
100381710038171003817100381711990911003817100381710038171003817
⟩ ⩽ 1
(113)
We deduce that
((119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
(119892lowast
1 119909
lowast
1)) (
(0 1199091)
100381710038171003817100381711990911003817100381710038171003817
(119892lowast
1 119909
lowast
1))
isin Π (1198711(120583|
119860) oplus
1119883)
(114)
Since 1198781((0 119909
1)119909
1) = (119878
1minus 119878
0)((0 119909
1)119909
1) lt 120576 we
get that
1 =1003816100381610038161003816⟨(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)⟩1003816100381610038161003816
=
100381610038161003816100381610038161003816100381610038161003816
100381710038171003817100381711989211003817100381710038171003817⟨(119892
lowast
1 119909
lowast
1) 119878
1((119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
)⟩
+100381710038171003817100381711990911003817100381710038171003817⟨(119892
lowast
1 119909
lowast
1) 119878
1((0 119909
1)
100381710038171003817100381711990911003817100381710038171003817
)⟩
100381610038161003816100381610038161003816100381610038161003816
⩽100381710038171003817100381711989211003817100381710038171003817 V (1198781) + 120576
100381710038171003817100381711990911003817100381710038171003817 lt100381710038171003817100381711989211003817100381710038171003817 +100381710038171003817100381711990911003817100381710038171003817 = 1
(115)
a contradiction This proves the claimWe define the operator 119878
2 119871
1(120583|
119860)oplus
1119883 rarr 119871
1(120583|
119860)oplus
1119883
by 1198782(119891 119909) = 119878
1(119891 0) for every 119891 isin 119871
1(120583|
119860) and for every
119909 isin 119883 Then we have
V (1198782) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816 = 1
10038171003817100381710038171198781 minus 11987821003817100381710038171003817 ⩽ 120576 (116)
Indeed from Claim 1 we have
V (1198781) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)1003816100381610038161003816 =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816 ⩽ V (119878
2)
(117)
On the other hand we see that1003816100381610038161003816(119891
lowast 119909
lowast) 119878
2(119891 119909)
1003816100381610038161003816
=1003816100381610038161003816(119891
lowast 119909
lowast) 119878
1(119891 0)
1003816100381610038161003816 ⩽10038171003817100381710038171198911003817100381710038171003817 V (1198781) ⩽ V (119878
1)
(118)
for every ((119891lowast 119909lowast) (119891 119909)) isin Π(1198711(120583|
119860)oplus
1119883) Therefore
V(1198782) ⩽ V(119878
1) Also
10038171003817100381710038171198781 minus 11987821003817100381710038171003817 ⩽ sup
119909isin119878119883
10038171003817100381710038171198781 (0 119909)1003817100381710038171003817
= sup119909isin119878119883
10038171003817100381710038171198781 (0 119909) minus 1198780 (0 119909)1003817100381710038171003817 ⩽ 120576
(119)
Claim 2 There exists 1198783 119871
1(120583|
119860)oplus
1119883 rarr 119871
1(120583|
119860)oplus
1119883 such
that 1198783(119892
1 0) = 119878
3 = 1 119878
3(0 119909) = 0 119878
3(119891 119909) isin 0oplus
1119883
for every (119891 119909) isin 1198711(120583|
119860)oplus
1119883 and 119878
3minus 119878
2 lt 4120576
Indeed write 1198781= (119863
1 119863
2) where 119863
1 119871
1(120583|
119860)oplus
1119883 rarr
1198711(120583|
119860) and119863
2 119871
1(120583|
119860)oplus
1119883 rarr 119883 We have that
sup 1003816100381610038161003816119892lowast119863
1(119892
1 0) + 119909
lowast119863
2(119892
1 0)1003816100381610038161003816
119909lowastisin 119878
119883lowast ⟨119892
lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860)
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816 +10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
⟨119892lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860)
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816
⟨119892lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860) +10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
⩽ V (1198782) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816119892
lowast
1119863
1(119892
1 0) + 119909
lowast
1119863
2(119892
1 0)1003816100381610038161003816
(120)
This implies that
1003816100381610038161003816119909lowast
1119863
2(119892
1 0)1003816100381610038161003816 =10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
1003816100381610038161003816119892lowast
1119863
1(119892
1 0)1003816100381610038161003816
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816 ⟨119892
lowast 119892
1⟩ = 1 119892
lowastisin 119871
infin(120583|
119860)
(121)
Therefore |119892lowast1| equals 1 on the support of 119863
1(119892
1 0) As
|⟨119892lowast
1 119892
1⟩| = 1 we also have that |119892lowast
1| equals 1 on the support
of 1198921 Changing the values of 119892lowast
1by the ones of 119891lowast
0on 119860
(supp(1198631(119892
1 0)) cup supp(g
1)) we may and do suppose that
|119892lowast
1| = 1 on the whole 119860We also have 119863
2(119892
1 0) gt 0 Indeed
10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198920 0)1003817100381710038171003817
⩽10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198921 0)
1003817100381710038171003817 +10038171003817100381710038171198780 (1198921 0) minus 1198780 (1198920 0)
1003817100381710038171003817
lt 2120576 + 120576 = 3120576
(122)
So we have
10038171003817100381710038171198632(119892
1 0) minus 119879
1198601198920
1003817100381710038171003817
⩽10038171003817100381710038171198631(119892
1 0)1003817100381710038171003817 +10038171003817100381710038171198632(119892
1 0) minus 119879
1198601198920
1003817100381710038171003817
14 Abstract and Applied Analysis
=1003817100381710038171003817(1198631
(1198921 0) 119863
2(119892
1 0)) minus (0 119879
1198601198920)1003817100381710038171003817
=10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198920 0)
1003817100381710038171003817 lt 3120576
(123)
and 1198632(119892
1 0) gt 119879
1198601198920 minus 3120576 ⩾ 1 minus 120578(120576
0) minus 3120576 gt 0
Finally define the operator 1198783by
1198783(119891 119909) = (0119863
2(119891 0) + 119892
lowast
1(119863
1(119891 0))
1198632(119892
1 0)
119909lowast
1119863
2(119892
1 0))
(124)
for (119891 119909) isin 1198711(120583|
119860)oplus
1119883 It is clear that
100381710038171003817100381711987831003817100381710038171003817 ⩽ sup
119891isin1198781198711(120583|119860)
(10038171003817100381710038171198632(119891 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119891 0)
1003816100381610038161003816) (125)
Notice also that
10038171003817100381710038171198631(119891 0)
1003817100381710038171003817
⩽10038171003817100381710038171198631(119891 0)
1003817100381710038171003817 +10038171003817100381710038171198632(119891 0) minus 119879
1198601198911003817100381710038171003817
=1003817100381710038171003817(1198631
(119891 0) 1198632(119891 0)) minus (0 119879
119860119891)1003817100381710038171003817
=10038171003817100381710038171198782 (119891 119909) minus 1198780 (119891 119909)
1003817100381710038171003817
(126)
for all (119891 119909) isin 1198711(120583|
119860)oplus
1119883 Hence we have
10038171003817100381710038171198783 minus 11987821003817100381710038171003817
= 2 sup119891isin1198781198711(120583|119860)
10038171003817100381710038171198631(119891 0)
1003817100381710038171003817 ⩽ 210038171003817100381710038171198782 minus 1198780
1003817100381710038171003817 lt 4120576(127)
On the other hand let 119866 1198711(120583|
119860) rarr 119871
1(120583|
119860) be defined by
119866(119891) = 119892lowast
1119891 for every 119891 isin 119871
1(120583|
119860) Then we have
V (1198782)
= sup 1003816100381610038161003816119911lowast11987821199111003816100381610038161003816 (119911 119911
lowast) isin Π (119871
1(120583|
119860) oplus
1119883)
⩾ sup10038161003816100381610038161003816100381610038161003816
119909lowast119863
2(119866(
1
120583 (119862)120594119862) 0)
+119892lowast
1119863
1(119866(
1
120583 (119862)120594119862) 0)
10038161003816100381610038161003816100381610038161003816
119909lowastisin 119878
119883lowast 119862 isin Σ
119860 120583 (119862) gt 0
= sup10038171003817100381710038171003817100381710038171003817
1198632(119866(
1
120583 (119862)120594119862) 0)
10038171003817100381710038171003817100381710038171003817
+
10038161003816100381610038161003816100381610038161003816
119892lowast
1119863
1(119866(
1
120583 (119862)120594119862) 0)
10038161003816100381610038161003816100381610038161003816
119862 isin Σ119860 120583 (119862) gt 0
(128)
where Σ119860is the family of measurable subsets of 119860
Hence for any simple function 119904 = sum119899
119894=1((120572
119894120583(119860
119894))120594
119860119894
) isin
1198781198711(120583|119860) where 119860
119894119894is a family of disjoint measurable subsets
with strictly positive measure we have
V (1198782)
⩾
119899
sum
119894=1
10038161003816100381610038161205721198941003816100381610038161003816 (
100381710038171003817100381710038171003817100381710038171003817
1198632(119866(
1
120583 (119860119894)120594119860119894
) 0)
100381710038171003817100381710038171003817100381710038171003817
+
100381610038161003816100381610038161003816100381610038161003816
119892lowast
1119863
1(119866(
1
120583 (119860119894)120594119860119894
) 0)
100381610038161003816100381610038161003816100381610038161003816
)
⩾10038171003817100381710038171198632(119866 (119904) 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119866 (119904) 0)
1003816100381610038161003816
(129)
Since |119892lowast1| = 1119866 is an isometric isomorphism so for each119891 isin
1198781198711(120583|119860)there exists a sequence of norm-one simple functions
(119904119896) such that 119866(119904
119896) converges to 119891 Therefore
V (1198782) ⩾ sup
119891isin1198781198711(120583|119860)
(10038171003817100381710038171198632(119891 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119891 0)
1003816100381610038161003816) ⩾100381710038171003817100381711987831003817100381710038171003817
(130)
On the other hand we see that100381710038171003817100381711987831003817100381710038171003817 ⩾1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
3(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816119909
lowast
1119863
2(119892
1 0) + 119892
lowast
1119863
1(119892
1 0)1003816100381610038161003816 = V (119878
2) = 1
(131)
Therefore 1 = 1198783 = 119878
3(119892
1 0) which proves Claim 2
Finally set 1198783= (0 ) for a suitable 119871
1(120583|
119860)oplus
1119883 rarr
119883 and define the operator 1198791 119871
1(120583) rarr 119883 by
1198791(119891) = 119879
0(119891120594
119860119888) + (119875
119860119891 0) (132)
for every 119891 isin 1198711(120583) Then we have
10038171003817100381710038171198791 (119891)1003817100381710038171003817 ⩽100381710038171003817100381711987901003817100381710038171003817
10038171003817100381710038171198911205941198601198881003817100381710038171003817 +1003817100381710038171003817100381710038171003817100381710038171003817
10038171003817100381710038171198911205941198601003817100381710038171003817 =10038171003817100381710038171198911003817100381710038171003817 (133)
for every 119891 isin 1198711(120583) so 119879
1 ⩽ 1 Also
10038171003817100381710038171198791 (1198691198601198921)1003817100381710038171003817 =10038171003817100381710038171198783 (1198921 0)
1003817100381710038171003817 =100381710038171003817100381711987831003817100381710038171003817 = 1 (134)
so 1198791attains its norm at 119869
1198601198921isin 119871
1(120583) and
10038171003817100381710038171198691198601198921 minus 11989101003817100381710038171003817 =10038171003817100381710038171198921 minus 1198920
1003817100381710038171003817 lt 120576 (135)
We also have that for any 119891 isin 1198781198711(120583)
10038171003817100381710038171198790 (119891) minus 1198791 (119891)1003817100381710038171003817 =100381710038171003817100381710038171198790(119891120594
119860) minus (119875
119860119891 0)10038171003817100381710038171003817
⩽10038171003817100381710038171198790 (119869119860119875119860119891) minus 119879119860 (119875119860119891)
1003817100381710038171003817
+10038171003817100381710038171003817119879119860(119875
119860119891) minus (119875
119860119891 0)10038171003817100381710038171003817
⩽10038171003817100381710038171198790119869119860 minus 119879119860
1003817100381710038171003817 +10038171003817100381710038171198780 minus 1198783
1003817100381710038171003817
lt 120578 (1205760) + 6120576
(136)
Hence 1198790minus 119879
1 ⩽ 120578(120576
0) + 6120576 lt 120576
0
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Abstract and Applied Analysis 15
Acknowledgments
The authors thank Gilles Godefroy and Rafael Paya forfruitful conversations about the content of this paper andVladimir Kadets for providing Lemma 18 They also appre-ciate anonymous referees for careful reading and fruitfulsuggestions about revision The second author was sup-ported by Basic Science Research Program through theNational Research Foundation of Korea (NRF) fundedby the Ministry of Education Science and Technology(NRF-2012R1A1A1006869) The third author was partiallysupported by Spanish MICINN and FEDER project noMTM2012-31755 and by Junta de Andalucıa and FEDERGrants FQM-185 and P09-FQM-4911
References
[1] F F Bonsall and J Duncan Numerical Ranges of Operators onNormed Spaces and of Elements of Normed Algebras vol 2 ofLondon Mathematical Society Lecture Note Series 2 CambridgeUniversity Press London UK 1971
[2] F F Bonsall and J Duncan Numerical Ranges II CambridgeUniversity Press London UK 1973 London MathematicalSociety Lecture Notes Series 10
[3] B Sims On numerical range and its applications to Banachalgebras [PhD thesis] University of Newcastle New SouthWales Australia 1972
[4] I D Berg and B Sims ldquoDenseness of operators which attaintheir numerical radiusrdquoAustralian Mathematical Society A vol36 no 1 pp 130ndash133 1984
[5] C S Cardassi ldquoDensity of numerical radius attaining operatorson some reflexive spacesrdquo Bulletin of the Australian Mathemat-ical Society vol 31 no 1 pp 1ndash3 1985
[6] C S Cardassi ldquoNumerical radius attaining operatorsrdquo inBanach Spaces (Columbia Mo 1984) vol 1166 of Lecture Notesin Mathematics pp 11ndash14 Springer Berlin Germany 1985
[7] C S Cardassi ldquoNumerical radius-attaining operators on119862(119870)rdquoProceedings of the AmericanMathematical Society vol 95 no 4pp 537ndash543 1985
[8] MDAcostaOperadores que alcanzan su radio numerico [PhDthesis] Universidad de Granada Granada Spain 1990
[9] M D Acosta and R Paya ldquoNumerical radius attaining oper-ators and the Radon-Nikodym propertyrdquo The Bulletin of theLondon Mathematical Society vol 25 no 1 pp 67ndash73 1993
[10] R Paya ldquoA counterexample on numerical radius attainingoperatorsrdquo Israel Journal of Mathematics vol 79 no 1 pp 83ndash101 1992
[11] Y S Choi and S G Kim ldquoNorm or numerical radius attainingmultilinear mappings and polynomialsrdquo Journal of the LondonMathematical Society vol 54 no 1 pp 135ndash147 1996
[12] M D Acosta J Becerra Guerrero and M Ruiz GalanldquoNumerical-radius-attaining polynomialsrdquo The Quarterly Jour-nal of Mathematics vol 54 no 1 pp 1ndash10 2003
[13] M D Acosta and S G Kim ldquoDenseness of holomorphicfunctions attaining their numerical radiirdquo Israel Journal ofMathematics vol 161 pp 373ndash386 2007
[14] S G Kim and H J Lee ldquoNumerical peak holomorphicfunctions on Banach spacesrdquo Journal of Mathematical Analysisand Applications vol 364 no 2 pp 437ndash452 2010
[15] M D Acosta R M Aron D Garcıa and M MaestreldquoThe Bishop-Phelps-Bollobas theorem for operatorsrdquo Journal ofFunctional Analysis vol 254 no 11 pp 2780ndash2799 2008
[16] A J Guirao and O Kozhushkina ldquoThe Bishop-Phelps-Bollobasproperty for numerical radius in ℓ
1(119862)rdquo Studia Mathematica
vol 218 no 1 pp 41ndash54 2013[17] J Falco ldquoThe Bishop-Phelps-Bollobas property for numerical
radius on 1198711rdquo Journal of Mathematical Analysis and Applica-
tions vol 414 no 1 pp 125ndash133 2014[18] A Aviles A J Guirao and J Rodriguez ldquoOn the Bishop-
Phelps-Bollobas property for numerical radius in119862(119870)-spacesrdquoPreprint
[19] RM Aron Y S Choi S K Kim H J Lee andMMartin ldquoTheBishop-Phelps-Bollobas version of Lindenstrauss properties Aand Brdquo Preprint
[20] V Kadets M Martın and R Paya ldquoRecent progress and openquestions on the numerical index of Banach spacesrdquo Revista dela Real Academia de Ciencias Exactas Fısicas y Naturales A vol100 no 1-2 pp 155ndash182 2006
[21] M Martın J Merı and M Popov ldquoOn the numerical index ofreal 119871
119901(120583)-spacesrdquo Israel Journal of Mathematics vol 184 pp
183ndash192 2011[22] A Iwanik ldquoNorm attaining operators on Lebesgue spacesrdquo
Pacific Journal of Mathematics vol 83 no 2 pp 381ndash386 1979[23] HH SchaeferBanach Lattices and Positive Operators Springer
New York NY USA 1974[24] J Duncan C M McGregor J D Pryce and A J White ldquoThe
numerical index of a normed spacerdquo Journal of the LondonMathematical Society Second Series vol 2 pp 481ndash488 1970
[25] Y S Choi S K Kim H J Lee and M Martin ldquoThe Bishop-Phelps-Bollobas theorem for operators on 119871
1(120583)rdquo Preprint
[26] H E Lacey The Isometric Theory of Classical Banach SpacesSpringer New York NY USA 1974
[27] G Godefroy ldquoUniversal spaces for strictly convex Banachspacesrdquo Revista de la Real Academia de Ciencias Exactas Fısicasy Naturales A vol 100 no 1-2 pp 137ndash146 2006
[28] C Finet M Martın and R Paya ldquoNumerical index andrenormingrdquo Proceedings of the American Mathematical Societyvol 131 no 3 pp 871ndash877 2003
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical Problems in Engineering
Hindawi Publishing Corporationhttpwwwhindawicom
Differential EquationsInternational Journal of
Volume 2014
Applied MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Probability and StatisticsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical PhysicsAdvances in
Complex AnalysisJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
OptimizationJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CombinatoricsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Operations ResearchAdvances in
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Function Spaces
Abstract and Applied AnalysisHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of Mathematics and Mathematical Sciences
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Algebra
Discrete Dynamics in Nature and Society
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Decision SciencesAdvances in
Discrete MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom
Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of
Abstract and Applied Analysis 9
= intΩ
120594119861119895
(119909)
1198981(119861
119895)
(1198891003816100381610038161003816]1198881003816100381610038161003816
1
1198891198981
(119909))
minus1
1198891003816100381610038161003816]1198881003816100381610038161003816
1
(119909)
= intΩ
120594119861119895
(119909)
1198981(119861
119895)
1198891198981(119909) = 1
(70)
Hence we get ⟨119879]119891 119892⟩ = 1 which implies that 119879]119891 =119879] = 1 Finally10038171003817100381710038171003817119891 minus 119891
10038171003817100381710038171003817
⩽10038171003817100381710038171003817119891 minus 119891
1
10038171003817100381710038171003817+10038171003817100381710038171198911 minus 119891
1003817100381710038171003817
=
10038171003817100381710038171003817100381710038171003817100381710038171003817
sum
119895isin119869
120573119895
120594119861119895
1198981(119861
119895)
minus sum
119895isin119869
120573119895
120594119861119895
1198981(119861
119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+ 120576
⩽ sum
119895isin119869
120573119895(
10038171003817100381710038171003817100381710038171003817100381710038171003817
120594119861119895
1198981(119861
119895)
minus
120594119861119895
1198981(119861
119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
+
10038171003817100381710038171003817100381710038171003817100381710038171003817
120594119861119895
1198981(119861
119895)
minus
120594119861119895
1198981(119861
119895)
10038171003817100381710038171003817100381710038171003817100381710038171003817
) + 120576
= 2sum
119895isin119869
120573119895
1198981(119861
119895 119861
119895)
1198981(119861
119895)
+ 120576
⩽ 2sum
119895isin119869
120573119895
(1205762)1198981(119861
119895)
1198981(119861
119895)
+ 120576 ⩽120576
1 minus 1205762+ 120576 lt 3120576
(71)
We are now ready to present the proof of the main resultin the case of finite regular positive Borel measures
Proof of Proposition 10 Let 1205751= 120575
3
2(5sdot2
4) 120575
2= 120575
12
3(3
2sdot2
14)
and 1205753= (12057610)
2 for some 0 lt 120576 lt 1 Suppose that 119879 isinL(119871
1(119898)) with 119879 = 1 and that there is an 119891
0isin 119878
1198711(119898)
and1198920isin 119878
119871infin(119898)
such that ⟨1198910 119892
0⟩ = 1 and |⟨119879119891
0 119892
0⟩| gt 1minus120575
3
12
6Then there is an isometric isomorphism Ψ from 119871
1(119898) onto
itself such thatΨ(1198910) = |119891
0| and there is a scalar number 120572 in
119878R such that |⟨1198791198910 119892
0⟩| = ⟨120572119879119891
0 119892
0⟩ Then letting 119891
1= Ψ119891
0
1198921= (Ψ
minus1)lowast1198920 and 119879
1= 120572Ψ119879Ψ
minus1 we have
⟨1198781198911 119892
1⟩ = ⟨120572Ψ119879
0Ψ
minus1Ψ119891
0 (Ψ
minus1)lowast
1198920⟩
= ⟨1205721198791198910 119892
0⟩ gt 1 minus
1205753
1
26
⟨1198911 119892
1⟩ = ⟨Ψ119891
0 (Ψ
minus1)lowast
1198920⟩ = 1
(72)
Since 11987911198911 gt 1 minus 120575(120575
3
12
6) by Lemma 13 there exists a
norm-one bounded operator 119879] and a nonnegative simplefunction119891
2isin 119878
1198711(119898)
such that 1198791minus119879] ⩽ 1205751 1198912minus1198911 ⩽ 31205751
and (119889|]|11198891198981)(119909) = 1 for all 119909 isin supp(119891
2) Then
⟨119879]1198912 1198921⟩
= ⟨11987911198911 119892
1⟩ minus ⟨119879
11198911minus 119879
11198912 119892
1⟩ minus ⟨119879
11198912minus 119879]1198912 1198921⟩
⩾ ⟨11987911198911 119892
1⟩ minus10038171003817100381710038171198911 minus 1198912
1003817100381710038171003817 minus10038171003817100381710038171198791 minus 119879]
1003817100381710038171003817
⩾ 1 minus1205753
1
26minus 3120575
1minus 120575
1⩾ 1 minus 5120575
1= 1 minus
1205753
2
16
(73)
Notice also that
⟨1198912 119892
1⟩ = ⟨119891
1 119892
1⟩ minus ⟨119891
1minus 119891
2 119892
1⟩ ⩾ 1 minus
10038171003817100381710038171198911 minus 11989121003817100381710038171003817
⩾ 1 minus 31205751⩾ 1 minus 5120575
1= 1 minus
1205753
2
16
(74)
By Lemma 12 there are a nonnegative simple function 1198913isin
1198781198711(119898)
and a function 1198923isin 119878
119871infin(119898)
such that
1198923(119909) = 120594supp119891
3
(119909) + 1198922(119909) 120594
Ωsupp1198913
(119909)
10038171003817100381710038171198912 minus 11989131003817100381710038171003817 ⩽ 1205752
10038171003817100381710038171198923 minus 11989211003817100381710038171003817 ⩽radic120575
2 ⟨119891
3 119892
3⟩ = 1
(75)
So we have
⟨119879]1198913 1198923⟩ = ⟨119879]1198912 1198921⟩ minus ⟨119879]1198912 minus 119879]1198913 1198921⟩
minus ⟨119879]1198913 1198921 minus 1198923⟩
⩾ 1 minus1205753
2
16minus 2radic120575
2⩾ 1 minus 3radic120575
2= 1 minus
1205756
3
27
(76)
By Lemma 14 there exist 1198914isin 119878
1198711(119898)
and 1198924isin 119878
119871infin(119898)
and anoperator 119879
4such that
⟨1198924 119879
41198914⟩ = 1 =
100381710038171003817100381711987941003817100381710038171003817
10038171003817100381710038171198794 minus 119879]1003817100381710038171003817 ⩽ 21205753
10038171003817100381710038171198914 minus 11989131003817100381710038171003817 ⩽ 31205753
10038171003817100381710038171198924 minus 11989231003817100381710038171003817 ⩽radic120575
3 ⟨119891
4 119892
4⟩ = 1
(77)
So we have
10038171003817100381710038171198794 minus 11987911003817100381710038171003817 ⩽10038171003817100381710038171198794 minus 119879]
1003817100381710038171003817 +1003817100381710038171003817119879] minus 1198791
1003817100381710038171003817
⩽ 1205751+ 2120575
3⩽ 3120575
3
10038171003817100381710038171198911 minus 11989141003817100381710038171003817 ⩽10038171003817100381710038171198911 minus 1198912
1003817100381710038171003817 +10038171003817100381710038171198912 minus 1198913
1003817100381710038171003817 +10038171003817100381710038171198913 minus 1198914
1003817100381710038171003817
⩽ 31205751+ 120575
2+ 3120575
3⩽ 10120575
3
10038171003817100381710038171198921 minus 11989241003817100381710038171003817 ⩽10038171003817100381710038171198921 minus 1198923
1003817100381710038171003817 + 1198923 minus 1198924
⩽ 1205752+ radic120575
3⩽ 2radic120575
3
(78)
10 Abstract and Applied Analysis
Let 119878 = 120572Ψminus11198794Ψ 119891 = Ψminus1
1198914 and 119892 = Ψlowast
1198924 then we have
119879 minus 119878 =10038171003817100381710038171003817119879 minus 120572Ψ
minus11198794Ψ10038171003817100381710038171003817=10038171003817100381710038171003817120572Ψ119879Ψ
minus1minus 119879
4
10038171003817100381710038171003817
=10038171003817100381710038171198791 minus 1198794
1003817100381710038171003817 ⩽ 31205753
100381710038171003817100381710038171198910minus 11989110038171003817100381710038171003817=100381710038171003817100381710038171198910minus Ψ
minus11198914
10038171003817100381710038171003817=10038171003817100381710038171198911 minus 1198914
1003817100381710038171003817 ⩽ 101205753
100381710038171003817100381710038171198920minus 11989110038171003817100381710038171003817=10038171003817100381710038171198920 minus Ψ
lowast1198924
1003817100381710038171003817 =100381710038171003817100381710038171003817(Ψ
minus1)lowast
1198920minus 119892
4
100381710038171003817100381710038171003817
=10038171003817100381710038171198921 minus 1198924
1003817100381710038171003817 ⩽ 2radic120575
3
⟨119891 119892⟩ = ⟨Ψminus11198914 Ψ
lowast1198924⟩ = ⟨119891
4 119892
4⟩ = 1
10038161003816100381610038161003816⟨119878119891 119892⟩
10038161003816100381610038161003816=10038161003816100381610038161003816⟨120572Ψ
minus11198794ΨΨ
minus11198914 Ψ
lowast1198924⟩10038161003816100381610038161003816= |120572| = 1
(79)
This completes the proof
Finally we may give the proof of the main result in fullgenerality
Proof of Theorem 9 Notice that the Kakutani representationtheorem (see [26] for a reference) says that for every 120590-finitemeasure ] the space 119871
1(]) is isometrically isomorphic to
1198711(119898) for some positive Borel regular measure on a compact
Hausdorff space Then by Proposition 10 there is a universalfunction 120576 997891rarr 120578(120576) gt 0 which gives the BPBp-nu for 119871
1(]) for
every 120590-finite measure ]Fix 120576 gt 0 Suppose that 119879
0isin L(119871
1(120583)) with V(119879
0) = 1
and (1198910 119891
lowast
0) isin Π(119871
1(120583)) satisfy
1003816100381610038161003816⟨119891lowast
0 119879
01198910⟩1003816100381610038161003816 gt 1 minus 120578 (120576) (80)
Choose a sequence 119891119899 in 119871
1(120583) such that sup
119899119879
0119891119899 = 1
and let 119866 be the closed linear span of
119879119899
0119891119898 119899 119898 isin N cup 0 (81)
As 119866 is separable there is a dense subset 119892119899 119899 isin N of 119866
and let 119864 = ⋃infin
119899=1supp119892
119899 where supp119892
119899is the support of 119892
119899
Then the measure 120583|119864is 120590-finite Let
119884 = 119891 isin 1198711(120583) supp (119891) sub 119864 (82)
be a closed subspace of 1198711(120583) It is clear that 119871
1(120583) = 119884oplus
1119885
and 119884 is isometrically isomorphic to 1198711(120583|
119864) So 119884 has the
BPBp-nu with 120578(120576)Now write 119878
0= 119879
0|119884 119884 rarr 119884 consider 119910
0= 119891
0isin
119878119884 119910lowast
0= 119891
lowast
0|119884isin 119878
119884lowast and observe that 119910lowast
0(119910
0) = 1 and
|119910lowast
0(119878
01199100)| = |119891
lowast
0(119879
01198910)| gt 1 minus 120578(120576) Hence there exist 119878 isin
L(119884) and (1199100 119910
lowast
0) isin Π(119884) such that
1003816100381610038161003816119910lowast
0(119878119910
0)1003816100381610038161003816 = 1 = V (119878) 1003817100381710038171003817119878 minus 1198780
1003817100381710038171003817 lt 120576
10038171003817100381710038171199100 minus 11991001003817100381710038171003817 lt 120576
1003817100381710038171003817119910lowast
0minus 119910
lowast
0
1003817100381710038171003817 lt 120576
(83)
Finally consider the operator 119879 isinL(1198711(120583)) given by
119879 (119910 119911)
= (119878119910 0) + 1198790(0 119911) ((119910 119911) isin 119871
1(120583) equiv 119884oplus
1119885)
(84)
We have 119879 = 1 (and so V(119879) = 1) Indeed1003817100381710038171003817119879 (119910 119911)
1003817100381710038171003817 =1003817100381710038171003817(119878119910 0)
1003817100381710038171003817 +10038171003817100381710038171198790 (0 119911)
1003817100381710038171003817 ⩽10038171003817100381710038171199101003817100381710038171003817 + 119911 =
1003817100381710038171003817(119910 119911)1003817100381710038171003817
(85)
for all (119910 119911) isin 1198711(120583) and 119879(119910
0 0) = (119878119910
0 0) = 119878119910
0 = 1
Let 119909 = (1199100 0) and 119909lowast = (119910lowast
0 119891
0|119885) Then (119909 119909lowast) isin Π(119871
1(120583))
Moreover we have1003816100381610038161003816119909
lowast1198791199091003816100381610038161003816 =1003816100381610038161003816119910
lowast
0119878119910
0
1003816100381610038161003816 = 1 = V (119879) 1003817100381710038171003817119909 minus 1198910
1003817100381710038171003817 =1003817100381710038171003817119910 minus 1199100
1003817100381710038171003817 lt 120576
1003817100381710038171003817119909lowast
0minus 119891
lowast
0
1003817100381710038171003817
= max 1003817100381710038171003817119910 minus 119891lowast
0|119884
1003817100381710038171003817 1003817100381710038171003817119891
lowast
0|119885minus 119891
lowast
0|119885
1003817100381710038171003817
=1003817100381710038171003817119910
lowastminus 119910
lowast
0
1003817100381710038171003817 lt 120576
1003817100381710038171003817119879 minus 11987901003817100381710038171003817 = sup119910+119911⩽1
1003817100381710038171003817119879 (119910 119911) minus 1198790 (119910 119911)1003817100381710038171003817
= sup119910⩽1
1003817100381710038171003817119878119910 minus 11987801199101003817100381710038171003817 =1003817100381710038171003817119878 minus 1198780
1003817100381710038171003817 lt 120576
(86)
This completes the proof
5 Examples of Spaces Failing theBishop-Phelps-Bollobaacutes Property forNumerical Radius
Our goal here is to prove that the density of numerical radiusattaining operators does not imply the BPBp-nu Actuallywe will show that among separable spaces there is noisomorphic property implying the BPBp-nu other than finite-dimensionality
We need to relate the BPBp-nu to the Bishop-Phelps-Bollobas property for operators which as mentioned in theintroduction was introduced in [15] A pair (119883 119884) of Banachspaces has the Bishop-Phelps-Bollobas property for operators(in short BPBp) if given 120576 gt 0 there exists 120578(120576) gt 0 suchthat given 119879 isin L(119883 119884) with 119879 = 1 and 119909 isin 119878
119883such that
119879119909 gt 1 minus 120578(120576) then there exist 119911 isin 119878119883and 119878 isin L(119883 119884)
satisfying
119878 = 119878119911 = 1 119909 minus 119911 lt 120576
119879 minus 119878 lt 120576
(87)
We refer the reader to [15 19 25] and references therein formore information and background Among the interestingresults on the BPBp we emphasize that a pair (119883 119884) when119883is finite-dimensional does not necessarily have the BPBp Forinstance if119884 is a strictly convex space which is not uniformlyconvex then the pair (ℓ(2)
1 119884) fails to have the BPBp (this is
contained in [15] see [19 Section 3])
Abstract and Applied Analysis 11
The next result relates the BPBp-nu to the BPBp foroperators in a particular case We will deduce our examplefrom it
Theorem 15 If 1198711(120583)oplus
1119883 has the BPBp-119899119906 then the pair
(1198711(120583) 119883) has the BPBp for operators
Before proving this proposition we will use it to get themain examples of this section The first example shows thatthe density of numerical radius attaining operators does notimply the BPBp-nu
Example 16 There is a reflexive space (and so numerical radiusattaining operators on it are dense) which fails to have theBPBp-nu Indeed let 119884 be a reflexive separable space whichis not superreflexive and we may suppose that 119884 is strictlyconvex Observe that 119884 cannot be uniformly convex since itis not superreflexive Now 119883 = ℓ(2)
1oplus1119884 is reflexive but the
pair (ℓ(2)1 119884) fails the BPBp since 119884 is strictly convex but not
uniformly convex [19 Corollary 33] Therefore Theorem 15gives us that119883 does not have the BPBp-nu
The example above can be extended to get the result thatevery infinite-dimensional separable Banach space can berenormed to fail the BPBp-nu This follows from the factthat every infinite-dimensional separable Banach space canbe renormed to be strictly convex but not uniformly convex(this result can be proved ldquoby handrdquo an alternative categoricalargument for it can be found in [27] and references therein)With a little more of effort we may get the main result of thesection
Theorem 17 Every infinite-dimensional separable Banachspace can be renormed to fail the weak-BPBp-nu (and so inparticular to fail the BPBp-nu)
We need the following result which is surely well knownAs we have not found a reference we include a nice and easyproof kindly given to us by Vladimir Kadets We recall thatgiven a Banach space 119884 the set of all equivalent norms on119884 can be viewed as a metric space using the Banach-Mazurdistance
Lemma 18 Let119884 be an infinite-dimensional separable Banachspace Then the set of equivalent norms on 119884 which are strictlyconvex and are not (locally) uniformly convex is dense in the setof all equivalent norms on119884 (with respect to the Banach-Mazurdistance)
Proof Fix 119890 isin 119878119884and 119890lowast
1isin 119878
119884lowast such that 119890lowast
1(119890) = 1 For a fixed
120576 isin (0 12) denote
119902 (119910) = max (1 minus 120576) 10038171003817100381710038171199101003817100381710038171003817 1003816100381610038161003816119890lowast
1(119910)1003816100381610038161003816 (119910 isin 119884) (88)
Evidently (1 minus 120576)119910 ⩽ 119902(119910) ⩽ 119910 for every 119910 isin 119884 Fix asequence 119890lowast
119896 119896 ⩾ 2 of norm-one functionals separating
the points of 119884 and denote
119901 (119910) = radic
infin
sum
119896=1
1
2119896
1003816100381610038161003816119890lowast
119896(119910)1003816100381610038161003816
2
(119910 isin 119884) (89)
Then 119901 is a strictly convex norm on 119884 119901(119890) ⩾ 1radic2 and119901(119910) ⩽ 119910 for all 119910 isin 119883 Finally write
100381710038171003817100381711991010038171003817100381710038171= (1 minus 120576) 119902 (119910) + 120576
119901 (119910)
119901 (119890)(119910 isin 119884) (90)
Then sdot 1is a strictly convex norm on 119884 and
(1 minus 120576)2 10038171003817100381710038171199101003817100381710038171003817 ⩽100381710038171003817100381711991010038171003817100381710038171⩽ (1 + 120576)
10038171003817100381710038171199101003817100381710038171003817 (119910 isin 119884) (91)
Wewill finish the proof by showing that sdot 1is not uniformly
convex (actually it is not locally uniformly convex) Indeedfor each 119899 isin N we select 119910
119899isin ⋂
119899
119896=1ker 119890lowast
119896with 119910
119899 = 1 and
consider 119890119899= 119890 + (1205764)119910
119899 Then 119902(119890) = 1 119902(119890
119899) = 1 and
119902(119890+119890119899) = 2 At the same time 119901(119910
119899) rarr 0 so 119901(119890
119899) rarr 119901(119890)
and 119901(119890 + 119890119899) rarr 2119901(119890) Consequently
1198901 = 1100381710038171003817100381711989011989910038171003817100381710038171997888rarr 1
1003817100381710038171003817119890 + 11989011989910038171003817100381710038171997888rarr 2 (92)
but 119890 minus 1198901198991= (1205764)119910
1198991⩾ (1 minus 120576)
2(1205764) which means the
absence of local uniform convexity at 119890
Proof of Theorem 17 Let 119883 be an infinite-dimensional sepa-rable Banach space Take a closed subspace 119884 of 119883 of codi-mension two By [28 Proposition 2] the map carrying everyequivalent norm on 119884 to its numerical index is continuousand so the set of values of the numerical index of 119884 upto reforming is a nontrivial interval [28 Theorem 9] ThenLemma 18 allows us to find an equivalent norm | sdot | on 119884 insuch a way that (119884 | sdot |) is strictly convex and is not uniformlyconvex and 119899(119884 | sdot |) gt 0 Now the space 119883 = ℓ(2)
1oplus1(119884 | sdot |)
is an equivalent renorming of 119883 which does not have theBPBp-119899119906 (indeed otherwise the pair (ℓ(2)
1 (119884 | sdot |)) would
have the BPBp for the operator norm and so (119884 | sdot |) wouldbe uniformly convex by [19 Corollary 33] a contradiction)Moreover as
119899 (119883) = min 119899 (ℓ(2)1) 119899 (119884 |sdot|) gt 0 (93)
(see [20 Proposition 2] for instance) 119883 also fails the weak-BPBp-nu by Proposition 6
To finish the section with the promised proof ofTheorem 15 we first see the following stability result
Lemma 19 Let119883 = [⨁infin
119896=1119883
119896]1198880
or [⨁infin
119896=1119883
119896]ℓ1
If119883 has theBishop-Phelps-Bollobas property for numerical radius with afunction 120578 then each Banach space 119883
119894has the Bishop-Phelps-
Bollobas property for numerical radius with 120578nu(119883119894) ⩾ 120578 That
is inf119894120578nu(119883119894
)(120576) ⩾ 120578nu(119883)(120576) for all 0 lt 120576 lt 1
Proof Let 119875119894 119883 rarr 119883
119894and 1198751015840
119894 119883
lowastrarr 119883
lowast
119894be the natural
projections and let 119876119894 119883
119894rarr 119883 and 1198761015840
119894 119883
lowast
119894rarr 119883
lowast be thenatural embeddings
Assume that an operator 119879119894 119883
119894rarr 119883
119894and a pair
(119909119894 119909
lowast
119894) isin Π(119883
119894) satisfy that
V (119879119894) = 1
1003816100381610038161003816119909lowast
119894119879119894119909119894
1003816100381610038161003816 gt 1 minus 120578 (120576) (94)
We define an operator 119879 119883 rarr 119883 and (119909 119909lowast) isin Π(119883) by
119879 = 119876119894∘ 119879
119894∘ 119875
119894 (119909 119909
lowast) = (119876
119894119909119894 119876
1015840
119894119909lowast
119894) (95)
12 Abstract and Applied Analysis
then clearly we see that1003816100381610038161003816119909
lowast1198791199091003816100381610038161003816 =1003816100381610038161003816119909
lowast
119894119879119894119909119894
1003816100381610038161003816 gt 1 minus 120578 (120576) (96)
From the assumption there exist 119878 119883 rarr 119883 and a pair(119910 119910
lowast) isin Π(119883) such that
1003816100381610038161003816119910lowast1198781199101003816100381610038161003816 = 1 = V (119878) 119878 minus 119879 lt 120576
1003817100381710038171003817119910lowastminus 119909
lowast1003817100381710038171003817 lt 1205761003817100381710038171003817119910 minus 119909
1003817100381710038171003817 lt 120576
(97)
Since this clearly shows that1003817100381710038171003817119875119894 ∘ 119878 ∘ 119876119894
minus 119879119894
1003817100381710038171003817 lt 120576100381710038171003817100381710038171198751015840
119894119910lowastminus 119909
lowast
119894
10038171003817100381710038171003817lt 120576
1003817100381710038171003817119875119894119910 minus 1199091198941003817100381710038171003817 lt 120576
(98)
we only need to show that |1198751015840119894119910lowast(119875
119894∘ 119878 ∘ 119876
119894)119875
119894119910| = 1
We first show the case of 1198880sum Since
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817=10038171003817100381710038171003817119875119895119910 minus 119875
11989511990910038171003817100381710038171003817⩽1003817100381710038171003817119910 minus 119909
1003817100381710038171003817 lt 120576 (99)
for every 119895 = 119894 we have
1 = 119910lowast(119910) = sum
119895isinN
1198751015840
119895119910lowast(119875
119895119910) ⩽ sum
119895isinN
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817
⩽100381710038171003817100381710038171198751015840
119894119910lowast10038171003817100381710038171003817+ 120576 sum
119895isinN119895 = 119894
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817⩽1003817100381710038171003817119910
lowast1003817100381710038171003817 = 1
(100)
This shows that 1198751015840119894119910lowast = 1 and 1198751015840
119895119910lowast= 0 for every 119895 = 119894
So 119910lowast = 1198761015840
1198941198751015840
119894119910lowast and 1198751015840
119894119910lowast(119875
119894119910) = 1 This and the fact that
119910 minus 119876119894119875119894119910 lt 120576 imply that
(119876119894119875119894119910 + (
1
120576) (119910 minus 119876
119894119875119894119910) 119876
1015840
1198941198751015840
119894119910lowast) isin Π (119883) (101)
So we get that (1198761015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910 + (1120576)(119910 minus 119876
119894119875119894119910))V(119878) = 1
Hence we have
1 =1003816100381610038161003816119910
lowast1198781199101003816100381610038161003816 =10038161003816100381610038161003816(119876
1015840
1198941198751015840
119894119910lowast) 11987811991010038161003816100381610038161003816
=
1003816100381610038161003816100381610038161003816(1 minus 120576) (119876
1015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910)
+120576 (1198761015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910 +1
120576(119910 minus 119876
119894119875119894119910))
1003816100381610038161003816100381610038161003816⩽ 1
(102)
and so we get |1198751015840119894119910lowast(119875
119894∘119878 ∘119876
119894)119875
119894119910| = |(119876
1015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910)| = 1
We next show the case of ℓ1sum The proof is almost
the same as that of the 1198880case However for the sake of
completeness we provide it hereSince 1198751015840
119895119910lowast = 119875
1015840
119895119910lowastminus 119875
1015840
119895119909lowast ⩽ 119910
lowastminus 119909
lowast lt 120576 for every
119895 = 119894 we have
1 = 119910lowast(119910) = sum
119895isinN
1198751015840
119895119910lowast(119875
119895119910) ⩽ sum
119895isinN
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817
⩽10038171003817100381710038171198751198941199101003817100381710038171003817 + 120576 sum
119895isinN119895 = 119894
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817⩽10038171003817100381710038171199101003817100381710038171003817 = 1
(103)
which shows 119875119894119910 = 1 and 119875
119895119910 = 0 for every 119895 = 119894 Since this
implies (119876119894119875119894119910119876
1015840
1198941198751015840
119894119910lowast+(1120576)(119910
lowastminus119876
1015840
1198941198751015840
119894119910lowast)) isin Π(119883) we get
that |(1198761015840
1198941198751015840
119894119910lowast+ (1120576)(119910
lowastminus 119876
1015840
1198941198751015840
119894119910lowast))119878(119876
119894119875119894119910)| ⩽ V(119878) = 1
Hence we have
1 =1003816100381610038161003816119910
lowast1198781199101003816100381610038161003816 =1003816100381610038161003816119910
lowast119878 (119876
119894119875119894119910)1003816100381610038161003816
=
1003816100381610038161003816100381610038161003816(1 minus 120576) (119876
1015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910)
+120576 (1198761015840
1198941198751015840
119894119910lowast+1
120576(119910
lowastminus 119876
1015840
1198941198751015840
119894119910lowast)) 119878 (119876
119894119875119894119910)
1003816100381610038161003816100381610038161003816⩽ 1
(104)
and so |1198751015840119894119910lowast(119875
119894∘ 119878 ∘ 119876
119894)119875
119894119910| = |(119876
1015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910)| = 1
Proof of Theorem 15 Note that 120578nu(1198711(120583)oplus1119883)(120576) rarr 0 as120576 rarr 0 Fix 0 lt 120576
0lt 1 and choose 0 lt 120576 lt 1 such that
6120576 + 120578nu(1198711(120583)oplus1119883)(120576) lt 1205760 Let 120578(1205760) = 120578nu(1198711(120583)oplus1119883)(120576)Suppose that 119879
0isin L(119871
1(120583) 119883) with 119879
0 = 1 and 119891
0isin
1198781198711(120583)
satisfy
1003817100381710038171003817119879011989101003817100381710038171003817 gt 1 minus 120578 (1205760) (105)
For any measurable subset 119861 let
1198711(120583|
119861) = 119891|
119861 119891 isin 119871
1(120583) (106)
with the norm 119891|119861 = 119891120594
1198611 Then it is easy to see
that 1198711(120583|
119861) is isometrically isomorphic to a complemented
subspace of 1198711(120583) Let 119875
119861 119871
1(120583) rarr 119871
1(120583|
119861) be the
restriction defined by 119875119861(119891) = 119891|
119861for all 119891 isin 119871
1(120583) and
let 119869119861 119871
1(120583|
119861) rarr 119871
1(120583) be the extension defined by
119869119861(119891)(120596) = 119891(120596) if 120596 isin 119861 and 119869
119861(119891)(120596) = 0 otherwise
It is clear that 119875119861119869119861= Id
1198711(120583|119861)and 119869
119861119875119861(119891) = 119891120594
119861for all
119891 isin 1198711(120583) Notice also that 119871
1(120583) is isometrically isomorphic
to 1198711(120583|
119861)oplus
11198711(120583|
119861119888)
Let 119860 = supp1198910and 119892
0= 119875
1198601198910 Then
1003817100381710038171003817119879011986911986011989201003817100381710038171003817 =100381710038171003817100381711987901198910
1003817100381710038171003817 gt 1 minus 120578 (1205760) gt 0 (107)
and define the operator 119879119860 119871
1(120583|
119860) rarr 119883 by 119879
119860119891 =
1198790119869119860119891119879
0119869119860 for every 119891 isin 119871
1(120583|
119860) Then
100381710038171003817100381711987911986011989201003817100381710038171003817 ⩾100381710038171003817100381711987901198910
1003817100381710038171003817 gt 1 minus 120578 (1205760) (108)
Since 120583|119860is 120590-finite 119871
1(120583|
119860)lowast= 119871
infin(120583|
119860) Let 119892lowast
0isin 119878
119871infin(120583|119860)
be a function such that ⟨119892lowast0 119892
0⟩ = 1 and choose 119909lowast
0isin 119878
119883lowast
such that 119909lowast0(119879
1198601198920) = 119879
1198601198920 Define the operator 119878
0isin
L(1198711(120583|
119860)oplus
1119883) by
1198780(119891 119909) = (0 119879
119860119891) ((119891 119909) isin 119871
1(120583|
119860) oplus
1119883) (109)
and observe that 1198780 = V(119878
0) = 1 Indeed
100381710038171003817100381711987801003817100381710038171003817 ⩽ 1 =
10038171003817100381710038171198791198601003817100381710038171003817
= sup 1003816100381610038161003816119909lowast1198791198601198911003816100381610038161003816 119909
lowastisin 119878
119883lowast 119891 isin 119878
1198711(120583|119860)
= sup 1003816100381610038161003816(119891lowast 119909
lowast) 119878
0(119891 119909)
1003816100381610038161003816
((119891lowast 119909
lowast) (119891 119909)) isin Π (119871
1(120583|
119860) oplus
1119883)
= V (1198780) ⩽100381710038171003817100381711987801003817100381710038171003817
(110)
Abstract and Applied Analysis 13
It is immediate that
(119892lowast
0 119909
lowast
0) 119878
0(119892
0 0) = 119909
lowast
0(119879
1198601198920) =10038171003817100381710038171198791198601198920
1003817100381710038171003817 gt 1 minus 120578 (1205760)
(111)
By Lemma 19 1198711(120583|
119860)oplus
1119883 has the BPBp-nu with the func-
tion 120578 Therefore there exist 1198781isin L(119871
1(120583|
119860)oplus
1119883) (119892
1 119909
1) isin
1198781198711(120583|119860)oplus1119883 and (119892lowast
1 119909
lowast
1) isin 119878
119871infin(120583|119860)oplusinfin119883lowast such that
1003817100381710038171003817(1198921 1199091) minus (1198920 0)1003817100381710038171003817 lt 120576
1003817100381710038171003817(119892lowast
1 119909
lowast
1) minus (119891
lowast
0 119909
lowast
0)1003817100381710038171003817 lt 120576
10038171003817100381710038171198781 minus 11987801003817100381710038171003817 lt 120576 ⟨(119892
lowast
1 119909
lowast
1) (119892
1 119909
1)⟩ = 1
1003816100381610038161003816(119892lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)1003816100381610038161003816 = V (119878
1) = 1
(112)
Claim 1 We claim that 1199091= 0
Otherwise
1 = Re ⟨(119892lowast1 119909
lowast
1) (119892
1 119909
1)⟩
=100381710038171003817100381711989211003817100381710038171003817Re⟨(119892
lowast
1 119909
lowast
1) (119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
⟩
+100381710038171003817100381711990911003817100381710038171003817Re⟨(119892
lowast
1 119909
lowast
1) (0 119909
1)
100381710038171003817100381711990911003817100381710038171003817
⟩ ⩽ 1
(113)
We deduce that
((119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
(119892lowast
1 119909
lowast
1)) (
(0 1199091)
100381710038171003817100381711990911003817100381710038171003817
(119892lowast
1 119909
lowast
1))
isin Π (1198711(120583|
119860) oplus
1119883)
(114)
Since 1198781((0 119909
1)119909
1) = (119878
1minus 119878
0)((0 119909
1)119909
1) lt 120576 we
get that
1 =1003816100381610038161003816⟨(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)⟩1003816100381610038161003816
=
100381610038161003816100381610038161003816100381610038161003816
100381710038171003817100381711989211003817100381710038171003817⟨(119892
lowast
1 119909
lowast
1) 119878
1((119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
)⟩
+100381710038171003817100381711990911003817100381710038171003817⟨(119892
lowast
1 119909
lowast
1) 119878
1((0 119909
1)
100381710038171003817100381711990911003817100381710038171003817
)⟩
100381610038161003816100381610038161003816100381610038161003816
⩽100381710038171003817100381711989211003817100381710038171003817 V (1198781) + 120576
100381710038171003817100381711990911003817100381710038171003817 lt100381710038171003817100381711989211003817100381710038171003817 +100381710038171003817100381711990911003817100381710038171003817 = 1
(115)
a contradiction This proves the claimWe define the operator 119878
2 119871
1(120583|
119860)oplus
1119883 rarr 119871
1(120583|
119860)oplus
1119883
by 1198782(119891 119909) = 119878
1(119891 0) for every 119891 isin 119871
1(120583|
119860) and for every
119909 isin 119883 Then we have
V (1198782) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816 = 1
10038171003817100381710038171198781 minus 11987821003817100381710038171003817 ⩽ 120576 (116)
Indeed from Claim 1 we have
V (1198781) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)1003816100381610038161003816 =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816 ⩽ V (119878
2)
(117)
On the other hand we see that1003816100381610038161003816(119891
lowast 119909
lowast) 119878
2(119891 119909)
1003816100381610038161003816
=1003816100381610038161003816(119891
lowast 119909
lowast) 119878
1(119891 0)
1003816100381610038161003816 ⩽10038171003817100381710038171198911003817100381710038171003817 V (1198781) ⩽ V (119878
1)
(118)
for every ((119891lowast 119909lowast) (119891 119909)) isin Π(1198711(120583|
119860)oplus
1119883) Therefore
V(1198782) ⩽ V(119878
1) Also
10038171003817100381710038171198781 minus 11987821003817100381710038171003817 ⩽ sup
119909isin119878119883
10038171003817100381710038171198781 (0 119909)1003817100381710038171003817
= sup119909isin119878119883
10038171003817100381710038171198781 (0 119909) minus 1198780 (0 119909)1003817100381710038171003817 ⩽ 120576
(119)
Claim 2 There exists 1198783 119871
1(120583|
119860)oplus
1119883 rarr 119871
1(120583|
119860)oplus
1119883 such
that 1198783(119892
1 0) = 119878
3 = 1 119878
3(0 119909) = 0 119878
3(119891 119909) isin 0oplus
1119883
for every (119891 119909) isin 1198711(120583|
119860)oplus
1119883 and 119878
3minus 119878
2 lt 4120576
Indeed write 1198781= (119863
1 119863
2) where 119863
1 119871
1(120583|
119860)oplus
1119883 rarr
1198711(120583|
119860) and119863
2 119871
1(120583|
119860)oplus
1119883 rarr 119883 We have that
sup 1003816100381610038161003816119892lowast119863
1(119892
1 0) + 119909
lowast119863
2(119892
1 0)1003816100381610038161003816
119909lowastisin 119878
119883lowast ⟨119892
lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860)
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816 +10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
⟨119892lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860)
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816
⟨119892lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860) +10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
⩽ V (1198782) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816119892
lowast
1119863
1(119892
1 0) + 119909
lowast
1119863
2(119892
1 0)1003816100381610038161003816
(120)
This implies that
1003816100381610038161003816119909lowast
1119863
2(119892
1 0)1003816100381610038161003816 =10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
1003816100381610038161003816119892lowast
1119863
1(119892
1 0)1003816100381610038161003816
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816 ⟨119892
lowast 119892
1⟩ = 1 119892
lowastisin 119871
infin(120583|
119860)
(121)
Therefore |119892lowast1| equals 1 on the support of 119863
1(119892
1 0) As
|⟨119892lowast
1 119892
1⟩| = 1 we also have that |119892lowast
1| equals 1 on the support
of 1198921 Changing the values of 119892lowast
1by the ones of 119891lowast
0on 119860
(supp(1198631(119892
1 0)) cup supp(g
1)) we may and do suppose that
|119892lowast
1| = 1 on the whole 119860We also have 119863
2(119892
1 0) gt 0 Indeed
10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198920 0)1003817100381710038171003817
⩽10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198921 0)
1003817100381710038171003817 +10038171003817100381710038171198780 (1198921 0) minus 1198780 (1198920 0)
1003817100381710038171003817
lt 2120576 + 120576 = 3120576
(122)
So we have
10038171003817100381710038171198632(119892
1 0) minus 119879
1198601198920
1003817100381710038171003817
⩽10038171003817100381710038171198631(119892
1 0)1003817100381710038171003817 +10038171003817100381710038171198632(119892
1 0) minus 119879
1198601198920
1003817100381710038171003817
14 Abstract and Applied Analysis
=1003817100381710038171003817(1198631
(1198921 0) 119863
2(119892
1 0)) minus (0 119879
1198601198920)1003817100381710038171003817
=10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198920 0)
1003817100381710038171003817 lt 3120576
(123)
and 1198632(119892
1 0) gt 119879
1198601198920 minus 3120576 ⩾ 1 minus 120578(120576
0) minus 3120576 gt 0
Finally define the operator 1198783by
1198783(119891 119909) = (0119863
2(119891 0) + 119892
lowast
1(119863
1(119891 0))
1198632(119892
1 0)
119909lowast
1119863
2(119892
1 0))
(124)
for (119891 119909) isin 1198711(120583|
119860)oplus
1119883 It is clear that
100381710038171003817100381711987831003817100381710038171003817 ⩽ sup
119891isin1198781198711(120583|119860)
(10038171003817100381710038171198632(119891 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119891 0)
1003816100381610038161003816) (125)
Notice also that
10038171003817100381710038171198631(119891 0)
1003817100381710038171003817
⩽10038171003817100381710038171198631(119891 0)
1003817100381710038171003817 +10038171003817100381710038171198632(119891 0) minus 119879
1198601198911003817100381710038171003817
=1003817100381710038171003817(1198631
(119891 0) 1198632(119891 0)) minus (0 119879
119860119891)1003817100381710038171003817
=10038171003817100381710038171198782 (119891 119909) minus 1198780 (119891 119909)
1003817100381710038171003817
(126)
for all (119891 119909) isin 1198711(120583|
119860)oplus
1119883 Hence we have
10038171003817100381710038171198783 minus 11987821003817100381710038171003817
= 2 sup119891isin1198781198711(120583|119860)
10038171003817100381710038171198631(119891 0)
1003817100381710038171003817 ⩽ 210038171003817100381710038171198782 minus 1198780
1003817100381710038171003817 lt 4120576(127)
On the other hand let 119866 1198711(120583|
119860) rarr 119871
1(120583|
119860) be defined by
119866(119891) = 119892lowast
1119891 for every 119891 isin 119871
1(120583|
119860) Then we have
V (1198782)
= sup 1003816100381610038161003816119911lowast11987821199111003816100381610038161003816 (119911 119911
lowast) isin Π (119871
1(120583|
119860) oplus
1119883)
⩾ sup10038161003816100381610038161003816100381610038161003816
119909lowast119863
2(119866(
1
120583 (119862)120594119862) 0)
+119892lowast
1119863
1(119866(
1
120583 (119862)120594119862) 0)
10038161003816100381610038161003816100381610038161003816
119909lowastisin 119878
119883lowast 119862 isin Σ
119860 120583 (119862) gt 0
= sup10038171003817100381710038171003817100381710038171003817
1198632(119866(
1
120583 (119862)120594119862) 0)
10038171003817100381710038171003817100381710038171003817
+
10038161003816100381610038161003816100381610038161003816
119892lowast
1119863
1(119866(
1
120583 (119862)120594119862) 0)
10038161003816100381610038161003816100381610038161003816
119862 isin Σ119860 120583 (119862) gt 0
(128)
where Σ119860is the family of measurable subsets of 119860
Hence for any simple function 119904 = sum119899
119894=1((120572
119894120583(119860
119894))120594
119860119894
) isin
1198781198711(120583|119860) where 119860
119894119894is a family of disjoint measurable subsets
with strictly positive measure we have
V (1198782)
⩾
119899
sum
119894=1
10038161003816100381610038161205721198941003816100381610038161003816 (
100381710038171003817100381710038171003817100381710038171003817
1198632(119866(
1
120583 (119860119894)120594119860119894
) 0)
100381710038171003817100381710038171003817100381710038171003817
+
100381610038161003816100381610038161003816100381610038161003816
119892lowast
1119863
1(119866(
1
120583 (119860119894)120594119860119894
) 0)
100381610038161003816100381610038161003816100381610038161003816
)
⩾10038171003817100381710038171198632(119866 (119904) 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119866 (119904) 0)
1003816100381610038161003816
(129)
Since |119892lowast1| = 1119866 is an isometric isomorphism so for each119891 isin
1198781198711(120583|119860)there exists a sequence of norm-one simple functions
(119904119896) such that 119866(119904
119896) converges to 119891 Therefore
V (1198782) ⩾ sup
119891isin1198781198711(120583|119860)
(10038171003817100381710038171198632(119891 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119891 0)
1003816100381610038161003816) ⩾100381710038171003817100381711987831003817100381710038171003817
(130)
On the other hand we see that100381710038171003817100381711987831003817100381710038171003817 ⩾1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
3(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816119909
lowast
1119863
2(119892
1 0) + 119892
lowast
1119863
1(119892
1 0)1003816100381610038161003816 = V (119878
2) = 1
(131)
Therefore 1 = 1198783 = 119878
3(119892
1 0) which proves Claim 2
Finally set 1198783= (0 ) for a suitable 119871
1(120583|
119860)oplus
1119883 rarr
119883 and define the operator 1198791 119871
1(120583) rarr 119883 by
1198791(119891) = 119879
0(119891120594
119860119888) + (119875
119860119891 0) (132)
for every 119891 isin 1198711(120583) Then we have
10038171003817100381710038171198791 (119891)1003817100381710038171003817 ⩽100381710038171003817100381711987901003817100381710038171003817
10038171003817100381710038171198911205941198601198881003817100381710038171003817 +1003817100381710038171003817100381710038171003817100381710038171003817
10038171003817100381710038171198911205941198601003817100381710038171003817 =10038171003817100381710038171198911003817100381710038171003817 (133)
for every 119891 isin 1198711(120583) so 119879
1 ⩽ 1 Also
10038171003817100381710038171198791 (1198691198601198921)1003817100381710038171003817 =10038171003817100381710038171198783 (1198921 0)
1003817100381710038171003817 =100381710038171003817100381711987831003817100381710038171003817 = 1 (134)
so 1198791attains its norm at 119869
1198601198921isin 119871
1(120583) and
10038171003817100381710038171198691198601198921 minus 11989101003817100381710038171003817 =10038171003817100381710038171198921 minus 1198920
1003817100381710038171003817 lt 120576 (135)
We also have that for any 119891 isin 1198781198711(120583)
10038171003817100381710038171198790 (119891) minus 1198791 (119891)1003817100381710038171003817 =100381710038171003817100381710038171198790(119891120594
119860) minus (119875
119860119891 0)10038171003817100381710038171003817
⩽10038171003817100381710038171198790 (119869119860119875119860119891) minus 119879119860 (119875119860119891)
1003817100381710038171003817
+10038171003817100381710038171003817119879119860(119875
119860119891) minus (119875
119860119891 0)10038171003817100381710038171003817
⩽10038171003817100381710038171198790119869119860 minus 119879119860
1003817100381710038171003817 +10038171003817100381710038171198780 minus 1198783
1003817100381710038171003817
lt 120578 (1205760) + 6120576
(136)
Hence 1198790minus 119879
1 ⩽ 120578(120576
0) + 6120576 lt 120576
0
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Abstract and Applied Analysis 15
Acknowledgments
The authors thank Gilles Godefroy and Rafael Paya forfruitful conversations about the content of this paper andVladimir Kadets for providing Lemma 18 They also appre-ciate anonymous referees for careful reading and fruitfulsuggestions about revision The second author was sup-ported by Basic Science Research Program through theNational Research Foundation of Korea (NRF) fundedby the Ministry of Education Science and Technology(NRF-2012R1A1A1006869) The third author was partiallysupported by Spanish MICINN and FEDER project noMTM2012-31755 and by Junta de Andalucıa and FEDERGrants FQM-185 and P09-FQM-4911
References
[1] F F Bonsall and J Duncan Numerical Ranges of Operators onNormed Spaces and of Elements of Normed Algebras vol 2 ofLondon Mathematical Society Lecture Note Series 2 CambridgeUniversity Press London UK 1971
[2] F F Bonsall and J Duncan Numerical Ranges II CambridgeUniversity Press London UK 1973 London MathematicalSociety Lecture Notes Series 10
[3] B Sims On numerical range and its applications to Banachalgebras [PhD thesis] University of Newcastle New SouthWales Australia 1972
[4] I D Berg and B Sims ldquoDenseness of operators which attaintheir numerical radiusrdquoAustralian Mathematical Society A vol36 no 1 pp 130ndash133 1984
[5] C S Cardassi ldquoDensity of numerical radius attaining operatorson some reflexive spacesrdquo Bulletin of the Australian Mathemat-ical Society vol 31 no 1 pp 1ndash3 1985
[6] C S Cardassi ldquoNumerical radius attaining operatorsrdquo inBanach Spaces (Columbia Mo 1984) vol 1166 of Lecture Notesin Mathematics pp 11ndash14 Springer Berlin Germany 1985
[7] C S Cardassi ldquoNumerical radius-attaining operators on119862(119870)rdquoProceedings of the AmericanMathematical Society vol 95 no 4pp 537ndash543 1985
[8] MDAcostaOperadores que alcanzan su radio numerico [PhDthesis] Universidad de Granada Granada Spain 1990
[9] M D Acosta and R Paya ldquoNumerical radius attaining oper-ators and the Radon-Nikodym propertyrdquo The Bulletin of theLondon Mathematical Society vol 25 no 1 pp 67ndash73 1993
[10] R Paya ldquoA counterexample on numerical radius attainingoperatorsrdquo Israel Journal of Mathematics vol 79 no 1 pp 83ndash101 1992
[11] Y S Choi and S G Kim ldquoNorm or numerical radius attainingmultilinear mappings and polynomialsrdquo Journal of the LondonMathematical Society vol 54 no 1 pp 135ndash147 1996
[12] M D Acosta J Becerra Guerrero and M Ruiz GalanldquoNumerical-radius-attaining polynomialsrdquo The Quarterly Jour-nal of Mathematics vol 54 no 1 pp 1ndash10 2003
[13] M D Acosta and S G Kim ldquoDenseness of holomorphicfunctions attaining their numerical radiirdquo Israel Journal ofMathematics vol 161 pp 373ndash386 2007
[14] S G Kim and H J Lee ldquoNumerical peak holomorphicfunctions on Banach spacesrdquo Journal of Mathematical Analysisand Applications vol 364 no 2 pp 437ndash452 2010
[15] M D Acosta R M Aron D Garcıa and M MaestreldquoThe Bishop-Phelps-Bollobas theorem for operatorsrdquo Journal ofFunctional Analysis vol 254 no 11 pp 2780ndash2799 2008
[16] A J Guirao and O Kozhushkina ldquoThe Bishop-Phelps-Bollobasproperty for numerical radius in ℓ
1(119862)rdquo Studia Mathematica
vol 218 no 1 pp 41ndash54 2013[17] J Falco ldquoThe Bishop-Phelps-Bollobas property for numerical
radius on 1198711rdquo Journal of Mathematical Analysis and Applica-
tions vol 414 no 1 pp 125ndash133 2014[18] A Aviles A J Guirao and J Rodriguez ldquoOn the Bishop-
Phelps-Bollobas property for numerical radius in119862(119870)-spacesrdquoPreprint
[19] RM Aron Y S Choi S K Kim H J Lee andMMartin ldquoTheBishop-Phelps-Bollobas version of Lindenstrauss properties Aand Brdquo Preprint
[20] V Kadets M Martın and R Paya ldquoRecent progress and openquestions on the numerical index of Banach spacesrdquo Revista dela Real Academia de Ciencias Exactas Fısicas y Naturales A vol100 no 1-2 pp 155ndash182 2006
[21] M Martın J Merı and M Popov ldquoOn the numerical index ofreal 119871
119901(120583)-spacesrdquo Israel Journal of Mathematics vol 184 pp
183ndash192 2011[22] A Iwanik ldquoNorm attaining operators on Lebesgue spacesrdquo
Pacific Journal of Mathematics vol 83 no 2 pp 381ndash386 1979[23] HH SchaeferBanach Lattices and Positive Operators Springer
New York NY USA 1974[24] J Duncan C M McGregor J D Pryce and A J White ldquoThe
numerical index of a normed spacerdquo Journal of the LondonMathematical Society Second Series vol 2 pp 481ndash488 1970
[25] Y S Choi S K Kim H J Lee and M Martin ldquoThe Bishop-Phelps-Bollobas theorem for operators on 119871
1(120583)rdquo Preprint
[26] H E Lacey The Isometric Theory of Classical Banach SpacesSpringer New York NY USA 1974
[27] G Godefroy ldquoUniversal spaces for strictly convex Banachspacesrdquo Revista de la Real Academia de Ciencias Exactas Fısicasy Naturales A vol 100 no 1-2 pp 137ndash146 2006
[28] C Finet M Martın and R Paya ldquoNumerical index andrenormingrdquo Proceedings of the American Mathematical Societyvol 131 no 3 pp 871ndash877 2003
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical Problems in Engineering
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Differential EquationsInternational Journal of
Volume 2014
Applied MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Probability and StatisticsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical PhysicsAdvances in
Complex AnalysisJournal of
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OptimizationJournal of
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CombinatoricsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
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Operations ResearchAdvances in
Journal of
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Function Spaces
Abstract and Applied AnalysisHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of Mathematics and Mathematical Sciences
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Algebra
Discrete Dynamics in Nature and Society
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Decision SciencesAdvances in
Discrete MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom
Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of
10 Abstract and Applied Analysis
Let 119878 = 120572Ψminus11198794Ψ 119891 = Ψminus1
1198914 and 119892 = Ψlowast
1198924 then we have
119879 minus 119878 =10038171003817100381710038171003817119879 minus 120572Ψ
minus11198794Ψ10038171003817100381710038171003817=10038171003817100381710038171003817120572Ψ119879Ψ
minus1minus 119879
4
10038171003817100381710038171003817
=10038171003817100381710038171198791 minus 1198794
1003817100381710038171003817 ⩽ 31205753
100381710038171003817100381710038171198910minus 11989110038171003817100381710038171003817=100381710038171003817100381710038171198910minus Ψ
minus11198914
10038171003817100381710038171003817=10038171003817100381710038171198911 minus 1198914
1003817100381710038171003817 ⩽ 101205753
100381710038171003817100381710038171198920minus 11989110038171003817100381710038171003817=10038171003817100381710038171198920 minus Ψ
lowast1198924
1003817100381710038171003817 =100381710038171003817100381710038171003817(Ψ
minus1)lowast
1198920minus 119892
4
100381710038171003817100381710038171003817
=10038171003817100381710038171198921 minus 1198924
1003817100381710038171003817 ⩽ 2radic120575
3
⟨119891 119892⟩ = ⟨Ψminus11198914 Ψ
lowast1198924⟩ = ⟨119891
4 119892
4⟩ = 1
10038161003816100381610038161003816⟨119878119891 119892⟩
10038161003816100381610038161003816=10038161003816100381610038161003816⟨120572Ψ
minus11198794ΨΨ
minus11198914 Ψ
lowast1198924⟩10038161003816100381610038161003816= |120572| = 1
(79)
This completes the proof
Finally we may give the proof of the main result in fullgenerality
Proof of Theorem 9 Notice that the Kakutani representationtheorem (see [26] for a reference) says that for every 120590-finitemeasure ] the space 119871
1(]) is isometrically isomorphic to
1198711(119898) for some positive Borel regular measure on a compact
Hausdorff space Then by Proposition 10 there is a universalfunction 120576 997891rarr 120578(120576) gt 0 which gives the BPBp-nu for 119871
1(]) for
every 120590-finite measure ]Fix 120576 gt 0 Suppose that 119879
0isin L(119871
1(120583)) with V(119879
0) = 1
and (1198910 119891
lowast
0) isin Π(119871
1(120583)) satisfy
1003816100381610038161003816⟨119891lowast
0 119879
01198910⟩1003816100381610038161003816 gt 1 minus 120578 (120576) (80)
Choose a sequence 119891119899 in 119871
1(120583) such that sup
119899119879
0119891119899 = 1
and let 119866 be the closed linear span of
119879119899
0119891119898 119899 119898 isin N cup 0 (81)
As 119866 is separable there is a dense subset 119892119899 119899 isin N of 119866
and let 119864 = ⋃infin
119899=1supp119892
119899 where supp119892
119899is the support of 119892
119899
Then the measure 120583|119864is 120590-finite Let
119884 = 119891 isin 1198711(120583) supp (119891) sub 119864 (82)
be a closed subspace of 1198711(120583) It is clear that 119871
1(120583) = 119884oplus
1119885
and 119884 is isometrically isomorphic to 1198711(120583|
119864) So 119884 has the
BPBp-nu with 120578(120576)Now write 119878
0= 119879
0|119884 119884 rarr 119884 consider 119910
0= 119891
0isin
119878119884 119910lowast
0= 119891
lowast
0|119884isin 119878
119884lowast and observe that 119910lowast
0(119910
0) = 1 and
|119910lowast
0(119878
01199100)| = |119891
lowast
0(119879
01198910)| gt 1 minus 120578(120576) Hence there exist 119878 isin
L(119884) and (1199100 119910
lowast
0) isin Π(119884) such that
1003816100381610038161003816119910lowast
0(119878119910
0)1003816100381610038161003816 = 1 = V (119878) 1003817100381710038171003817119878 minus 1198780
1003817100381710038171003817 lt 120576
10038171003817100381710038171199100 minus 11991001003817100381710038171003817 lt 120576
1003817100381710038171003817119910lowast
0minus 119910
lowast
0
1003817100381710038171003817 lt 120576
(83)
Finally consider the operator 119879 isinL(1198711(120583)) given by
119879 (119910 119911)
= (119878119910 0) + 1198790(0 119911) ((119910 119911) isin 119871
1(120583) equiv 119884oplus
1119885)
(84)
We have 119879 = 1 (and so V(119879) = 1) Indeed1003817100381710038171003817119879 (119910 119911)
1003817100381710038171003817 =1003817100381710038171003817(119878119910 0)
1003817100381710038171003817 +10038171003817100381710038171198790 (0 119911)
1003817100381710038171003817 ⩽10038171003817100381710038171199101003817100381710038171003817 + 119911 =
1003817100381710038171003817(119910 119911)1003817100381710038171003817
(85)
for all (119910 119911) isin 1198711(120583) and 119879(119910
0 0) = (119878119910
0 0) = 119878119910
0 = 1
Let 119909 = (1199100 0) and 119909lowast = (119910lowast
0 119891
0|119885) Then (119909 119909lowast) isin Π(119871
1(120583))
Moreover we have1003816100381610038161003816119909
lowast1198791199091003816100381610038161003816 =1003816100381610038161003816119910
lowast
0119878119910
0
1003816100381610038161003816 = 1 = V (119879) 1003817100381710038171003817119909 minus 1198910
1003817100381710038171003817 =1003817100381710038171003817119910 minus 1199100
1003817100381710038171003817 lt 120576
1003817100381710038171003817119909lowast
0minus 119891
lowast
0
1003817100381710038171003817
= max 1003817100381710038171003817119910 minus 119891lowast
0|119884
1003817100381710038171003817 1003817100381710038171003817119891
lowast
0|119885minus 119891
lowast
0|119885
1003817100381710038171003817
=1003817100381710038171003817119910
lowastminus 119910
lowast
0
1003817100381710038171003817 lt 120576
1003817100381710038171003817119879 minus 11987901003817100381710038171003817 = sup119910+119911⩽1
1003817100381710038171003817119879 (119910 119911) minus 1198790 (119910 119911)1003817100381710038171003817
= sup119910⩽1
1003817100381710038171003817119878119910 minus 11987801199101003817100381710038171003817 =1003817100381710038171003817119878 minus 1198780
1003817100381710038171003817 lt 120576
(86)
This completes the proof
5 Examples of Spaces Failing theBishop-Phelps-Bollobaacutes Property forNumerical Radius
Our goal here is to prove that the density of numerical radiusattaining operators does not imply the BPBp-nu Actuallywe will show that among separable spaces there is noisomorphic property implying the BPBp-nu other than finite-dimensionality
We need to relate the BPBp-nu to the Bishop-Phelps-Bollobas property for operators which as mentioned in theintroduction was introduced in [15] A pair (119883 119884) of Banachspaces has the Bishop-Phelps-Bollobas property for operators(in short BPBp) if given 120576 gt 0 there exists 120578(120576) gt 0 suchthat given 119879 isin L(119883 119884) with 119879 = 1 and 119909 isin 119878
119883such that
119879119909 gt 1 minus 120578(120576) then there exist 119911 isin 119878119883and 119878 isin L(119883 119884)
satisfying
119878 = 119878119911 = 1 119909 minus 119911 lt 120576
119879 minus 119878 lt 120576
(87)
We refer the reader to [15 19 25] and references therein formore information and background Among the interestingresults on the BPBp we emphasize that a pair (119883 119884) when119883is finite-dimensional does not necessarily have the BPBp Forinstance if119884 is a strictly convex space which is not uniformlyconvex then the pair (ℓ(2)
1 119884) fails to have the BPBp (this is
contained in [15] see [19 Section 3])
Abstract and Applied Analysis 11
The next result relates the BPBp-nu to the BPBp foroperators in a particular case We will deduce our examplefrom it
Theorem 15 If 1198711(120583)oplus
1119883 has the BPBp-119899119906 then the pair
(1198711(120583) 119883) has the BPBp for operators
Before proving this proposition we will use it to get themain examples of this section The first example shows thatthe density of numerical radius attaining operators does notimply the BPBp-nu
Example 16 There is a reflexive space (and so numerical radiusattaining operators on it are dense) which fails to have theBPBp-nu Indeed let 119884 be a reflexive separable space whichis not superreflexive and we may suppose that 119884 is strictlyconvex Observe that 119884 cannot be uniformly convex since itis not superreflexive Now 119883 = ℓ(2)
1oplus1119884 is reflexive but the
pair (ℓ(2)1 119884) fails the BPBp since 119884 is strictly convex but not
uniformly convex [19 Corollary 33] Therefore Theorem 15gives us that119883 does not have the BPBp-nu
The example above can be extended to get the result thatevery infinite-dimensional separable Banach space can berenormed to fail the BPBp-nu This follows from the factthat every infinite-dimensional separable Banach space canbe renormed to be strictly convex but not uniformly convex(this result can be proved ldquoby handrdquo an alternative categoricalargument for it can be found in [27] and references therein)With a little more of effort we may get the main result of thesection
Theorem 17 Every infinite-dimensional separable Banachspace can be renormed to fail the weak-BPBp-nu (and so inparticular to fail the BPBp-nu)
We need the following result which is surely well knownAs we have not found a reference we include a nice and easyproof kindly given to us by Vladimir Kadets We recall thatgiven a Banach space 119884 the set of all equivalent norms on119884 can be viewed as a metric space using the Banach-Mazurdistance
Lemma 18 Let119884 be an infinite-dimensional separable Banachspace Then the set of equivalent norms on 119884 which are strictlyconvex and are not (locally) uniformly convex is dense in the setof all equivalent norms on119884 (with respect to the Banach-Mazurdistance)
Proof Fix 119890 isin 119878119884and 119890lowast
1isin 119878
119884lowast such that 119890lowast
1(119890) = 1 For a fixed
120576 isin (0 12) denote
119902 (119910) = max (1 minus 120576) 10038171003817100381710038171199101003817100381710038171003817 1003816100381610038161003816119890lowast
1(119910)1003816100381610038161003816 (119910 isin 119884) (88)
Evidently (1 minus 120576)119910 ⩽ 119902(119910) ⩽ 119910 for every 119910 isin 119884 Fix asequence 119890lowast
119896 119896 ⩾ 2 of norm-one functionals separating
the points of 119884 and denote
119901 (119910) = radic
infin
sum
119896=1
1
2119896
1003816100381610038161003816119890lowast
119896(119910)1003816100381610038161003816
2
(119910 isin 119884) (89)
Then 119901 is a strictly convex norm on 119884 119901(119890) ⩾ 1radic2 and119901(119910) ⩽ 119910 for all 119910 isin 119883 Finally write
100381710038171003817100381711991010038171003817100381710038171= (1 minus 120576) 119902 (119910) + 120576
119901 (119910)
119901 (119890)(119910 isin 119884) (90)
Then sdot 1is a strictly convex norm on 119884 and
(1 minus 120576)2 10038171003817100381710038171199101003817100381710038171003817 ⩽100381710038171003817100381711991010038171003817100381710038171⩽ (1 + 120576)
10038171003817100381710038171199101003817100381710038171003817 (119910 isin 119884) (91)
Wewill finish the proof by showing that sdot 1is not uniformly
convex (actually it is not locally uniformly convex) Indeedfor each 119899 isin N we select 119910
119899isin ⋂
119899
119896=1ker 119890lowast
119896with 119910
119899 = 1 and
consider 119890119899= 119890 + (1205764)119910
119899 Then 119902(119890) = 1 119902(119890
119899) = 1 and
119902(119890+119890119899) = 2 At the same time 119901(119910
119899) rarr 0 so 119901(119890
119899) rarr 119901(119890)
and 119901(119890 + 119890119899) rarr 2119901(119890) Consequently
1198901 = 1100381710038171003817100381711989011989910038171003817100381710038171997888rarr 1
1003817100381710038171003817119890 + 11989011989910038171003817100381710038171997888rarr 2 (92)
but 119890 minus 1198901198991= (1205764)119910
1198991⩾ (1 minus 120576)
2(1205764) which means the
absence of local uniform convexity at 119890
Proof of Theorem 17 Let 119883 be an infinite-dimensional sepa-rable Banach space Take a closed subspace 119884 of 119883 of codi-mension two By [28 Proposition 2] the map carrying everyequivalent norm on 119884 to its numerical index is continuousand so the set of values of the numerical index of 119884 upto reforming is a nontrivial interval [28 Theorem 9] ThenLemma 18 allows us to find an equivalent norm | sdot | on 119884 insuch a way that (119884 | sdot |) is strictly convex and is not uniformlyconvex and 119899(119884 | sdot |) gt 0 Now the space 119883 = ℓ(2)
1oplus1(119884 | sdot |)
is an equivalent renorming of 119883 which does not have theBPBp-119899119906 (indeed otherwise the pair (ℓ(2)
1 (119884 | sdot |)) would
have the BPBp for the operator norm and so (119884 | sdot |) wouldbe uniformly convex by [19 Corollary 33] a contradiction)Moreover as
119899 (119883) = min 119899 (ℓ(2)1) 119899 (119884 |sdot|) gt 0 (93)
(see [20 Proposition 2] for instance) 119883 also fails the weak-BPBp-nu by Proposition 6
To finish the section with the promised proof ofTheorem 15 we first see the following stability result
Lemma 19 Let119883 = [⨁infin
119896=1119883
119896]1198880
or [⨁infin
119896=1119883
119896]ℓ1
If119883 has theBishop-Phelps-Bollobas property for numerical radius with afunction 120578 then each Banach space 119883
119894has the Bishop-Phelps-
Bollobas property for numerical radius with 120578nu(119883119894) ⩾ 120578 That
is inf119894120578nu(119883119894
)(120576) ⩾ 120578nu(119883)(120576) for all 0 lt 120576 lt 1
Proof Let 119875119894 119883 rarr 119883
119894and 1198751015840
119894 119883
lowastrarr 119883
lowast
119894be the natural
projections and let 119876119894 119883
119894rarr 119883 and 1198761015840
119894 119883
lowast
119894rarr 119883
lowast be thenatural embeddings
Assume that an operator 119879119894 119883
119894rarr 119883
119894and a pair
(119909119894 119909
lowast
119894) isin Π(119883
119894) satisfy that
V (119879119894) = 1
1003816100381610038161003816119909lowast
119894119879119894119909119894
1003816100381610038161003816 gt 1 minus 120578 (120576) (94)
We define an operator 119879 119883 rarr 119883 and (119909 119909lowast) isin Π(119883) by
119879 = 119876119894∘ 119879
119894∘ 119875
119894 (119909 119909
lowast) = (119876
119894119909119894 119876
1015840
119894119909lowast
119894) (95)
12 Abstract and Applied Analysis
then clearly we see that1003816100381610038161003816119909
lowast1198791199091003816100381610038161003816 =1003816100381610038161003816119909
lowast
119894119879119894119909119894
1003816100381610038161003816 gt 1 minus 120578 (120576) (96)
From the assumption there exist 119878 119883 rarr 119883 and a pair(119910 119910
lowast) isin Π(119883) such that
1003816100381610038161003816119910lowast1198781199101003816100381610038161003816 = 1 = V (119878) 119878 minus 119879 lt 120576
1003817100381710038171003817119910lowastminus 119909
lowast1003817100381710038171003817 lt 1205761003817100381710038171003817119910 minus 119909
1003817100381710038171003817 lt 120576
(97)
Since this clearly shows that1003817100381710038171003817119875119894 ∘ 119878 ∘ 119876119894
minus 119879119894
1003817100381710038171003817 lt 120576100381710038171003817100381710038171198751015840
119894119910lowastminus 119909
lowast
119894
10038171003817100381710038171003817lt 120576
1003817100381710038171003817119875119894119910 minus 1199091198941003817100381710038171003817 lt 120576
(98)
we only need to show that |1198751015840119894119910lowast(119875
119894∘ 119878 ∘ 119876
119894)119875
119894119910| = 1
We first show the case of 1198880sum Since
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817=10038171003817100381710038171003817119875119895119910 minus 119875
11989511990910038171003817100381710038171003817⩽1003817100381710038171003817119910 minus 119909
1003817100381710038171003817 lt 120576 (99)
for every 119895 = 119894 we have
1 = 119910lowast(119910) = sum
119895isinN
1198751015840
119895119910lowast(119875
119895119910) ⩽ sum
119895isinN
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817
⩽100381710038171003817100381710038171198751015840
119894119910lowast10038171003817100381710038171003817+ 120576 sum
119895isinN119895 = 119894
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817⩽1003817100381710038171003817119910
lowast1003817100381710038171003817 = 1
(100)
This shows that 1198751015840119894119910lowast = 1 and 1198751015840
119895119910lowast= 0 for every 119895 = 119894
So 119910lowast = 1198761015840
1198941198751015840
119894119910lowast and 1198751015840
119894119910lowast(119875
119894119910) = 1 This and the fact that
119910 minus 119876119894119875119894119910 lt 120576 imply that
(119876119894119875119894119910 + (
1
120576) (119910 minus 119876
119894119875119894119910) 119876
1015840
1198941198751015840
119894119910lowast) isin Π (119883) (101)
So we get that (1198761015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910 + (1120576)(119910 minus 119876
119894119875119894119910))V(119878) = 1
Hence we have
1 =1003816100381610038161003816119910
lowast1198781199101003816100381610038161003816 =10038161003816100381610038161003816(119876
1015840
1198941198751015840
119894119910lowast) 11987811991010038161003816100381610038161003816
=
1003816100381610038161003816100381610038161003816(1 minus 120576) (119876
1015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910)
+120576 (1198761015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910 +1
120576(119910 minus 119876
119894119875119894119910))
1003816100381610038161003816100381610038161003816⩽ 1
(102)
and so we get |1198751015840119894119910lowast(119875
119894∘119878 ∘119876
119894)119875
119894119910| = |(119876
1015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910)| = 1
We next show the case of ℓ1sum The proof is almost
the same as that of the 1198880case However for the sake of
completeness we provide it hereSince 1198751015840
119895119910lowast = 119875
1015840
119895119910lowastminus 119875
1015840
119895119909lowast ⩽ 119910
lowastminus 119909
lowast lt 120576 for every
119895 = 119894 we have
1 = 119910lowast(119910) = sum
119895isinN
1198751015840
119895119910lowast(119875
119895119910) ⩽ sum
119895isinN
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817
⩽10038171003817100381710038171198751198941199101003817100381710038171003817 + 120576 sum
119895isinN119895 = 119894
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817⩽10038171003817100381710038171199101003817100381710038171003817 = 1
(103)
which shows 119875119894119910 = 1 and 119875
119895119910 = 0 for every 119895 = 119894 Since this
implies (119876119894119875119894119910119876
1015840
1198941198751015840
119894119910lowast+(1120576)(119910
lowastminus119876
1015840
1198941198751015840
119894119910lowast)) isin Π(119883) we get
that |(1198761015840
1198941198751015840
119894119910lowast+ (1120576)(119910
lowastminus 119876
1015840
1198941198751015840
119894119910lowast))119878(119876
119894119875119894119910)| ⩽ V(119878) = 1
Hence we have
1 =1003816100381610038161003816119910
lowast1198781199101003816100381610038161003816 =1003816100381610038161003816119910
lowast119878 (119876
119894119875119894119910)1003816100381610038161003816
=
1003816100381610038161003816100381610038161003816(1 minus 120576) (119876
1015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910)
+120576 (1198761015840
1198941198751015840
119894119910lowast+1
120576(119910
lowastminus 119876
1015840
1198941198751015840
119894119910lowast)) 119878 (119876
119894119875119894119910)
1003816100381610038161003816100381610038161003816⩽ 1
(104)
and so |1198751015840119894119910lowast(119875
119894∘ 119878 ∘ 119876
119894)119875
119894119910| = |(119876
1015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910)| = 1
Proof of Theorem 15 Note that 120578nu(1198711(120583)oplus1119883)(120576) rarr 0 as120576 rarr 0 Fix 0 lt 120576
0lt 1 and choose 0 lt 120576 lt 1 such that
6120576 + 120578nu(1198711(120583)oplus1119883)(120576) lt 1205760 Let 120578(1205760) = 120578nu(1198711(120583)oplus1119883)(120576)Suppose that 119879
0isin L(119871
1(120583) 119883) with 119879
0 = 1 and 119891
0isin
1198781198711(120583)
satisfy
1003817100381710038171003817119879011989101003817100381710038171003817 gt 1 minus 120578 (1205760) (105)
For any measurable subset 119861 let
1198711(120583|
119861) = 119891|
119861 119891 isin 119871
1(120583) (106)
with the norm 119891|119861 = 119891120594
1198611 Then it is easy to see
that 1198711(120583|
119861) is isometrically isomorphic to a complemented
subspace of 1198711(120583) Let 119875
119861 119871
1(120583) rarr 119871
1(120583|
119861) be the
restriction defined by 119875119861(119891) = 119891|
119861for all 119891 isin 119871
1(120583) and
let 119869119861 119871
1(120583|
119861) rarr 119871
1(120583) be the extension defined by
119869119861(119891)(120596) = 119891(120596) if 120596 isin 119861 and 119869
119861(119891)(120596) = 0 otherwise
It is clear that 119875119861119869119861= Id
1198711(120583|119861)and 119869
119861119875119861(119891) = 119891120594
119861for all
119891 isin 1198711(120583) Notice also that 119871
1(120583) is isometrically isomorphic
to 1198711(120583|
119861)oplus
11198711(120583|
119861119888)
Let 119860 = supp1198910and 119892
0= 119875
1198601198910 Then
1003817100381710038171003817119879011986911986011989201003817100381710038171003817 =100381710038171003817100381711987901198910
1003817100381710038171003817 gt 1 minus 120578 (1205760) gt 0 (107)
and define the operator 119879119860 119871
1(120583|
119860) rarr 119883 by 119879
119860119891 =
1198790119869119860119891119879
0119869119860 for every 119891 isin 119871
1(120583|
119860) Then
100381710038171003817100381711987911986011989201003817100381710038171003817 ⩾100381710038171003817100381711987901198910
1003817100381710038171003817 gt 1 minus 120578 (1205760) (108)
Since 120583|119860is 120590-finite 119871
1(120583|
119860)lowast= 119871
infin(120583|
119860) Let 119892lowast
0isin 119878
119871infin(120583|119860)
be a function such that ⟨119892lowast0 119892
0⟩ = 1 and choose 119909lowast
0isin 119878
119883lowast
such that 119909lowast0(119879
1198601198920) = 119879
1198601198920 Define the operator 119878
0isin
L(1198711(120583|
119860)oplus
1119883) by
1198780(119891 119909) = (0 119879
119860119891) ((119891 119909) isin 119871
1(120583|
119860) oplus
1119883) (109)
and observe that 1198780 = V(119878
0) = 1 Indeed
100381710038171003817100381711987801003817100381710038171003817 ⩽ 1 =
10038171003817100381710038171198791198601003817100381710038171003817
= sup 1003816100381610038161003816119909lowast1198791198601198911003816100381610038161003816 119909
lowastisin 119878
119883lowast 119891 isin 119878
1198711(120583|119860)
= sup 1003816100381610038161003816(119891lowast 119909
lowast) 119878
0(119891 119909)
1003816100381610038161003816
((119891lowast 119909
lowast) (119891 119909)) isin Π (119871
1(120583|
119860) oplus
1119883)
= V (1198780) ⩽100381710038171003817100381711987801003817100381710038171003817
(110)
Abstract and Applied Analysis 13
It is immediate that
(119892lowast
0 119909
lowast
0) 119878
0(119892
0 0) = 119909
lowast
0(119879
1198601198920) =10038171003817100381710038171198791198601198920
1003817100381710038171003817 gt 1 minus 120578 (1205760)
(111)
By Lemma 19 1198711(120583|
119860)oplus
1119883 has the BPBp-nu with the func-
tion 120578 Therefore there exist 1198781isin L(119871
1(120583|
119860)oplus
1119883) (119892
1 119909
1) isin
1198781198711(120583|119860)oplus1119883 and (119892lowast
1 119909
lowast
1) isin 119878
119871infin(120583|119860)oplusinfin119883lowast such that
1003817100381710038171003817(1198921 1199091) minus (1198920 0)1003817100381710038171003817 lt 120576
1003817100381710038171003817(119892lowast
1 119909
lowast
1) minus (119891
lowast
0 119909
lowast
0)1003817100381710038171003817 lt 120576
10038171003817100381710038171198781 minus 11987801003817100381710038171003817 lt 120576 ⟨(119892
lowast
1 119909
lowast
1) (119892
1 119909
1)⟩ = 1
1003816100381610038161003816(119892lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)1003816100381610038161003816 = V (119878
1) = 1
(112)
Claim 1 We claim that 1199091= 0
Otherwise
1 = Re ⟨(119892lowast1 119909
lowast
1) (119892
1 119909
1)⟩
=100381710038171003817100381711989211003817100381710038171003817Re⟨(119892
lowast
1 119909
lowast
1) (119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
⟩
+100381710038171003817100381711990911003817100381710038171003817Re⟨(119892
lowast
1 119909
lowast
1) (0 119909
1)
100381710038171003817100381711990911003817100381710038171003817
⟩ ⩽ 1
(113)
We deduce that
((119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
(119892lowast
1 119909
lowast
1)) (
(0 1199091)
100381710038171003817100381711990911003817100381710038171003817
(119892lowast
1 119909
lowast
1))
isin Π (1198711(120583|
119860) oplus
1119883)
(114)
Since 1198781((0 119909
1)119909
1) = (119878
1minus 119878
0)((0 119909
1)119909
1) lt 120576 we
get that
1 =1003816100381610038161003816⟨(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)⟩1003816100381610038161003816
=
100381610038161003816100381610038161003816100381610038161003816
100381710038171003817100381711989211003817100381710038171003817⟨(119892
lowast
1 119909
lowast
1) 119878
1((119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
)⟩
+100381710038171003817100381711990911003817100381710038171003817⟨(119892
lowast
1 119909
lowast
1) 119878
1((0 119909
1)
100381710038171003817100381711990911003817100381710038171003817
)⟩
100381610038161003816100381610038161003816100381610038161003816
⩽100381710038171003817100381711989211003817100381710038171003817 V (1198781) + 120576
100381710038171003817100381711990911003817100381710038171003817 lt100381710038171003817100381711989211003817100381710038171003817 +100381710038171003817100381711990911003817100381710038171003817 = 1
(115)
a contradiction This proves the claimWe define the operator 119878
2 119871
1(120583|
119860)oplus
1119883 rarr 119871
1(120583|
119860)oplus
1119883
by 1198782(119891 119909) = 119878
1(119891 0) for every 119891 isin 119871
1(120583|
119860) and for every
119909 isin 119883 Then we have
V (1198782) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816 = 1
10038171003817100381710038171198781 minus 11987821003817100381710038171003817 ⩽ 120576 (116)
Indeed from Claim 1 we have
V (1198781) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)1003816100381610038161003816 =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816 ⩽ V (119878
2)
(117)
On the other hand we see that1003816100381610038161003816(119891
lowast 119909
lowast) 119878
2(119891 119909)
1003816100381610038161003816
=1003816100381610038161003816(119891
lowast 119909
lowast) 119878
1(119891 0)
1003816100381610038161003816 ⩽10038171003817100381710038171198911003817100381710038171003817 V (1198781) ⩽ V (119878
1)
(118)
for every ((119891lowast 119909lowast) (119891 119909)) isin Π(1198711(120583|
119860)oplus
1119883) Therefore
V(1198782) ⩽ V(119878
1) Also
10038171003817100381710038171198781 minus 11987821003817100381710038171003817 ⩽ sup
119909isin119878119883
10038171003817100381710038171198781 (0 119909)1003817100381710038171003817
= sup119909isin119878119883
10038171003817100381710038171198781 (0 119909) minus 1198780 (0 119909)1003817100381710038171003817 ⩽ 120576
(119)
Claim 2 There exists 1198783 119871
1(120583|
119860)oplus
1119883 rarr 119871
1(120583|
119860)oplus
1119883 such
that 1198783(119892
1 0) = 119878
3 = 1 119878
3(0 119909) = 0 119878
3(119891 119909) isin 0oplus
1119883
for every (119891 119909) isin 1198711(120583|
119860)oplus
1119883 and 119878
3minus 119878
2 lt 4120576
Indeed write 1198781= (119863
1 119863
2) where 119863
1 119871
1(120583|
119860)oplus
1119883 rarr
1198711(120583|
119860) and119863
2 119871
1(120583|
119860)oplus
1119883 rarr 119883 We have that
sup 1003816100381610038161003816119892lowast119863
1(119892
1 0) + 119909
lowast119863
2(119892
1 0)1003816100381610038161003816
119909lowastisin 119878
119883lowast ⟨119892
lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860)
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816 +10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
⟨119892lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860)
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816
⟨119892lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860) +10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
⩽ V (1198782) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816119892
lowast
1119863
1(119892
1 0) + 119909
lowast
1119863
2(119892
1 0)1003816100381610038161003816
(120)
This implies that
1003816100381610038161003816119909lowast
1119863
2(119892
1 0)1003816100381610038161003816 =10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
1003816100381610038161003816119892lowast
1119863
1(119892
1 0)1003816100381610038161003816
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816 ⟨119892
lowast 119892
1⟩ = 1 119892
lowastisin 119871
infin(120583|
119860)
(121)
Therefore |119892lowast1| equals 1 on the support of 119863
1(119892
1 0) As
|⟨119892lowast
1 119892
1⟩| = 1 we also have that |119892lowast
1| equals 1 on the support
of 1198921 Changing the values of 119892lowast
1by the ones of 119891lowast
0on 119860
(supp(1198631(119892
1 0)) cup supp(g
1)) we may and do suppose that
|119892lowast
1| = 1 on the whole 119860We also have 119863
2(119892
1 0) gt 0 Indeed
10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198920 0)1003817100381710038171003817
⩽10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198921 0)
1003817100381710038171003817 +10038171003817100381710038171198780 (1198921 0) minus 1198780 (1198920 0)
1003817100381710038171003817
lt 2120576 + 120576 = 3120576
(122)
So we have
10038171003817100381710038171198632(119892
1 0) minus 119879
1198601198920
1003817100381710038171003817
⩽10038171003817100381710038171198631(119892
1 0)1003817100381710038171003817 +10038171003817100381710038171198632(119892
1 0) minus 119879
1198601198920
1003817100381710038171003817
14 Abstract and Applied Analysis
=1003817100381710038171003817(1198631
(1198921 0) 119863
2(119892
1 0)) minus (0 119879
1198601198920)1003817100381710038171003817
=10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198920 0)
1003817100381710038171003817 lt 3120576
(123)
and 1198632(119892
1 0) gt 119879
1198601198920 minus 3120576 ⩾ 1 minus 120578(120576
0) minus 3120576 gt 0
Finally define the operator 1198783by
1198783(119891 119909) = (0119863
2(119891 0) + 119892
lowast
1(119863
1(119891 0))
1198632(119892
1 0)
119909lowast
1119863
2(119892
1 0))
(124)
for (119891 119909) isin 1198711(120583|
119860)oplus
1119883 It is clear that
100381710038171003817100381711987831003817100381710038171003817 ⩽ sup
119891isin1198781198711(120583|119860)
(10038171003817100381710038171198632(119891 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119891 0)
1003816100381610038161003816) (125)
Notice also that
10038171003817100381710038171198631(119891 0)
1003817100381710038171003817
⩽10038171003817100381710038171198631(119891 0)
1003817100381710038171003817 +10038171003817100381710038171198632(119891 0) minus 119879
1198601198911003817100381710038171003817
=1003817100381710038171003817(1198631
(119891 0) 1198632(119891 0)) minus (0 119879
119860119891)1003817100381710038171003817
=10038171003817100381710038171198782 (119891 119909) minus 1198780 (119891 119909)
1003817100381710038171003817
(126)
for all (119891 119909) isin 1198711(120583|
119860)oplus
1119883 Hence we have
10038171003817100381710038171198783 minus 11987821003817100381710038171003817
= 2 sup119891isin1198781198711(120583|119860)
10038171003817100381710038171198631(119891 0)
1003817100381710038171003817 ⩽ 210038171003817100381710038171198782 minus 1198780
1003817100381710038171003817 lt 4120576(127)
On the other hand let 119866 1198711(120583|
119860) rarr 119871
1(120583|
119860) be defined by
119866(119891) = 119892lowast
1119891 for every 119891 isin 119871
1(120583|
119860) Then we have
V (1198782)
= sup 1003816100381610038161003816119911lowast11987821199111003816100381610038161003816 (119911 119911
lowast) isin Π (119871
1(120583|
119860) oplus
1119883)
⩾ sup10038161003816100381610038161003816100381610038161003816
119909lowast119863
2(119866(
1
120583 (119862)120594119862) 0)
+119892lowast
1119863
1(119866(
1
120583 (119862)120594119862) 0)
10038161003816100381610038161003816100381610038161003816
119909lowastisin 119878
119883lowast 119862 isin Σ
119860 120583 (119862) gt 0
= sup10038171003817100381710038171003817100381710038171003817
1198632(119866(
1
120583 (119862)120594119862) 0)
10038171003817100381710038171003817100381710038171003817
+
10038161003816100381610038161003816100381610038161003816
119892lowast
1119863
1(119866(
1
120583 (119862)120594119862) 0)
10038161003816100381610038161003816100381610038161003816
119862 isin Σ119860 120583 (119862) gt 0
(128)
where Σ119860is the family of measurable subsets of 119860
Hence for any simple function 119904 = sum119899
119894=1((120572
119894120583(119860
119894))120594
119860119894
) isin
1198781198711(120583|119860) where 119860
119894119894is a family of disjoint measurable subsets
with strictly positive measure we have
V (1198782)
⩾
119899
sum
119894=1
10038161003816100381610038161205721198941003816100381610038161003816 (
100381710038171003817100381710038171003817100381710038171003817
1198632(119866(
1
120583 (119860119894)120594119860119894
) 0)
100381710038171003817100381710038171003817100381710038171003817
+
100381610038161003816100381610038161003816100381610038161003816
119892lowast
1119863
1(119866(
1
120583 (119860119894)120594119860119894
) 0)
100381610038161003816100381610038161003816100381610038161003816
)
⩾10038171003817100381710038171198632(119866 (119904) 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119866 (119904) 0)
1003816100381610038161003816
(129)
Since |119892lowast1| = 1119866 is an isometric isomorphism so for each119891 isin
1198781198711(120583|119860)there exists a sequence of norm-one simple functions
(119904119896) such that 119866(119904
119896) converges to 119891 Therefore
V (1198782) ⩾ sup
119891isin1198781198711(120583|119860)
(10038171003817100381710038171198632(119891 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119891 0)
1003816100381610038161003816) ⩾100381710038171003817100381711987831003817100381710038171003817
(130)
On the other hand we see that100381710038171003817100381711987831003817100381710038171003817 ⩾1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
3(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816119909
lowast
1119863
2(119892
1 0) + 119892
lowast
1119863
1(119892
1 0)1003816100381610038161003816 = V (119878
2) = 1
(131)
Therefore 1 = 1198783 = 119878
3(119892
1 0) which proves Claim 2
Finally set 1198783= (0 ) for a suitable 119871
1(120583|
119860)oplus
1119883 rarr
119883 and define the operator 1198791 119871
1(120583) rarr 119883 by
1198791(119891) = 119879
0(119891120594
119860119888) + (119875
119860119891 0) (132)
for every 119891 isin 1198711(120583) Then we have
10038171003817100381710038171198791 (119891)1003817100381710038171003817 ⩽100381710038171003817100381711987901003817100381710038171003817
10038171003817100381710038171198911205941198601198881003817100381710038171003817 +1003817100381710038171003817100381710038171003817100381710038171003817
10038171003817100381710038171198911205941198601003817100381710038171003817 =10038171003817100381710038171198911003817100381710038171003817 (133)
for every 119891 isin 1198711(120583) so 119879
1 ⩽ 1 Also
10038171003817100381710038171198791 (1198691198601198921)1003817100381710038171003817 =10038171003817100381710038171198783 (1198921 0)
1003817100381710038171003817 =100381710038171003817100381711987831003817100381710038171003817 = 1 (134)
so 1198791attains its norm at 119869
1198601198921isin 119871
1(120583) and
10038171003817100381710038171198691198601198921 minus 11989101003817100381710038171003817 =10038171003817100381710038171198921 minus 1198920
1003817100381710038171003817 lt 120576 (135)
We also have that for any 119891 isin 1198781198711(120583)
10038171003817100381710038171198790 (119891) minus 1198791 (119891)1003817100381710038171003817 =100381710038171003817100381710038171198790(119891120594
119860) minus (119875
119860119891 0)10038171003817100381710038171003817
⩽10038171003817100381710038171198790 (119869119860119875119860119891) minus 119879119860 (119875119860119891)
1003817100381710038171003817
+10038171003817100381710038171003817119879119860(119875
119860119891) minus (119875
119860119891 0)10038171003817100381710038171003817
⩽10038171003817100381710038171198790119869119860 minus 119879119860
1003817100381710038171003817 +10038171003817100381710038171198780 minus 1198783
1003817100381710038171003817
lt 120578 (1205760) + 6120576
(136)
Hence 1198790minus 119879
1 ⩽ 120578(120576
0) + 6120576 lt 120576
0
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Abstract and Applied Analysis 15
Acknowledgments
The authors thank Gilles Godefroy and Rafael Paya forfruitful conversations about the content of this paper andVladimir Kadets for providing Lemma 18 They also appre-ciate anonymous referees for careful reading and fruitfulsuggestions about revision The second author was sup-ported by Basic Science Research Program through theNational Research Foundation of Korea (NRF) fundedby the Ministry of Education Science and Technology(NRF-2012R1A1A1006869) The third author was partiallysupported by Spanish MICINN and FEDER project noMTM2012-31755 and by Junta de Andalucıa and FEDERGrants FQM-185 and P09-FQM-4911
References
[1] F F Bonsall and J Duncan Numerical Ranges of Operators onNormed Spaces and of Elements of Normed Algebras vol 2 ofLondon Mathematical Society Lecture Note Series 2 CambridgeUniversity Press London UK 1971
[2] F F Bonsall and J Duncan Numerical Ranges II CambridgeUniversity Press London UK 1973 London MathematicalSociety Lecture Notes Series 10
[3] B Sims On numerical range and its applications to Banachalgebras [PhD thesis] University of Newcastle New SouthWales Australia 1972
[4] I D Berg and B Sims ldquoDenseness of operators which attaintheir numerical radiusrdquoAustralian Mathematical Society A vol36 no 1 pp 130ndash133 1984
[5] C S Cardassi ldquoDensity of numerical radius attaining operatorson some reflexive spacesrdquo Bulletin of the Australian Mathemat-ical Society vol 31 no 1 pp 1ndash3 1985
[6] C S Cardassi ldquoNumerical radius attaining operatorsrdquo inBanach Spaces (Columbia Mo 1984) vol 1166 of Lecture Notesin Mathematics pp 11ndash14 Springer Berlin Germany 1985
[7] C S Cardassi ldquoNumerical radius-attaining operators on119862(119870)rdquoProceedings of the AmericanMathematical Society vol 95 no 4pp 537ndash543 1985
[8] MDAcostaOperadores que alcanzan su radio numerico [PhDthesis] Universidad de Granada Granada Spain 1990
[9] M D Acosta and R Paya ldquoNumerical radius attaining oper-ators and the Radon-Nikodym propertyrdquo The Bulletin of theLondon Mathematical Society vol 25 no 1 pp 67ndash73 1993
[10] R Paya ldquoA counterexample on numerical radius attainingoperatorsrdquo Israel Journal of Mathematics vol 79 no 1 pp 83ndash101 1992
[11] Y S Choi and S G Kim ldquoNorm or numerical radius attainingmultilinear mappings and polynomialsrdquo Journal of the LondonMathematical Society vol 54 no 1 pp 135ndash147 1996
[12] M D Acosta J Becerra Guerrero and M Ruiz GalanldquoNumerical-radius-attaining polynomialsrdquo The Quarterly Jour-nal of Mathematics vol 54 no 1 pp 1ndash10 2003
[13] M D Acosta and S G Kim ldquoDenseness of holomorphicfunctions attaining their numerical radiirdquo Israel Journal ofMathematics vol 161 pp 373ndash386 2007
[14] S G Kim and H J Lee ldquoNumerical peak holomorphicfunctions on Banach spacesrdquo Journal of Mathematical Analysisand Applications vol 364 no 2 pp 437ndash452 2010
[15] M D Acosta R M Aron D Garcıa and M MaestreldquoThe Bishop-Phelps-Bollobas theorem for operatorsrdquo Journal ofFunctional Analysis vol 254 no 11 pp 2780ndash2799 2008
[16] A J Guirao and O Kozhushkina ldquoThe Bishop-Phelps-Bollobasproperty for numerical radius in ℓ
1(119862)rdquo Studia Mathematica
vol 218 no 1 pp 41ndash54 2013[17] J Falco ldquoThe Bishop-Phelps-Bollobas property for numerical
radius on 1198711rdquo Journal of Mathematical Analysis and Applica-
tions vol 414 no 1 pp 125ndash133 2014[18] A Aviles A J Guirao and J Rodriguez ldquoOn the Bishop-
Phelps-Bollobas property for numerical radius in119862(119870)-spacesrdquoPreprint
[19] RM Aron Y S Choi S K Kim H J Lee andMMartin ldquoTheBishop-Phelps-Bollobas version of Lindenstrauss properties Aand Brdquo Preprint
[20] V Kadets M Martın and R Paya ldquoRecent progress and openquestions on the numerical index of Banach spacesrdquo Revista dela Real Academia de Ciencias Exactas Fısicas y Naturales A vol100 no 1-2 pp 155ndash182 2006
[21] M Martın J Merı and M Popov ldquoOn the numerical index ofreal 119871
119901(120583)-spacesrdquo Israel Journal of Mathematics vol 184 pp
183ndash192 2011[22] A Iwanik ldquoNorm attaining operators on Lebesgue spacesrdquo
Pacific Journal of Mathematics vol 83 no 2 pp 381ndash386 1979[23] HH SchaeferBanach Lattices and Positive Operators Springer
New York NY USA 1974[24] J Duncan C M McGregor J D Pryce and A J White ldquoThe
numerical index of a normed spacerdquo Journal of the LondonMathematical Society Second Series vol 2 pp 481ndash488 1970
[25] Y S Choi S K Kim H J Lee and M Martin ldquoThe Bishop-Phelps-Bollobas theorem for operators on 119871
1(120583)rdquo Preprint
[26] H E Lacey The Isometric Theory of Classical Banach SpacesSpringer New York NY USA 1974
[27] G Godefroy ldquoUniversal spaces for strictly convex Banachspacesrdquo Revista de la Real Academia de Ciencias Exactas Fısicasy Naturales A vol 100 no 1-2 pp 137ndash146 2006
[28] C Finet M Martın and R Paya ldquoNumerical index andrenormingrdquo Proceedings of the American Mathematical Societyvol 131 no 3 pp 871ndash877 2003
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of
Abstract and Applied Analysis 11
The next result relates the BPBp-nu to the BPBp foroperators in a particular case We will deduce our examplefrom it
Theorem 15 If 1198711(120583)oplus
1119883 has the BPBp-119899119906 then the pair
(1198711(120583) 119883) has the BPBp for operators
Before proving this proposition we will use it to get themain examples of this section The first example shows thatthe density of numerical radius attaining operators does notimply the BPBp-nu
Example 16 There is a reflexive space (and so numerical radiusattaining operators on it are dense) which fails to have theBPBp-nu Indeed let 119884 be a reflexive separable space whichis not superreflexive and we may suppose that 119884 is strictlyconvex Observe that 119884 cannot be uniformly convex since itis not superreflexive Now 119883 = ℓ(2)
1oplus1119884 is reflexive but the
pair (ℓ(2)1 119884) fails the BPBp since 119884 is strictly convex but not
uniformly convex [19 Corollary 33] Therefore Theorem 15gives us that119883 does not have the BPBp-nu
The example above can be extended to get the result thatevery infinite-dimensional separable Banach space can berenormed to fail the BPBp-nu This follows from the factthat every infinite-dimensional separable Banach space canbe renormed to be strictly convex but not uniformly convex(this result can be proved ldquoby handrdquo an alternative categoricalargument for it can be found in [27] and references therein)With a little more of effort we may get the main result of thesection
Theorem 17 Every infinite-dimensional separable Banachspace can be renormed to fail the weak-BPBp-nu (and so inparticular to fail the BPBp-nu)
We need the following result which is surely well knownAs we have not found a reference we include a nice and easyproof kindly given to us by Vladimir Kadets We recall thatgiven a Banach space 119884 the set of all equivalent norms on119884 can be viewed as a metric space using the Banach-Mazurdistance
Lemma 18 Let119884 be an infinite-dimensional separable Banachspace Then the set of equivalent norms on 119884 which are strictlyconvex and are not (locally) uniformly convex is dense in the setof all equivalent norms on119884 (with respect to the Banach-Mazurdistance)
Proof Fix 119890 isin 119878119884and 119890lowast
1isin 119878
119884lowast such that 119890lowast
1(119890) = 1 For a fixed
120576 isin (0 12) denote
119902 (119910) = max (1 minus 120576) 10038171003817100381710038171199101003817100381710038171003817 1003816100381610038161003816119890lowast
1(119910)1003816100381610038161003816 (119910 isin 119884) (88)
Evidently (1 minus 120576)119910 ⩽ 119902(119910) ⩽ 119910 for every 119910 isin 119884 Fix asequence 119890lowast
119896 119896 ⩾ 2 of norm-one functionals separating
the points of 119884 and denote
119901 (119910) = radic
infin
sum
119896=1
1
2119896
1003816100381610038161003816119890lowast
119896(119910)1003816100381610038161003816
2
(119910 isin 119884) (89)
Then 119901 is a strictly convex norm on 119884 119901(119890) ⩾ 1radic2 and119901(119910) ⩽ 119910 for all 119910 isin 119883 Finally write
100381710038171003817100381711991010038171003817100381710038171= (1 minus 120576) 119902 (119910) + 120576
119901 (119910)
119901 (119890)(119910 isin 119884) (90)
Then sdot 1is a strictly convex norm on 119884 and
(1 minus 120576)2 10038171003817100381710038171199101003817100381710038171003817 ⩽100381710038171003817100381711991010038171003817100381710038171⩽ (1 + 120576)
10038171003817100381710038171199101003817100381710038171003817 (119910 isin 119884) (91)
Wewill finish the proof by showing that sdot 1is not uniformly
convex (actually it is not locally uniformly convex) Indeedfor each 119899 isin N we select 119910
119899isin ⋂
119899
119896=1ker 119890lowast
119896with 119910
119899 = 1 and
consider 119890119899= 119890 + (1205764)119910
119899 Then 119902(119890) = 1 119902(119890
119899) = 1 and
119902(119890+119890119899) = 2 At the same time 119901(119910
119899) rarr 0 so 119901(119890
119899) rarr 119901(119890)
and 119901(119890 + 119890119899) rarr 2119901(119890) Consequently
1198901 = 1100381710038171003817100381711989011989910038171003817100381710038171997888rarr 1
1003817100381710038171003817119890 + 11989011989910038171003817100381710038171997888rarr 2 (92)
but 119890 minus 1198901198991= (1205764)119910
1198991⩾ (1 minus 120576)
2(1205764) which means the
absence of local uniform convexity at 119890
Proof of Theorem 17 Let 119883 be an infinite-dimensional sepa-rable Banach space Take a closed subspace 119884 of 119883 of codi-mension two By [28 Proposition 2] the map carrying everyequivalent norm on 119884 to its numerical index is continuousand so the set of values of the numerical index of 119884 upto reforming is a nontrivial interval [28 Theorem 9] ThenLemma 18 allows us to find an equivalent norm | sdot | on 119884 insuch a way that (119884 | sdot |) is strictly convex and is not uniformlyconvex and 119899(119884 | sdot |) gt 0 Now the space 119883 = ℓ(2)
1oplus1(119884 | sdot |)
is an equivalent renorming of 119883 which does not have theBPBp-119899119906 (indeed otherwise the pair (ℓ(2)
1 (119884 | sdot |)) would
have the BPBp for the operator norm and so (119884 | sdot |) wouldbe uniformly convex by [19 Corollary 33] a contradiction)Moreover as
119899 (119883) = min 119899 (ℓ(2)1) 119899 (119884 |sdot|) gt 0 (93)
(see [20 Proposition 2] for instance) 119883 also fails the weak-BPBp-nu by Proposition 6
To finish the section with the promised proof ofTheorem 15 we first see the following stability result
Lemma 19 Let119883 = [⨁infin
119896=1119883
119896]1198880
or [⨁infin
119896=1119883
119896]ℓ1
If119883 has theBishop-Phelps-Bollobas property for numerical radius with afunction 120578 then each Banach space 119883
119894has the Bishop-Phelps-
Bollobas property for numerical radius with 120578nu(119883119894) ⩾ 120578 That
is inf119894120578nu(119883119894
)(120576) ⩾ 120578nu(119883)(120576) for all 0 lt 120576 lt 1
Proof Let 119875119894 119883 rarr 119883
119894and 1198751015840
119894 119883
lowastrarr 119883
lowast
119894be the natural
projections and let 119876119894 119883
119894rarr 119883 and 1198761015840
119894 119883
lowast
119894rarr 119883
lowast be thenatural embeddings
Assume that an operator 119879119894 119883
119894rarr 119883
119894and a pair
(119909119894 119909
lowast
119894) isin Π(119883
119894) satisfy that
V (119879119894) = 1
1003816100381610038161003816119909lowast
119894119879119894119909119894
1003816100381610038161003816 gt 1 minus 120578 (120576) (94)
We define an operator 119879 119883 rarr 119883 and (119909 119909lowast) isin Π(119883) by
119879 = 119876119894∘ 119879
119894∘ 119875
119894 (119909 119909
lowast) = (119876
119894119909119894 119876
1015840
119894119909lowast
119894) (95)
12 Abstract and Applied Analysis
then clearly we see that1003816100381610038161003816119909
lowast1198791199091003816100381610038161003816 =1003816100381610038161003816119909
lowast
119894119879119894119909119894
1003816100381610038161003816 gt 1 minus 120578 (120576) (96)
From the assumption there exist 119878 119883 rarr 119883 and a pair(119910 119910
lowast) isin Π(119883) such that
1003816100381610038161003816119910lowast1198781199101003816100381610038161003816 = 1 = V (119878) 119878 minus 119879 lt 120576
1003817100381710038171003817119910lowastminus 119909
lowast1003817100381710038171003817 lt 1205761003817100381710038171003817119910 minus 119909
1003817100381710038171003817 lt 120576
(97)
Since this clearly shows that1003817100381710038171003817119875119894 ∘ 119878 ∘ 119876119894
minus 119879119894
1003817100381710038171003817 lt 120576100381710038171003817100381710038171198751015840
119894119910lowastminus 119909
lowast
119894
10038171003817100381710038171003817lt 120576
1003817100381710038171003817119875119894119910 minus 1199091198941003817100381710038171003817 lt 120576
(98)
we only need to show that |1198751015840119894119910lowast(119875
119894∘ 119878 ∘ 119876
119894)119875
119894119910| = 1
We first show the case of 1198880sum Since
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817=10038171003817100381710038171003817119875119895119910 minus 119875
11989511990910038171003817100381710038171003817⩽1003817100381710038171003817119910 minus 119909
1003817100381710038171003817 lt 120576 (99)
for every 119895 = 119894 we have
1 = 119910lowast(119910) = sum
119895isinN
1198751015840
119895119910lowast(119875
119895119910) ⩽ sum
119895isinN
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817
⩽100381710038171003817100381710038171198751015840
119894119910lowast10038171003817100381710038171003817+ 120576 sum
119895isinN119895 = 119894
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817⩽1003817100381710038171003817119910
lowast1003817100381710038171003817 = 1
(100)
This shows that 1198751015840119894119910lowast = 1 and 1198751015840
119895119910lowast= 0 for every 119895 = 119894
So 119910lowast = 1198761015840
1198941198751015840
119894119910lowast and 1198751015840
119894119910lowast(119875
119894119910) = 1 This and the fact that
119910 minus 119876119894119875119894119910 lt 120576 imply that
(119876119894119875119894119910 + (
1
120576) (119910 minus 119876
119894119875119894119910) 119876
1015840
1198941198751015840
119894119910lowast) isin Π (119883) (101)
So we get that (1198761015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910 + (1120576)(119910 minus 119876
119894119875119894119910))V(119878) = 1
Hence we have
1 =1003816100381610038161003816119910
lowast1198781199101003816100381610038161003816 =10038161003816100381610038161003816(119876
1015840
1198941198751015840
119894119910lowast) 11987811991010038161003816100381610038161003816
=
1003816100381610038161003816100381610038161003816(1 minus 120576) (119876
1015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910)
+120576 (1198761015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910 +1
120576(119910 minus 119876
119894119875119894119910))
1003816100381610038161003816100381610038161003816⩽ 1
(102)
and so we get |1198751015840119894119910lowast(119875
119894∘119878 ∘119876
119894)119875
119894119910| = |(119876
1015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910)| = 1
We next show the case of ℓ1sum The proof is almost
the same as that of the 1198880case However for the sake of
completeness we provide it hereSince 1198751015840
119895119910lowast = 119875
1015840
119895119910lowastminus 119875
1015840
119895119909lowast ⩽ 119910
lowastminus 119909
lowast lt 120576 for every
119895 = 119894 we have
1 = 119910lowast(119910) = sum
119895isinN
1198751015840
119895119910lowast(119875
119895119910) ⩽ sum
119895isinN
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817
⩽10038171003817100381710038171198751198941199101003817100381710038171003817 + 120576 sum
119895isinN119895 = 119894
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817⩽10038171003817100381710038171199101003817100381710038171003817 = 1
(103)
which shows 119875119894119910 = 1 and 119875
119895119910 = 0 for every 119895 = 119894 Since this
implies (119876119894119875119894119910119876
1015840
1198941198751015840
119894119910lowast+(1120576)(119910
lowastminus119876
1015840
1198941198751015840
119894119910lowast)) isin Π(119883) we get
that |(1198761015840
1198941198751015840
119894119910lowast+ (1120576)(119910
lowastminus 119876
1015840
1198941198751015840
119894119910lowast))119878(119876
119894119875119894119910)| ⩽ V(119878) = 1
Hence we have
1 =1003816100381610038161003816119910
lowast1198781199101003816100381610038161003816 =1003816100381610038161003816119910
lowast119878 (119876
119894119875119894119910)1003816100381610038161003816
=
1003816100381610038161003816100381610038161003816(1 minus 120576) (119876
1015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910)
+120576 (1198761015840
1198941198751015840
119894119910lowast+1
120576(119910
lowastminus 119876
1015840
1198941198751015840
119894119910lowast)) 119878 (119876
119894119875119894119910)
1003816100381610038161003816100381610038161003816⩽ 1
(104)
and so |1198751015840119894119910lowast(119875
119894∘ 119878 ∘ 119876
119894)119875
119894119910| = |(119876
1015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910)| = 1
Proof of Theorem 15 Note that 120578nu(1198711(120583)oplus1119883)(120576) rarr 0 as120576 rarr 0 Fix 0 lt 120576
0lt 1 and choose 0 lt 120576 lt 1 such that
6120576 + 120578nu(1198711(120583)oplus1119883)(120576) lt 1205760 Let 120578(1205760) = 120578nu(1198711(120583)oplus1119883)(120576)Suppose that 119879
0isin L(119871
1(120583) 119883) with 119879
0 = 1 and 119891
0isin
1198781198711(120583)
satisfy
1003817100381710038171003817119879011989101003817100381710038171003817 gt 1 minus 120578 (1205760) (105)
For any measurable subset 119861 let
1198711(120583|
119861) = 119891|
119861 119891 isin 119871
1(120583) (106)
with the norm 119891|119861 = 119891120594
1198611 Then it is easy to see
that 1198711(120583|
119861) is isometrically isomorphic to a complemented
subspace of 1198711(120583) Let 119875
119861 119871
1(120583) rarr 119871
1(120583|
119861) be the
restriction defined by 119875119861(119891) = 119891|
119861for all 119891 isin 119871
1(120583) and
let 119869119861 119871
1(120583|
119861) rarr 119871
1(120583) be the extension defined by
119869119861(119891)(120596) = 119891(120596) if 120596 isin 119861 and 119869
119861(119891)(120596) = 0 otherwise
It is clear that 119875119861119869119861= Id
1198711(120583|119861)and 119869
119861119875119861(119891) = 119891120594
119861for all
119891 isin 1198711(120583) Notice also that 119871
1(120583) is isometrically isomorphic
to 1198711(120583|
119861)oplus
11198711(120583|
119861119888)
Let 119860 = supp1198910and 119892
0= 119875
1198601198910 Then
1003817100381710038171003817119879011986911986011989201003817100381710038171003817 =100381710038171003817100381711987901198910
1003817100381710038171003817 gt 1 minus 120578 (1205760) gt 0 (107)
and define the operator 119879119860 119871
1(120583|
119860) rarr 119883 by 119879
119860119891 =
1198790119869119860119891119879
0119869119860 for every 119891 isin 119871
1(120583|
119860) Then
100381710038171003817100381711987911986011989201003817100381710038171003817 ⩾100381710038171003817100381711987901198910
1003817100381710038171003817 gt 1 minus 120578 (1205760) (108)
Since 120583|119860is 120590-finite 119871
1(120583|
119860)lowast= 119871
infin(120583|
119860) Let 119892lowast
0isin 119878
119871infin(120583|119860)
be a function such that ⟨119892lowast0 119892
0⟩ = 1 and choose 119909lowast
0isin 119878
119883lowast
such that 119909lowast0(119879
1198601198920) = 119879
1198601198920 Define the operator 119878
0isin
L(1198711(120583|
119860)oplus
1119883) by
1198780(119891 119909) = (0 119879
119860119891) ((119891 119909) isin 119871
1(120583|
119860) oplus
1119883) (109)
and observe that 1198780 = V(119878
0) = 1 Indeed
100381710038171003817100381711987801003817100381710038171003817 ⩽ 1 =
10038171003817100381710038171198791198601003817100381710038171003817
= sup 1003816100381610038161003816119909lowast1198791198601198911003816100381610038161003816 119909
lowastisin 119878
119883lowast 119891 isin 119878
1198711(120583|119860)
= sup 1003816100381610038161003816(119891lowast 119909
lowast) 119878
0(119891 119909)
1003816100381610038161003816
((119891lowast 119909
lowast) (119891 119909)) isin Π (119871
1(120583|
119860) oplus
1119883)
= V (1198780) ⩽100381710038171003817100381711987801003817100381710038171003817
(110)
Abstract and Applied Analysis 13
It is immediate that
(119892lowast
0 119909
lowast
0) 119878
0(119892
0 0) = 119909
lowast
0(119879
1198601198920) =10038171003817100381710038171198791198601198920
1003817100381710038171003817 gt 1 minus 120578 (1205760)
(111)
By Lemma 19 1198711(120583|
119860)oplus
1119883 has the BPBp-nu with the func-
tion 120578 Therefore there exist 1198781isin L(119871
1(120583|
119860)oplus
1119883) (119892
1 119909
1) isin
1198781198711(120583|119860)oplus1119883 and (119892lowast
1 119909
lowast
1) isin 119878
119871infin(120583|119860)oplusinfin119883lowast such that
1003817100381710038171003817(1198921 1199091) minus (1198920 0)1003817100381710038171003817 lt 120576
1003817100381710038171003817(119892lowast
1 119909
lowast
1) minus (119891
lowast
0 119909
lowast
0)1003817100381710038171003817 lt 120576
10038171003817100381710038171198781 minus 11987801003817100381710038171003817 lt 120576 ⟨(119892
lowast
1 119909
lowast
1) (119892
1 119909
1)⟩ = 1
1003816100381610038161003816(119892lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)1003816100381610038161003816 = V (119878
1) = 1
(112)
Claim 1 We claim that 1199091= 0
Otherwise
1 = Re ⟨(119892lowast1 119909
lowast
1) (119892
1 119909
1)⟩
=100381710038171003817100381711989211003817100381710038171003817Re⟨(119892
lowast
1 119909
lowast
1) (119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
⟩
+100381710038171003817100381711990911003817100381710038171003817Re⟨(119892
lowast
1 119909
lowast
1) (0 119909
1)
100381710038171003817100381711990911003817100381710038171003817
⟩ ⩽ 1
(113)
We deduce that
((119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
(119892lowast
1 119909
lowast
1)) (
(0 1199091)
100381710038171003817100381711990911003817100381710038171003817
(119892lowast
1 119909
lowast
1))
isin Π (1198711(120583|
119860) oplus
1119883)
(114)
Since 1198781((0 119909
1)119909
1) = (119878
1minus 119878
0)((0 119909
1)119909
1) lt 120576 we
get that
1 =1003816100381610038161003816⟨(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)⟩1003816100381610038161003816
=
100381610038161003816100381610038161003816100381610038161003816
100381710038171003817100381711989211003817100381710038171003817⟨(119892
lowast
1 119909
lowast
1) 119878
1((119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
)⟩
+100381710038171003817100381711990911003817100381710038171003817⟨(119892
lowast
1 119909
lowast
1) 119878
1((0 119909
1)
100381710038171003817100381711990911003817100381710038171003817
)⟩
100381610038161003816100381610038161003816100381610038161003816
⩽100381710038171003817100381711989211003817100381710038171003817 V (1198781) + 120576
100381710038171003817100381711990911003817100381710038171003817 lt100381710038171003817100381711989211003817100381710038171003817 +100381710038171003817100381711990911003817100381710038171003817 = 1
(115)
a contradiction This proves the claimWe define the operator 119878
2 119871
1(120583|
119860)oplus
1119883 rarr 119871
1(120583|
119860)oplus
1119883
by 1198782(119891 119909) = 119878
1(119891 0) for every 119891 isin 119871
1(120583|
119860) and for every
119909 isin 119883 Then we have
V (1198782) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816 = 1
10038171003817100381710038171198781 minus 11987821003817100381710038171003817 ⩽ 120576 (116)
Indeed from Claim 1 we have
V (1198781) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)1003816100381610038161003816 =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816 ⩽ V (119878
2)
(117)
On the other hand we see that1003816100381610038161003816(119891
lowast 119909
lowast) 119878
2(119891 119909)
1003816100381610038161003816
=1003816100381610038161003816(119891
lowast 119909
lowast) 119878
1(119891 0)
1003816100381610038161003816 ⩽10038171003817100381710038171198911003817100381710038171003817 V (1198781) ⩽ V (119878
1)
(118)
for every ((119891lowast 119909lowast) (119891 119909)) isin Π(1198711(120583|
119860)oplus
1119883) Therefore
V(1198782) ⩽ V(119878
1) Also
10038171003817100381710038171198781 minus 11987821003817100381710038171003817 ⩽ sup
119909isin119878119883
10038171003817100381710038171198781 (0 119909)1003817100381710038171003817
= sup119909isin119878119883
10038171003817100381710038171198781 (0 119909) minus 1198780 (0 119909)1003817100381710038171003817 ⩽ 120576
(119)
Claim 2 There exists 1198783 119871
1(120583|
119860)oplus
1119883 rarr 119871
1(120583|
119860)oplus
1119883 such
that 1198783(119892
1 0) = 119878
3 = 1 119878
3(0 119909) = 0 119878
3(119891 119909) isin 0oplus
1119883
for every (119891 119909) isin 1198711(120583|
119860)oplus
1119883 and 119878
3minus 119878
2 lt 4120576
Indeed write 1198781= (119863
1 119863
2) where 119863
1 119871
1(120583|
119860)oplus
1119883 rarr
1198711(120583|
119860) and119863
2 119871
1(120583|
119860)oplus
1119883 rarr 119883 We have that
sup 1003816100381610038161003816119892lowast119863
1(119892
1 0) + 119909
lowast119863
2(119892
1 0)1003816100381610038161003816
119909lowastisin 119878
119883lowast ⟨119892
lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860)
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816 +10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
⟨119892lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860)
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816
⟨119892lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860) +10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
⩽ V (1198782) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816119892
lowast
1119863
1(119892
1 0) + 119909
lowast
1119863
2(119892
1 0)1003816100381610038161003816
(120)
This implies that
1003816100381610038161003816119909lowast
1119863
2(119892
1 0)1003816100381610038161003816 =10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
1003816100381610038161003816119892lowast
1119863
1(119892
1 0)1003816100381610038161003816
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816 ⟨119892
lowast 119892
1⟩ = 1 119892
lowastisin 119871
infin(120583|
119860)
(121)
Therefore |119892lowast1| equals 1 on the support of 119863
1(119892
1 0) As
|⟨119892lowast
1 119892
1⟩| = 1 we also have that |119892lowast
1| equals 1 on the support
of 1198921 Changing the values of 119892lowast
1by the ones of 119891lowast
0on 119860
(supp(1198631(119892
1 0)) cup supp(g
1)) we may and do suppose that
|119892lowast
1| = 1 on the whole 119860We also have 119863
2(119892
1 0) gt 0 Indeed
10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198920 0)1003817100381710038171003817
⩽10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198921 0)
1003817100381710038171003817 +10038171003817100381710038171198780 (1198921 0) minus 1198780 (1198920 0)
1003817100381710038171003817
lt 2120576 + 120576 = 3120576
(122)
So we have
10038171003817100381710038171198632(119892
1 0) minus 119879
1198601198920
1003817100381710038171003817
⩽10038171003817100381710038171198631(119892
1 0)1003817100381710038171003817 +10038171003817100381710038171198632(119892
1 0) minus 119879
1198601198920
1003817100381710038171003817
14 Abstract and Applied Analysis
=1003817100381710038171003817(1198631
(1198921 0) 119863
2(119892
1 0)) minus (0 119879
1198601198920)1003817100381710038171003817
=10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198920 0)
1003817100381710038171003817 lt 3120576
(123)
and 1198632(119892
1 0) gt 119879
1198601198920 minus 3120576 ⩾ 1 minus 120578(120576
0) minus 3120576 gt 0
Finally define the operator 1198783by
1198783(119891 119909) = (0119863
2(119891 0) + 119892
lowast
1(119863
1(119891 0))
1198632(119892
1 0)
119909lowast
1119863
2(119892
1 0))
(124)
for (119891 119909) isin 1198711(120583|
119860)oplus
1119883 It is clear that
100381710038171003817100381711987831003817100381710038171003817 ⩽ sup
119891isin1198781198711(120583|119860)
(10038171003817100381710038171198632(119891 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119891 0)
1003816100381610038161003816) (125)
Notice also that
10038171003817100381710038171198631(119891 0)
1003817100381710038171003817
⩽10038171003817100381710038171198631(119891 0)
1003817100381710038171003817 +10038171003817100381710038171198632(119891 0) minus 119879
1198601198911003817100381710038171003817
=1003817100381710038171003817(1198631
(119891 0) 1198632(119891 0)) minus (0 119879
119860119891)1003817100381710038171003817
=10038171003817100381710038171198782 (119891 119909) minus 1198780 (119891 119909)
1003817100381710038171003817
(126)
for all (119891 119909) isin 1198711(120583|
119860)oplus
1119883 Hence we have
10038171003817100381710038171198783 minus 11987821003817100381710038171003817
= 2 sup119891isin1198781198711(120583|119860)
10038171003817100381710038171198631(119891 0)
1003817100381710038171003817 ⩽ 210038171003817100381710038171198782 minus 1198780
1003817100381710038171003817 lt 4120576(127)
On the other hand let 119866 1198711(120583|
119860) rarr 119871
1(120583|
119860) be defined by
119866(119891) = 119892lowast
1119891 for every 119891 isin 119871
1(120583|
119860) Then we have
V (1198782)
= sup 1003816100381610038161003816119911lowast11987821199111003816100381610038161003816 (119911 119911
lowast) isin Π (119871
1(120583|
119860) oplus
1119883)
⩾ sup10038161003816100381610038161003816100381610038161003816
119909lowast119863
2(119866(
1
120583 (119862)120594119862) 0)
+119892lowast
1119863
1(119866(
1
120583 (119862)120594119862) 0)
10038161003816100381610038161003816100381610038161003816
119909lowastisin 119878
119883lowast 119862 isin Σ
119860 120583 (119862) gt 0
= sup10038171003817100381710038171003817100381710038171003817
1198632(119866(
1
120583 (119862)120594119862) 0)
10038171003817100381710038171003817100381710038171003817
+
10038161003816100381610038161003816100381610038161003816
119892lowast
1119863
1(119866(
1
120583 (119862)120594119862) 0)
10038161003816100381610038161003816100381610038161003816
119862 isin Σ119860 120583 (119862) gt 0
(128)
where Σ119860is the family of measurable subsets of 119860
Hence for any simple function 119904 = sum119899
119894=1((120572
119894120583(119860
119894))120594
119860119894
) isin
1198781198711(120583|119860) where 119860
119894119894is a family of disjoint measurable subsets
with strictly positive measure we have
V (1198782)
⩾
119899
sum
119894=1
10038161003816100381610038161205721198941003816100381610038161003816 (
100381710038171003817100381710038171003817100381710038171003817
1198632(119866(
1
120583 (119860119894)120594119860119894
) 0)
100381710038171003817100381710038171003817100381710038171003817
+
100381610038161003816100381610038161003816100381610038161003816
119892lowast
1119863
1(119866(
1
120583 (119860119894)120594119860119894
) 0)
100381610038161003816100381610038161003816100381610038161003816
)
⩾10038171003817100381710038171198632(119866 (119904) 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119866 (119904) 0)
1003816100381610038161003816
(129)
Since |119892lowast1| = 1119866 is an isometric isomorphism so for each119891 isin
1198781198711(120583|119860)there exists a sequence of norm-one simple functions
(119904119896) such that 119866(119904
119896) converges to 119891 Therefore
V (1198782) ⩾ sup
119891isin1198781198711(120583|119860)
(10038171003817100381710038171198632(119891 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119891 0)
1003816100381610038161003816) ⩾100381710038171003817100381711987831003817100381710038171003817
(130)
On the other hand we see that100381710038171003817100381711987831003817100381710038171003817 ⩾1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
3(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816119909
lowast
1119863
2(119892
1 0) + 119892
lowast
1119863
1(119892
1 0)1003816100381610038161003816 = V (119878
2) = 1
(131)
Therefore 1 = 1198783 = 119878
3(119892
1 0) which proves Claim 2
Finally set 1198783= (0 ) for a suitable 119871
1(120583|
119860)oplus
1119883 rarr
119883 and define the operator 1198791 119871
1(120583) rarr 119883 by
1198791(119891) = 119879
0(119891120594
119860119888) + (119875
119860119891 0) (132)
for every 119891 isin 1198711(120583) Then we have
10038171003817100381710038171198791 (119891)1003817100381710038171003817 ⩽100381710038171003817100381711987901003817100381710038171003817
10038171003817100381710038171198911205941198601198881003817100381710038171003817 +1003817100381710038171003817100381710038171003817100381710038171003817
10038171003817100381710038171198911205941198601003817100381710038171003817 =10038171003817100381710038171198911003817100381710038171003817 (133)
for every 119891 isin 1198711(120583) so 119879
1 ⩽ 1 Also
10038171003817100381710038171198791 (1198691198601198921)1003817100381710038171003817 =10038171003817100381710038171198783 (1198921 0)
1003817100381710038171003817 =100381710038171003817100381711987831003817100381710038171003817 = 1 (134)
so 1198791attains its norm at 119869
1198601198921isin 119871
1(120583) and
10038171003817100381710038171198691198601198921 minus 11989101003817100381710038171003817 =10038171003817100381710038171198921 minus 1198920
1003817100381710038171003817 lt 120576 (135)
We also have that for any 119891 isin 1198781198711(120583)
10038171003817100381710038171198790 (119891) minus 1198791 (119891)1003817100381710038171003817 =100381710038171003817100381710038171198790(119891120594
119860) minus (119875
119860119891 0)10038171003817100381710038171003817
⩽10038171003817100381710038171198790 (119869119860119875119860119891) minus 119879119860 (119875119860119891)
1003817100381710038171003817
+10038171003817100381710038171003817119879119860(119875
119860119891) minus (119875
119860119891 0)10038171003817100381710038171003817
⩽10038171003817100381710038171198790119869119860 minus 119879119860
1003817100381710038171003817 +10038171003817100381710038171198780 minus 1198783
1003817100381710038171003817
lt 120578 (1205760) + 6120576
(136)
Hence 1198790minus 119879
1 ⩽ 120578(120576
0) + 6120576 lt 120576
0
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Abstract and Applied Analysis 15
Acknowledgments
The authors thank Gilles Godefroy and Rafael Paya forfruitful conversations about the content of this paper andVladimir Kadets for providing Lemma 18 They also appre-ciate anonymous referees for careful reading and fruitfulsuggestions about revision The second author was sup-ported by Basic Science Research Program through theNational Research Foundation of Korea (NRF) fundedby the Ministry of Education Science and Technology(NRF-2012R1A1A1006869) The third author was partiallysupported by Spanish MICINN and FEDER project noMTM2012-31755 and by Junta de Andalucıa and FEDERGrants FQM-185 and P09-FQM-4911
References
[1] F F Bonsall and J Duncan Numerical Ranges of Operators onNormed Spaces and of Elements of Normed Algebras vol 2 ofLondon Mathematical Society Lecture Note Series 2 CambridgeUniversity Press London UK 1971
[2] F F Bonsall and J Duncan Numerical Ranges II CambridgeUniversity Press London UK 1973 London MathematicalSociety Lecture Notes Series 10
[3] B Sims On numerical range and its applications to Banachalgebras [PhD thesis] University of Newcastle New SouthWales Australia 1972
[4] I D Berg and B Sims ldquoDenseness of operators which attaintheir numerical radiusrdquoAustralian Mathematical Society A vol36 no 1 pp 130ndash133 1984
[5] C S Cardassi ldquoDensity of numerical radius attaining operatorson some reflexive spacesrdquo Bulletin of the Australian Mathemat-ical Society vol 31 no 1 pp 1ndash3 1985
[6] C S Cardassi ldquoNumerical radius attaining operatorsrdquo inBanach Spaces (Columbia Mo 1984) vol 1166 of Lecture Notesin Mathematics pp 11ndash14 Springer Berlin Germany 1985
[7] C S Cardassi ldquoNumerical radius-attaining operators on119862(119870)rdquoProceedings of the AmericanMathematical Society vol 95 no 4pp 537ndash543 1985
[8] MDAcostaOperadores que alcanzan su radio numerico [PhDthesis] Universidad de Granada Granada Spain 1990
[9] M D Acosta and R Paya ldquoNumerical radius attaining oper-ators and the Radon-Nikodym propertyrdquo The Bulletin of theLondon Mathematical Society vol 25 no 1 pp 67ndash73 1993
[10] R Paya ldquoA counterexample on numerical radius attainingoperatorsrdquo Israel Journal of Mathematics vol 79 no 1 pp 83ndash101 1992
[11] Y S Choi and S G Kim ldquoNorm or numerical radius attainingmultilinear mappings and polynomialsrdquo Journal of the LondonMathematical Society vol 54 no 1 pp 135ndash147 1996
[12] M D Acosta J Becerra Guerrero and M Ruiz GalanldquoNumerical-radius-attaining polynomialsrdquo The Quarterly Jour-nal of Mathematics vol 54 no 1 pp 1ndash10 2003
[13] M D Acosta and S G Kim ldquoDenseness of holomorphicfunctions attaining their numerical radiirdquo Israel Journal ofMathematics vol 161 pp 373ndash386 2007
[14] S G Kim and H J Lee ldquoNumerical peak holomorphicfunctions on Banach spacesrdquo Journal of Mathematical Analysisand Applications vol 364 no 2 pp 437ndash452 2010
[15] M D Acosta R M Aron D Garcıa and M MaestreldquoThe Bishop-Phelps-Bollobas theorem for operatorsrdquo Journal ofFunctional Analysis vol 254 no 11 pp 2780ndash2799 2008
[16] A J Guirao and O Kozhushkina ldquoThe Bishop-Phelps-Bollobasproperty for numerical radius in ℓ
1(119862)rdquo Studia Mathematica
vol 218 no 1 pp 41ndash54 2013[17] J Falco ldquoThe Bishop-Phelps-Bollobas property for numerical
radius on 1198711rdquo Journal of Mathematical Analysis and Applica-
tions vol 414 no 1 pp 125ndash133 2014[18] A Aviles A J Guirao and J Rodriguez ldquoOn the Bishop-
Phelps-Bollobas property for numerical radius in119862(119870)-spacesrdquoPreprint
[19] RM Aron Y S Choi S K Kim H J Lee andMMartin ldquoTheBishop-Phelps-Bollobas version of Lindenstrauss properties Aand Brdquo Preprint
[20] V Kadets M Martın and R Paya ldquoRecent progress and openquestions on the numerical index of Banach spacesrdquo Revista dela Real Academia de Ciencias Exactas Fısicas y Naturales A vol100 no 1-2 pp 155ndash182 2006
[21] M Martın J Merı and M Popov ldquoOn the numerical index ofreal 119871
119901(120583)-spacesrdquo Israel Journal of Mathematics vol 184 pp
183ndash192 2011[22] A Iwanik ldquoNorm attaining operators on Lebesgue spacesrdquo
Pacific Journal of Mathematics vol 83 no 2 pp 381ndash386 1979[23] HH SchaeferBanach Lattices and Positive Operators Springer
New York NY USA 1974[24] J Duncan C M McGregor J D Pryce and A J White ldquoThe
numerical index of a normed spacerdquo Journal of the LondonMathematical Society Second Series vol 2 pp 481ndash488 1970
[25] Y S Choi S K Kim H J Lee and M Martin ldquoThe Bishop-Phelps-Bollobas theorem for operators on 119871
1(120583)rdquo Preprint
[26] H E Lacey The Isometric Theory of Classical Banach SpacesSpringer New York NY USA 1974
[27] G Godefroy ldquoUniversal spaces for strictly convex Banachspacesrdquo Revista de la Real Academia de Ciencias Exactas Fısicasy Naturales A vol 100 no 1-2 pp 137ndash146 2006
[28] C Finet M Martın and R Paya ldquoNumerical index andrenormingrdquo Proceedings of the American Mathematical Societyvol 131 no 3 pp 871ndash877 2003
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Mathematical Problems in Engineering
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Differential EquationsInternational Journal of
Volume 2014
Applied MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Journal of
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Mathematical PhysicsAdvances in
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OptimizationJournal of
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International Journal of
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Function Spaces
Abstract and Applied AnalysisHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of Mathematics and Mathematical Sciences
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Algebra
Discrete Dynamics in Nature and Society
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Decision SciencesAdvances in
Discrete MathematicsJournal of
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Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of
12 Abstract and Applied Analysis
then clearly we see that1003816100381610038161003816119909
lowast1198791199091003816100381610038161003816 =1003816100381610038161003816119909
lowast
119894119879119894119909119894
1003816100381610038161003816 gt 1 minus 120578 (120576) (96)
From the assumption there exist 119878 119883 rarr 119883 and a pair(119910 119910
lowast) isin Π(119883) such that
1003816100381610038161003816119910lowast1198781199101003816100381610038161003816 = 1 = V (119878) 119878 minus 119879 lt 120576
1003817100381710038171003817119910lowastminus 119909
lowast1003817100381710038171003817 lt 1205761003817100381710038171003817119910 minus 119909
1003817100381710038171003817 lt 120576
(97)
Since this clearly shows that1003817100381710038171003817119875119894 ∘ 119878 ∘ 119876119894
minus 119879119894
1003817100381710038171003817 lt 120576100381710038171003817100381710038171198751015840
119894119910lowastminus 119909
lowast
119894
10038171003817100381710038171003817lt 120576
1003817100381710038171003817119875119894119910 minus 1199091198941003817100381710038171003817 lt 120576
(98)
we only need to show that |1198751015840119894119910lowast(119875
119894∘ 119878 ∘ 119876
119894)119875
119894119910| = 1
We first show the case of 1198880sum Since
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817=10038171003817100381710038171003817119875119895119910 minus 119875
11989511990910038171003817100381710038171003817⩽1003817100381710038171003817119910 minus 119909
1003817100381710038171003817 lt 120576 (99)
for every 119895 = 119894 we have
1 = 119910lowast(119910) = sum
119895isinN
1198751015840
119895119910lowast(119875
119895119910) ⩽ sum
119895isinN
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817
⩽100381710038171003817100381710038171198751015840
119894119910lowast10038171003817100381710038171003817+ 120576 sum
119895isinN119895 = 119894
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817⩽1003817100381710038171003817119910
lowast1003817100381710038171003817 = 1
(100)
This shows that 1198751015840119894119910lowast = 1 and 1198751015840
119895119910lowast= 0 for every 119895 = 119894
So 119910lowast = 1198761015840
1198941198751015840
119894119910lowast and 1198751015840
119894119910lowast(119875
119894119910) = 1 This and the fact that
119910 minus 119876119894119875119894119910 lt 120576 imply that
(119876119894119875119894119910 + (
1
120576) (119910 minus 119876
119894119875119894119910) 119876
1015840
1198941198751015840
119894119910lowast) isin Π (119883) (101)
So we get that (1198761015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910 + (1120576)(119910 minus 119876
119894119875119894119910))V(119878) = 1
Hence we have
1 =1003816100381610038161003816119910
lowast1198781199101003816100381610038161003816 =10038161003816100381610038161003816(119876
1015840
1198941198751015840
119894119910lowast) 11987811991010038161003816100381610038161003816
=
1003816100381610038161003816100381610038161003816(1 minus 120576) (119876
1015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910)
+120576 (1198761015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910 +1
120576(119910 minus 119876
119894119875119894119910))
1003816100381610038161003816100381610038161003816⩽ 1
(102)
and so we get |1198751015840119894119910lowast(119875
119894∘119878 ∘119876
119894)119875
119894119910| = |(119876
1015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910)| = 1
We next show the case of ℓ1sum The proof is almost
the same as that of the 1198880case However for the sake of
completeness we provide it hereSince 1198751015840
119895119910lowast = 119875
1015840
119895119910lowastminus 119875
1015840
119895119909lowast ⩽ 119910
lowastminus 119909
lowast lt 120576 for every
119895 = 119894 we have
1 = 119910lowast(119910) = sum
119895isinN
1198751015840
119895119910lowast(119875
119895119910) ⩽ sum
119895isinN
100381710038171003817100381710038171198751015840
119895119910lowast10038171003817100381710038171003817
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817
⩽10038171003817100381710038171198751198941199101003817100381710038171003817 + 120576 sum
119895isinN119895 = 119894
1003817100381710038171003817100381711987511989511991010038171003817100381710038171003817⩽10038171003817100381710038171199101003817100381710038171003817 = 1
(103)
which shows 119875119894119910 = 1 and 119875
119895119910 = 0 for every 119895 = 119894 Since this
implies (119876119894119875119894119910119876
1015840
1198941198751015840
119894119910lowast+(1120576)(119910
lowastminus119876
1015840
1198941198751015840
119894119910lowast)) isin Π(119883) we get
that |(1198761015840
1198941198751015840
119894119910lowast+ (1120576)(119910
lowastminus 119876
1015840
1198941198751015840
119894119910lowast))119878(119876
119894119875119894119910)| ⩽ V(119878) = 1
Hence we have
1 =1003816100381610038161003816119910
lowast1198781199101003816100381610038161003816 =1003816100381610038161003816119910
lowast119878 (119876
119894119875119894119910)1003816100381610038161003816
=
1003816100381610038161003816100381610038161003816(1 minus 120576) (119876
1015840
1198941198751015840
119894119910lowast) 119878 (119876
119894119875119894119910)
+120576 (1198761015840
1198941198751015840
119894119910lowast+1
120576(119910
lowastminus 119876
1015840
1198941198751015840
119894119910lowast)) 119878 (119876
119894119875119894119910)
1003816100381610038161003816100381610038161003816⩽ 1
(104)
and so |1198751015840119894119910lowast(119875
119894∘ 119878 ∘ 119876
119894)119875
119894119910| = |(119876
1015840
1198941198751015840
119894119910lowast)119878(119876
119894119875119894119910)| = 1
Proof of Theorem 15 Note that 120578nu(1198711(120583)oplus1119883)(120576) rarr 0 as120576 rarr 0 Fix 0 lt 120576
0lt 1 and choose 0 lt 120576 lt 1 such that
6120576 + 120578nu(1198711(120583)oplus1119883)(120576) lt 1205760 Let 120578(1205760) = 120578nu(1198711(120583)oplus1119883)(120576)Suppose that 119879
0isin L(119871
1(120583) 119883) with 119879
0 = 1 and 119891
0isin
1198781198711(120583)
satisfy
1003817100381710038171003817119879011989101003817100381710038171003817 gt 1 minus 120578 (1205760) (105)
For any measurable subset 119861 let
1198711(120583|
119861) = 119891|
119861 119891 isin 119871
1(120583) (106)
with the norm 119891|119861 = 119891120594
1198611 Then it is easy to see
that 1198711(120583|
119861) is isometrically isomorphic to a complemented
subspace of 1198711(120583) Let 119875
119861 119871
1(120583) rarr 119871
1(120583|
119861) be the
restriction defined by 119875119861(119891) = 119891|
119861for all 119891 isin 119871
1(120583) and
let 119869119861 119871
1(120583|
119861) rarr 119871
1(120583) be the extension defined by
119869119861(119891)(120596) = 119891(120596) if 120596 isin 119861 and 119869
119861(119891)(120596) = 0 otherwise
It is clear that 119875119861119869119861= Id
1198711(120583|119861)and 119869
119861119875119861(119891) = 119891120594
119861for all
119891 isin 1198711(120583) Notice also that 119871
1(120583) is isometrically isomorphic
to 1198711(120583|
119861)oplus
11198711(120583|
119861119888)
Let 119860 = supp1198910and 119892
0= 119875
1198601198910 Then
1003817100381710038171003817119879011986911986011989201003817100381710038171003817 =100381710038171003817100381711987901198910
1003817100381710038171003817 gt 1 minus 120578 (1205760) gt 0 (107)
and define the operator 119879119860 119871
1(120583|
119860) rarr 119883 by 119879
119860119891 =
1198790119869119860119891119879
0119869119860 for every 119891 isin 119871
1(120583|
119860) Then
100381710038171003817100381711987911986011989201003817100381710038171003817 ⩾100381710038171003817100381711987901198910
1003817100381710038171003817 gt 1 minus 120578 (1205760) (108)
Since 120583|119860is 120590-finite 119871
1(120583|
119860)lowast= 119871
infin(120583|
119860) Let 119892lowast
0isin 119878
119871infin(120583|119860)
be a function such that ⟨119892lowast0 119892
0⟩ = 1 and choose 119909lowast
0isin 119878
119883lowast
such that 119909lowast0(119879
1198601198920) = 119879
1198601198920 Define the operator 119878
0isin
L(1198711(120583|
119860)oplus
1119883) by
1198780(119891 119909) = (0 119879
119860119891) ((119891 119909) isin 119871
1(120583|
119860) oplus
1119883) (109)
and observe that 1198780 = V(119878
0) = 1 Indeed
100381710038171003817100381711987801003817100381710038171003817 ⩽ 1 =
10038171003817100381710038171198791198601003817100381710038171003817
= sup 1003816100381610038161003816119909lowast1198791198601198911003816100381610038161003816 119909
lowastisin 119878
119883lowast 119891 isin 119878
1198711(120583|119860)
= sup 1003816100381610038161003816(119891lowast 119909
lowast) 119878
0(119891 119909)
1003816100381610038161003816
((119891lowast 119909
lowast) (119891 119909)) isin Π (119871
1(120583|
119860) oplus
1119883)
= V (1198780) ⩽100381710038171003817100381711987801003817100381710038171003817
(110)
Abstract and Applied Analysis 13
It is immediate that
(119892lowast
0 119909
lowast
0) 119878
0(119892
0 0) = 119909
lowast
0(119879
1198601198920) =10038171003817100381710038171198791198601198920
1003817100381710038171003817 gt 1 minus 120578 (1205760)
(111)
By Lemma 19 1198711(120583|
119860)oplus
1119883 has the BPBp-nu with the func-
tion 120578 Therefore there exist 1198781isin L(119871
1(120583|
119860)oplus
1119883) (119892
1 119909
1) isin
1198781198711(120583|119860)oplus1119883 and (119892lowast
1 119909
lowast
1) isin 119878
119871infin(120583|119860)oplusinfin119883lowast such that
1003817100381710038171003817(1198921 1199091) minus (1198920 0)1003817100381710038171003817 lt 120576
1003817100381710038171003817(119892lowast
1 119909
lowast
1) minus (119891
lowast
0 119909
lowast
0)1003817100381710038171003817 lt 120576
10038171003817100381710038171198781 minus 11987801003817100381710038171003817 lt 120576 ⟨(119892
lowast
1 119909
lowast
1) (119892
1 119909
1)⟩ = 1
1003816100381610038161003816(119892lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)1003816100381610038161003816 = V (119878
1) = 1
(112)
Claim 1 We claim that 1199091= 0
Otherwise
1 = Re ⟨(119892lowast1 119909
lowast
1) (119892
1 119909
1)⟩
=100381710038171003817100381711989211003817100381710038171003817Re⟨(119892
lowast
1 119909
lowast
1) (119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
⟩
+100381710038171003817100381711990911003817100381710038171003817Re⟨(119892
lowast
1 119909
lowast
1) (0 119909
1)
100381710038171003817100381711990911003817100381710038171003817
⟩ ⩽ 1
(113)
We deduce that
((119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
(119892lowast
1 119909
lowast
1)) (
(0 1199091)
100381710038171003817100381711990911003817100381710038171003817
(119892lowast
1 119909
lowast
1))
isin Π (1198711(120583|
119860) oplus
1119883)
(114)
Since 1198781((0 119909
1)119909
1) = (119878
1minus 119878
0)((0 119909
1)119909
1) lt 120576 we
get that
1 =1003816100381610038161003816⟨(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)⟩1003816100381610038161003816
=
100381610038161003816100381610038161003816100381610038161003816
100381710038171003817100381711989211003817100381710038171003817⟨(119892
lowast
1 119909
lowast
1) 119878
1((119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
)⟩
+100381710038171003817100381711990911003817100381710038171003817⟨(119892
lowast
1 119909
lowast
1) 119878
1((0 119909
1)
100381710038171003817100381711990911003817100381710038171003817
)⟩
100381610038161003816100381610038161003816100381610038161003816
⩽100381710038171003817100381711989211003817100381710038171003817 V (1198781) + 120576
100381710038171003817100381711990911003817100381710038171003817 lt100381710038171003817100381711989211003817100381710038171003817 +100381710038171003817100381711990911003817100381710038171003817 = 1
(115)
a contradiction This proves the claimWe define the operator 119878
2 119871
1(120583|
119860)oplus
1119883 rarr 119871
1(120583|
119860)oplus
1119883
by 1198782(119891 119909) = 119878
1(119891 0) for every 119891 isin 119871
1(120583|
119860) and for every
119909 isin 119883 Then we have
V (1198782) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816 = 1
10038171003817100381710038171198781 minus 11987821003817100381710038171003817 ⩽ 120576 (116)
Indeed from Claim 1 we have
V (1198781) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)1003816100381610038161003816 =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816 ⩽ V (119878
2)
(117)
On the other hand we see that1003816100381610038161003816(119891
lowast 119909
lowast) 119878
2(119891 119909)
1003816100381610038161003816
=1003816100381610038161003816(119891
lowast 119909
lowast) 119878
1(119891 0)
1003816100381610038161003816 ⩽10038171003817100381710038171198911003817100381710038171003817 V (1198781) ⩽ V (119878
1)
(118)
for every ((119891lowast 119909lowast) (119891 119909)) isin Π(1198711(120583|
119860)oplus
1119883) Therefore
V(1198782) ⩽ V(119878
1) Also
10038171003817100381710038171198781 minus 11987821003817100381710038171003817 ⩽ sup
119909isin119878119883
10038171003817100381710038171198781 (0 119909)1003817100381710038171003817
= sup119909isin119878119883
10038171003817100381710038171198781 (0 119909) minus 1198780 (0 119909)1003817100381710038171003817 ⩽ 120576
(119)
Claim 2 There exists 1198783 119871
1(120583|
119860)oplus
1119883 rarr 119871
1(120583|
119860)oplus
1119883 such
that 1198783(119892
1 0) = 119878
3 = 1 119878
3(0 119909) = 0 119878
3(119891 119909) isin 0oplus
1119883
for every (119891 119909) isin 1198711(120583|
119860)oplus
1119883 and 119878
3minus 119878
2 lt 4120576
Indeed write 1198781= (119863
1 119863
2) where 119863
1 119871
1(120583|
119860)oplus
1119883 rarr
1198711(120583|
119860) and119863
2 119871
1(120583|
119860)oplus
1119883 rarr 119883 We have that
sup 1003816100381610038161003816119892lowast119863
1(119892
1 0) + 119909
lowast119863
2(119892
1 0)1003816100381610038161003816
119909lowastisin 119878
119883lowast ⟨119892
lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860)
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816 +10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
⟨119892lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860)
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816
⟨119892lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860) +10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
⩽ V (1198782) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816119892
lowast
1119863
1(119892
1 0) + 119909
lowast
1119863
2(119892
1 0)1003816100381610038161003816
(120)
This implies that
1003816100381610038161003816119909lowast
1119863
2(119892
1 0)1003816100381610038161003816 =10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
1003816100381610038161003816119892lowast
1119863
1(119892
1 0)1003816100381610038161003816
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816 ⟨119892
lowast 119892
1⟩ = 1 119892
lowastisin 119871
infin(120583|
119860)
(121)
Therefore |119892lowast1| equals 1 on the support of 119863
1(119892
1 0) As
|⟨119892lowast
1 119892
1⟩| = 1 we also have that |119892lowast
1| equals 1 on the support
of 1198921 Changing the values of 119892lowast
1by the ones of 119891lowast
0on 119860
(supp(1198631(119892
1 0)) cup supp(g
1)) we may and do suppose that
|119892lowast
1| = 1 on the whole 119860We also have 119863
2(119892
1 0) gt 0 Indeed
10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198920 0)1003817100381710038171003817
⩽10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198921 0)
1003817100381710038171003817 +10038171003817100381710038171198780 (1198921 0) minus 1198780 (1198920 0)
1003817100381710038171003817
lt 2120576 + 120576 = 3120576
(122)
So we have
10038171003817100381710038171198632(119892
1 0) minus 119879
1198601198920
1003817100381710038171003817
⩽10038171003817100381710038171198631(119892
1 0)1003817100381710038171003817 +10038171003817100381710038171198632(119892
1 0) minus 119879
1198601198920
1003817100381710038171003817
14 Abstract and Applied Analysis
=1003817100381710038171003817(1198631
(1198921 0) 119863
2(119892
1 0)) minus (0 119879
1198601198920)1003817100381710038171003817
=10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198920 0)
1003817100381710038171003817 lt 3120576
(123)
and 1198632(119892
1 0) gt 119879
1198601198920 minus 3120576 ⩾ 1 minus 120578(120576
0) minus 3120576 gt 0
Finally define the operator 1198783by
1198783(119891 119909) = (0119863
2(119891 0) + 119892
lowast
1(119863
1(119891 0))
1198632(119892
1 0)
119909lowast
1119863
2(119892
1 0))
(124)
for (119891 119909) isin 1198711(120583|
119860)oplus
1119883 It is clear that
100381710038171003817100381711987831003817100381710038171003817 ⩽ sup
119891isin1198781198711(120583|119860)
(10038171003817100381710038171198632(119891 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119891 0)
1003816100381610038161003816) (125)
Notice also that
10038171003817100381710038171198631(119891 0)
1003817100381710038171003817
⩽10038171003817100381710038171198631(119891 0)
1003817100381710038171003817 +10038171003817100381710038171198632(119891 0) minus 119879
1198601198911003817100381710038171003817
=1003817100381710038171003817(1198631
(119891 0) 1198632(119891 0)) minus (0 119879
119860119891)1003817100381710038171003817
=10038171003817100381710038171198782 (119891 119909) minus 1198780 (119891 119909)
1003817100381710038171003817
(126)
for all (119891 119909) isin 1198711(120583|
119860)oplus
1119883 Hence we have
10038171003817100381710038171198783 minus 11987821003817100381710038171003817
= 2 sup119891isin1198781198711(120583|119860)
10038171003817100381710038171198631(119891 0)
1003817100381710038171003817 ⩽ 210038171003817100381710038171198782 minus 1198780
1003817100381710038171003817 lt 4120576(127)
On the other hand let 119866 1198711(120583|
119860) rarr 119871
1(120583|
119860) be defined by
119866(119891) = 119892lowast
1119891 for every 119891 isin 119871
1(120583|
119860) Then we have
V (1198782)
= sup 1003816100381610038161003816119911lowast11987821199111003816100381610038161003816 (119911 119911
lowast) isin Π (119871
1(120583|
119860) oplus
1119883)
⩾ sup10038161003816100381610038161003816100381610038161003816
119909lowast119863
2(119866(
1
120583 (119862)120594119862) 0)
+119892lowast
1119863
1(119866(
1
120583 (119862)120594119862) 0)
10038161003816100381610038161003816100381610038161003816
119909lowastisin 119878
119883lowast 119862 isin Σ
119860 120583 (119862) gt 0
= sup10038171003817100381710038171003817100381710038171003817
1198632(119866(
1
120583 (119862)120594119862) 0)
10038171003817100381710038171003817100381710038171003817
+
10038161003816100381610038161003816100381610038161003816
119892lowast
1119863
1(119866(
1
120583 (119862)120594119862) 0)
10038161003816100381610038161003816100381610038161003816
119862 isin Σ119860 120583 (119862) gt 0
(128)
where Σ119860is the family of measurable subsets of 119860
Hence for any simple function 119904 = sum119899
119894=1((120572
119894120583(119860
119894))120594
119860119894
) isin
1198781198711(120583|119860) where 119860
119894119894is a family of disjoint measurable subsets
with strictly positive measure we have
V (1198782)
⩾
119899
sum
119894=1
10038161003816100381610038161205721198941003816100381610038161003816 (
100381710038171003817100381710038171003817100381710038171003817
1198632(119866(
1
120583 (119860119894)120594119860119894
) 0)
100381710038171003817100381710038171003817100381710038171003817
+
100381610038161003816100381610038161003816100381610038161003816
119892lowast
1119863
1(119866(
1
120583 (119860119894)120594119860119894
) 0)
100381610038161003816100381610038161003816100381610038161003816
)
⩾10038171003817100381710038171198632(119866 (119904) 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119866 (119904) 0)
1003816100381610038161003816
(129)
Since |119892lowast1| = 1119866 is an isometric isomorphism so for each119891 isin
1198781198711(120583|119860)there exists a sequence of norm-one simple functions
(119904119896) such that 119866(119904
119896) converges to 119891 Therefore
V (1198782) ⩾ sup
119891isin1198781198711(120583|119860)
(10038171003817100381710038171198632(119891 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119891 0)
1003816100381610038161003816) ⩾100381710038171003817100381711987831003817100381710038171003817
(130)
On the other hand we see that100381710038171003817100381711987831003817100381710038171003817 ⩾1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
3(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816119909
lowast
1119863
2(119892
1 0) + 119892
lowast
1119863
1(119892
1 0)1003816100381610038161003816 = V (119878
2) = 1
(131)
Therefore 1 = 1198783 = 119878
3(119892
1 0) which proves Claim 2
Finally set 1198783= (0 ) for a suitable 119871
1(120583|
119860)oplus
1119883 rarr
119883 and define the operator 1198791 119871
1(120583) rarr 119883 by
1198791(119891) = 119879
0(119891120594
119860119888) + (119875
119860119891 0) (132)
for every 119891 isin 1198711(120583) Then we have
10038171003817100381710038171198791 (119891)1003817100381710038171003817 ⩽100381710038171003817100381711987901003817100381710038171003817
10038171003817100381710038171198911205941198601198881003817100381710038171003817 +1003817100381710038171003817100381710038171003817100381710038171003817
10038171003817100381710038171198911205941198601003817100381710038171003817 =10038171003817100381710038171198911003817100381710038171003817 (133)
for every 119891 isin 1198711(120583) so 119879
1 ⩽ 1 Also
10038171003817100381710038171198791 (1198691198601198921)1003817100381710038171003817 =10038171003817100381710038171198783 (1198921 0)
1003817100381710038171003817 =100381710038171003817100381711987831003817100381710038171003817 = 1 (134)
so 1198791attains its norm at 119869
1198601198921isin 119871
1(120583) and
10038171003817100381710038171198691198601198921 minus 11989101003817100381710038171003817 =10038171003817100381710038171198921 minus 1198920
1003817100381710038171003817 lt 120576 (135)
We also have that for any 119891 isin 1198781198711(120583)
10038171003817100381710038171198790 (119891) minus 1198791 (119891)1003817100381710038171003817 =100381710038171003817100381710038171198790(119891120594
119860) minus (119875
119860119891 0)10038171003817100381710038171003817
⩽10038171003817100381710038171198790 (119869119860119875119860119891) minus 119879119860 (119875119860119891)
1003817100381710038171003817
+10038171003817100381710038171003817119879119860(119875
119860119891) minus (119875
119860119891 0)10038171003817100381710038171003817
⩽10038171003817100381710038171198790119869119860 minus 119879119860
1003817100381710038171003817 +10038171003817100381710038171198780 minus 1198783
1003817100381710038171003817
lt 120578 (1205760) + 6120576
(136)
Hence 1198790minus 119879
1 ⩽ 120578(120576
0) + 6120576 lt 120576
0
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Abstract and Applied Analysis 15
Acknowledgments
The authors thank Gilles Godefroy and Rafael Paya forfruitful conversations about the content of this paper andVladimir Kadets for providing Lemma 18 They also appre-ciate anonymous referees for careful reading and fruitfulsuggestions about revision The second author was sup-ported by Basic Science Research Program through theNational Research Foundation of Korea (NRF) fundedby the Ministry of Education Science and Technology(NRF-2012R1A1A1006869) The third author was partiallysupported by Spanish MICINN and FEDER project noMTM2012-31755 and by Junta de Andalucıa and FEDERGrants FQM-185 and P09-FQM-4911
References
[1] F F Bonsall and J Duncan Numerical Ranges of Operators onNormed Spaces and of Elements of Normed Algebras vol 2 ofLondon Mathematical Society Lecture Note Series 2 CambridgeUniversity Press London UK 1971
[2] F F Bonsall and J Duncan Numerical Ranges II CambridgeUniversity Press London UK 1973 London MathematicalSociety Lecture Notes Series 10
[3] B Sims On numerical range and its applications to Banachalgebras [PhD thesis] University of Newcastle New SouthWales Australia 1972
[4] I D Berg and B Sims ldquoDenseness of operators which attaintheir numerical radiusrdquoAustralian Mathematical Society A vol36 no 1 pp 130ndash133 1984
[5] C S Cardassi ldquoDensity of numerical radius attaining operatorson some reflexive spacesrdquo Bulletin of the Australian Mathemat-ical Society vol 31 no 1 pp 1ndash3 1985
[6] C S Cardassi ldquoNumerical radius attaining operatorsrdquo inBanach Spaces (Columbia Mo 1984) vol 1166 of Lecture Notesin Mathematics pp 11ndash14 Springer Berlin Germany 1985
[7] C S Cardassi ldquoNumerical radius-attaining operators on119862(119870)rdquoProceedings of the AmericanMathematical Society vol 95 no 4pp 537ndash543 1985
[8] MDAcostaOperadores que alcanzan su radio numerico [PhDthesis] Universidad de Granada Granada Spain 1990
[9] M D Acosta and R Paya ldquoNumerical radius attaining oper-ators and the Radon-Nikodym propertyrdquo The Bulletin of theLondon Mathematical Society vol 25 no 1 pp 67ndash73 1993
[10] R Paya ldquoA counterexample on numerical radius attainingoperatorsrdquo Israel Journal of Mathematics vol 79 no 1 pp 83ndash101 1992
[11] Y S Choi and S G Kim ldquoNorm or numerical radius attainingmultilinear mappings and polynomialsrdquo Journal of the LondonMathematical Society vol 54 no 1 pp 135ndash147 1996
[12] M D Acosta J Becerra Guerrero and M Ruiz GalanldquoNumerical-radius-attaining polynomialsrdquo The Quarterly Jour-nal of Mathematics vol 54 no 1 pp 1ndash10 2003
[13] M D Acosta and S G Kim ldquoDenseness of holomorphicfunctions attaining their numerical radiirdquo Israel Journal ofMathematics vol 161 pp 373ndash386 2007
[14] S G Kim and H J Lee ldquoNumerical peak holomorphicfunctions on Banach spacesrdquo Journal of Mathematical Analysisand Applications vol 364 no 2 pp 437ndash452 2010
[15] M D Acosta R M Aron D Garcıa and M MaestreldquoThe Bishop-Phelps-Bollobas theorem for operatorsrdquo Journal ofFunctional Analysis vol 254 no 11 pp 2780ndash2799 2008
[16] A J Guirao and O Kozhushkina ldquoThe Bishop-Phelps-Bollobasproperty for numerical radius in ℓ
1(119862)rdquo Studia Mathematica
vol 218 no 1 pp 41ndash54 2013[17] J Falco ldquoThe Bishop-Phelps-Bollobas property for numerical
radius on 1198711rdquo Journal of Mathematical Analysis and Applica-
tions vol 414 no 1 pp 125ndash133 2014[18] A Aviles A J Guirao and J Rodriguez ldquoOn the Bishop-
Phelps-Bollobas property for numerical radius in119862(119870)-spacesrdquoPreprint
[19] RM Aron Y S Choi S K Kim H J Lee andMMartin ldquoTheBishop-Phelps-Bollobas version of Lindenstrauss properties Aand Brdquo Preprint
[20] V Kadets M Martın and R Paya ldquoRecent progress and openquestions on the numerical index of Banach spacesrdquo Revista dela Real Academia de Ciencias Exactas Fısicas y Naturales A vol100 no 1-2 pp 155ndash182 2006
[21] M Martın J Merı and M Popov ldquoOn the numerical index ofreal 119871
119901(120583)-spacesrdquo Israel Journal of Mathematics vol 184 pp
183ndash192 2011[22] A Iwanik ldquoNorm attaining operators on Lebesgue spacesrdquo
Pacific Journal of Mathematics vol 83 no 2 pp 381ndash386 1979[23] HH SchaeferBanach Lattices and Positive Operators Springer
New York NY USA 1974[24] J Duncan C M McGregor J D Pryce and A J White ldquoThe
numerical index of a normed spacerdquo Journal of the LondonMathematical Society Second Series vol 2 pp 481ndash488 1970
[25] Y S Choi S K Kim H J Lee and M Martin ldquoThe Bishop-Phelps-Bollobas theorem for operators on 119871
1(120583)rdquo Preprint
[26] H E Lacey The Isometric Theory of Classical Banach SpacesSpringer New York NY USA 1974
[27] G Godefroy ldquoUniversal spaces for strictly convex Banachspacesrdquo Revista de la Real Academia de Ciencias Exactas Fısicasy Naturales A vol 100 no 1-2 pp 137ndash146 2006
[28] C Finet M Martın and R Paya ldquoNumerical index andrenormingrdquo Proceedings of the American Mathematical Societyvol 131 no 3 pp 871ndash877 2003
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical Problems in Engineering
Hindawi Publishing Corporationhttpwwwhindawicom
Differential EquationsInternational Journal of
Volume 2014
Applied MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Probability and StatisticsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical PhysicsAdvances in
Complex AnalysisJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
OptimizationJournal of
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CombinatoricsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Operations ResearchAdvances in
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Function Spaces
Abstract and Applied AnalysisHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of Mathematics and Mathematical Sciences
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Algebra
Discrete Dynamics in Nature and Society
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Decision SciencesAdvances in
Discrete MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom
Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of
Abstract and Applied Analysis 13
It is immediate that
(119892lowast
0 119909
lowast
0) 119878
0(119892
0 0) = 119909
lowast
0(119879
1198601198920) =10038171003817100381710038171198791198601198920
1003817100381710038171003817 gt 1 minus 120578 (1205760)
(111)
By Lemma 19 1198711(120583|
119860)oplus
1119883 has the BPBp-nu with the func-
tion 120578 Therefore there exist 1198781isin L(119871
1(120583|
119860)oplus
1119883) (119892
1 119909
1) isin
1198781198711(120583|119860)oplus1119883 and (119892lowast
1 119909
lowast
1) isin 119878
119871infin(120583|119860)oplusinfin119883lowast such that
1003817100381710038171003817(1198921 1199091) minus (1198920 0)1003817100381710038171003817 lt 120576
1003817100381710038171003817(119892lowast
1 119909
lowast
1) minus (119891
lowast
0 119909
lowast
0)1003817100381710038171003817 lt 120576
10038171003817100381710038171198781 minus 11987801003817100381710038171003817 lt 120576 ⟨(119892
lowast
1 119909
lowast
1) (119892
1 119909
1)⟩ = 1
1003816100381610038161003816(119892lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)1003816100381610038161003816 = V (119878
1) = 1
(112)
Claim 1 We claim that 1199091= 0
Otherwise
1 = Re ⟨(119892lowast1 119909
lowast
1) (119892
1 119909
1)⟩
=100381710038171003817100381711989211003817100381710038171003817Re⟨(119892
lowast
1 119909
lowast
1) (119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
⟩
+100381710038171003817100381711990911003817100381710038171003817Re⟨(119892
lowast
1 119909
lowast
1) (0 119909
1)
100381710038171003817100381711990911003817100381710038171003817
⟩ ⩽ 1
(113)
We deduce that
((119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
(119892lowast
1 119909
lowast
1)) (
(0 1199091)
100381710038171003817100381711990911003817100381710038171003817
(119892lowast
1 119909
lowast
1))
isin Π (1198711(120583|
119860) oplus
1119883)
(114)
Since 1198781((0 119909
1)119909
1) = (119878
1minus 119878
0)((0 119909
1)119909
1) lt 120576 we
get that
1 =1003816100381610038161003816⟨(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)⟩1003816100381610038161003816
=
100381610038161003816100381610038161003816100381610038161003816
100381710038171003817100381711989211003817100381710038171003817⟨(119892
lowast
1 119909
lowast
1) 119878
1((119892
1 0)
100381710038171003817100381711989211003817100381710038171003817
)⟩
+100381710038171003817100381711990911003817100381710038171003817⟨(119892
lowast
1 119909
lowast
1) 119878
1((0 119909
1)
100381710038171003817100381711990911003817100381710038171003817
)⟩
100381610038161003816100381610038161003816100381610038161003816
⩽100381710038171003817100381711989211003817100381710038171003817 V (1198781) + 120576
100381710038171003817100381711990911003817100381710038171003817 lt100381710038171003817100381711989211003817100381710038171003817 +100381710038171003817100381711990911003817100381710038171003817 = 1
(115)
a contradiction This proves the claimWe define the operator 119878
2 119871
1(120583|
119860)oplus
1119883 rarr 119871
1(120583|
119860)oplus
1119883
by 1198782(119891 119909) = 119878
1(119891 0) for every 119891 isin 119871
1(120583|
119860) and for every
119909 isin 119883 Then we have
V (1198782) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816 = 1
10038171003817100381710038171198781 minus 11987821003817100381710038171003817 ⩽ 120576 (116)
Indeed from Claim 1 we have
V (1198781) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 119909
1)1003816100381610038161003816 =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
1(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816 ⩽ V (119878
2)
(117)
On the other hand we see that1003816100381610038161003816(119891
lowast 119909
lowast) 119878
2(119891 119909)
1003816100381610038161003816
=1003816100381610038161003816(119891
lowast 119909
lowast) 119878
1(119891 0)
1003816100381610038161003816 ⩽10038171003817100381710038171198911003817100381710038171003817 V (1198781) ⩽ V (119878
1)
(118)
for every ((119891lowast 119909lowast) (119891 119909)) isin Π(1198711(120583|
119860)oplus
1119883) Therefore
V(1198782) ⩽ V(119878
1) Also
10038171003817100381710038171198781 minus 11987821003817100381710038171003817 ⩽ sup
119909isin119878119883
10038171003817100381710038171198781 (0 119909)1003817100381710038171003817
= sup119909isin119878119883
10038171003817100381710038171198781 (0 119909) minus 1198780 (0 119909)1003817100381710038171003817 ⩽ 120576
(119)
Claim 2 There exists 1198783 119871
1(120583|
119860)oplus
1119883 rarr 119871
1(120583|
119860)oplus
1119883 such
that 1198783(119892
1 0) = 119878
3 = 1 119878
3(0 119909) = 0 119878
3(119891 119909) isin 0oplus
1119883
for every (119891 119909) isin 1198711(120583|
119860)oplus
1119883 and 119878
3minus 119878
2 lt 4120576
Indeed write 1198781= (119863
1 119863
2) where 119863
1 119871
1(120583|
119860)oplus
1119883 rarr
1198711(120583|
119860) and119863
2 119871
1(120583|
119860)oplus
1119883 rarr 119883 We have that
sup 1003816100381610038161003816119892lowast119863
1(119892
1 0) + 119909
lowast119863
2(119892
1 0)1003816100381610038161003816
119909lowastisin 119878
119883lowast ⟨119892
lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860)
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816 +10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
⟨119892lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860)
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816
⟨119892lowast 119892
1⟩ = 1 119892
lowastisin 119878
119871infin(120583|119860) +10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
⩽ V (1198782) =1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
2(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816119892
lowast
1119863
1(119892
1 0) + 119909
lowast
1119863
2(119892
1 0)1003816100381610038161003816
(120)
This implies that
1003816100381610038161003816119909lowast
1119863
2(119892
1 0)1003816100381610038161003816 =10038171003817100381710038171198632(119892
1 0)1003817100381710038171003817
1003816100381610038161003816119892lowast
1119863
1(119892
1 0)1003816100381610038161003816
= sup 1003816100381610038161003816119892lowast119863
1(119892
1 0)1003816100381610038161003816 ⟨119892
lowast 119892
1⟩ = 1 119892
lowastisin 119871
infin(120583|
119860)
(121)
Therefore |119892lowast1| equals 1 on the support of 119863
1(119892
1 0) As
|⟨119892lowast
1 119892
1⟩| = 1 we also have that |119892lowast
1| equals 1 on the support
of 1198921 Changing the values of 119892lowast
1by the ones of 119891lowast
0on 119860
(supp(1198631(119892
1 0)) cup supp(g
1)) we may and do suppose that
|119892lowast
1| = 1 on the whole 119860We also have 119863
2(119892
1 0) gt 0 Indeed
10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198920 0)1003817100381710038171003817
⩽10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198921 0)
1003817100381710038171003817 +10038171003817100381710038171198780 (1198921 0) minus 1198780 (1198920 0)
1003817100381710038171003817
lt 2120576 + 120576 = 3120576
(122)
So we have
10038171003817100381710038171198632(119892
1 0) minus 119879
1198601198920
1003817100381710038171003817
⩽10038171003817100381710038171198631(119892
1 0)1003817100381710038171003817 +10038171003817100381710038171198632(119892
1 0) minus 119879
1198601198920
1003817100381710038171003817
14 Abstract and Applied Analysis
=1003817100381710038171003817(1198631
(1198921 0) 119863
2(119892
1 0)) minus (0 119879
1198601198920)1003817100381710038171003817
=10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198920 0)
1003817100381710038171003817 lt 3120576
(123)
and 1198632(119892
1 0) gt 119879
1198601198920 minus 3120576 ⩾ 1 minus 120578(120576
0) minus 3120576 gt 0
Finally define the operator 1198783by
1198783(119891 119909) = (0119863
2(119891 0) + 119892
lowast
1(119863
1(119891 0))
1198632(119892
1 0)
119909lowast
1119863
2(119892
1 0))
(124)
for (119891 119909) isin 1198711(120583|
119860)oplus
1119883 It is clear that
100381710038171003817100381711987831003817100381710038171003817 ⩽ sup
119891isin1198781198711(120583|119860)
(10038171003817100381710038171198632(119891 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119891 0)
1003816100381610038161003816) (125)
Notice also that
10038171003817100381710038171198631(119891 0)
1003817100381710038171003817
⩽10038171003817100381710038171198631(119891 0)
1003817100381710038171003817 +10038171003817100381710038171198632(119891 0) minus 119879
1198601198911003817100381710038171003817
=1003817100381710038171003817(1198631
(119891 0) 1198632(119891 0)) minus (0 119879
119860119891)1003817100381710038171003817
=10038171003817100381710038171198782 (119891 119909) minus 1198780 (119891 119909)
1003817100381710038171003817
(126)
for all (119891 119909) isin 1198711(120583|
119860)oplus
1119883 Hence we have
10038171003817100381710038171198783 minus 11987821003817100381710038171003817
= 2 sup119891isin1198781198711(120583|119860)
10038171003817100381710038171198631(119891 0)
1003817100381710038171003817 ⩽ 210038171003817100381710038171198782 minus 1198780
1003817100381710038171003817 lt 4120576(127)
On the other hand let 119866 1198711(120583|
119860) rarr 119871
1(120583|
119860) be defined by
119866(119891) = 119892lowast
1119891 for every 119891 isin 119871
1(120583|
119860) Then we have
V (1198782)
= sup 1003816100381610038161003816119911lowast11987821199111003816100381610038161003816 (119911 119911
lowast) isin Π (119871
1(120583|
119860) oplus
1119883)
⩾ sup10038161003816100381610038161003816100381610038161003816
119909lowast119863
2(119866(
1
120583 (119862)120594119862) 0)
+119892lowast
1119863
1(119866(
1
120583 (119862)120594119862) 0)
10038161003816100381610038161003816100381610038161003816
119909lowastisin 119878
119883lowast 119862 isin Σ
119860 120583 (119862) gt 0
= sup10038171003817100381710038171003817100381710038171003817
1198632(119866(
1
120583 (119862)120594119862) 0)
10038171003817100381710038171003817100381710038171003817
+
10038161003816100381610038161003816100381610038161003816
119892lowast
1119863
1(119866(
1
120583 (119862)120594119862) 0)
10038161003816100381610038161003816100381610038161003816
119862 isin Σ119860 120583 (119862) gt 0
(128)
where Σ119860is the family of measurable subsets of 119860
Hence for any simple function 119904 = sum119899
119894=1((120572
119894120583(119860
119894))120594
119860119894
) isin
1198781198711(120583|119860) where 119860
119894119894is a family of disjoint measurable subsets
with strictly positive measure we have
V (1198782)
⩾
119899
sum
119894=1
10038161003816100381610038161205721198941003816100381610038161003816 (
100381710038171003817100381710038171003817100381710038171003817
1198632(119866(
1
120583 (119860119894)120594119860119894
) 0)
100381710038171003817100381710038171003817100381710038171003817
+
100381610038161003816100381610038161003816100381610038161003816
119892lowast
1119863
1(119866(
1
120583 (119860119894)120594119860119894
) 0)
100381610038161003816100381610038161003816100381610038161003816
)
⩾10038171003817100381710038171198632(119866 (119904) 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119866 (119904) 0)
1003816100381610038161003816
(129)
Since |119892lowast1| = 1119866 is an isometric isomorphism so for each119891 isin
1198781198711(120583|119860)there exists a sequence of norm-one simple functions
(119904119896) such that 119866(119904
119896) converges to 119891 Therefore
V (1198782) ⩾ sup
119891isin1198781198711(120583|119860)
(10038171003817100381710038171198632(119891 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119891 0)
1003816100381610038161003816) ⩾100381710038171003817100381711987831003817100381710038171003817
(130)
On the other hand we see that100381710038171003817100381711987831003817100381710038171003817 ⩾1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
3(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816119909
lowast
1119863
2(119892
1 0) + 119892
lowast
1119863
1(119892
1 0)1003816100381610038161003816 = V (119878
2) = 1
(131)
Therefore 1 = 1198783 = 119878
3(119892
1 0) which proves Claim 2
Finally set 1198783= (0 ) for a suitable 119871
1(120583|
119860)oplus
1119883 rarr
119883 and define the operator 1198791 119871
1(120583) rarr 119883 by
1198791(119891) = 119879
0(119891120594
119860119888) + (119875
119860119891 0) (132)
for every 119891 isin 1198711(120583) Then we have
10038171003817100381710038171198791 (119891)1003817100381710038171003817 ⩽100381710038171003817100381711987901003817100381710038171003817
10038171003817100381710038171198911205941198601198881003817100381710038171003817 +1003817100381710038171003817100381710038171003817100381710038171003817
10038171003817100381710038171198911205941198601003817100381710038171003817 =10038171003817100381710038171198911003817100381710038171003817 (133)
for every 119891 isin 1198711(120583) so 119879
1 ⩽ 1 Also
10038171003817100381710038171198791 (1198691198601198921)1003817100381710038171003817 =10038171003817100381710038171198783 (1198921 0)
1003817100381710038171003817 =100381710038171003817100381711987831003817100381710038171003817 = 1 (134)
so 1198791attains its norm at 119869
1198601198921isin 119871
1(120583) and
10038171003817100381710038171198691198601198921 minus 11989101003817100381710038171003817 =10038171003817100381710038171198921 minus 1198920
1003817100381710038171003817 lt 120576 (135)
We also have that for any 119891 isin 1198781198711(120583)
10038171003817100381710038171198790 (119891) minus 1198791 (119891)1003817100381710038171003817 =100381710038171003817100381710038171198790(119891120594
119860) minus (119875
119860119891 0)10038171003817100381710038171003817
⩽10038171003817100381710038171198790 (119869119860119875119860119891) minus 119879119860 (119875119860119891)
1003817100381710038171003817
+10038171003817100381710038171003817119879119860(119875
119860119891) minus (119875
119860119891 0)10038171003817100381710038171003817
⩽10038171003817100381710038171198790119869119860 minus 119879119860
1003817100381710038171003817 +10038171003817100381710038171198780 minus 1198783
1003817100381710038171003817
lt 120578 (1205760) + 6120576
(136)
Hence 1198790minus 119879
1 ⩽ 120578(120576
0) + 6120576 lt 120576
0
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Abstract and Applied Analysis 15
Acknowledgments
The authors thank Gilles Godefroy and Rafael Paya forfruitful conversations about the content of this paper andVladimir Kadets for providing Lemma 18 They also appre-ciate anonymous referees for careful reading and fruitfulsuggestions about revision The second author was sup-ported by Basic Science Research Program through theNational Research Foundation of Korea (NRF) fundedby the Ministry of Education Science and Technology(NRF-2012R1A1A1006869) The third author was partiallysupported by Spanish MICINN and FEDER project noMTM2012-31755 and by Junta de Andalucıa and FEDERGrants FQM-185 and P09-FQM-4911
References
[1] F F Bonsall and J Duncan Numerical Ranges of Operators onNormed Spaces and of Elements of Normed Algebras vol 2 ofLondon Mathematical Society Lecture Note Series 2 CambridgeUniversity Press London UK 1971
[2] F F Bonsall and J Duncan Numerical Ranges II CambridgeUniversity Press London UK 1973 London MathematicalSociety Lecture Notes Series 10
[3] B Sims On numerical range and its applications to Banachalgebras [PhD thesis] University of Newcastle New SouthWales Australia 1972
[4] I D Berg and B Sims ldquoDenseness of operators which attaintheir numerical radiusrdquoAustralian Mathematical Society A vol36 no 1 pp 130ndash133 1984
[5] C S Cardassi ldquoDensity of numerical radius attaining operatorson some reflexive spacesrdquo Bulletin of the Australian Mathemat-ical Society vol 31 no 1 pp 1ndash3 1985
[6] C S Cardassi ldquoNumerical radius attaining operatorsrdquo inBanach Spaces (Columbia Mo 1984) vol 1166 of Lecture Notesin Mathematics pp 11ndash14 Springer Berlin Germany 1985
[7] C S Cardassi ldquoNumerical radius-attaining operators on119862(119870)rdquoProceedings of the AmericanMathematical Society vol 95 no 4pp 537ndash543 1985
[8] MDAcostaOperadores que alcanzan su radio numerico [PhDthesis] Universidad de Granada Granada Spain 1990
[9] M D Acosta and R Paya ldquoNumerical radius attaining oper-ators and the Radon-Nikodym propertyrdquo The Bulletin of theLondon Mathematical Society vol 25 no 1 pp 67ndash73 1993
[10] R Paya ldquoA counterexample on numerical radius attainingoperatorsrdquo Israel Journal of Mathematics vol 79 no 1 pp 83ndash101 1992
[11] Y S Choi and S G Kim ldquoNorm or numerical radius attainingmultilinear mappings and polynomialsrdquo Journal of the LondonMathematical Society vol 54 no 1 pp 135ndash147 1996
[12] M D Acosta J Becerra Guerrero and M Ruiz GalanldquoNumerical-radius-attaining polynomialsrdquo The Quarterly Jour-nal of Mathematics vol 54 no 1 pp 1ndash10 2003
[13] M D Acosta and S G Kim ldquoDenseness of holomorphicfunctions attaining their numerical radiirdquo Israel Journal ofMathematics vol 161 pp 373ndash386 2007
[14] S G Kim and H J Lee ldquoNumerical peak holomorphicfunctions on Banach spacesrdquo Journal of Mathematical Analysisand Applications vol 364 no 2 pp 437ndash452 2010
[15] M D Acosta R M Aron D Garcıa and M MaestreldquoThe Bishop-Phelps-Bollobas theorem for operatorsrdquo Journal ofFunctional Analysis vol 254 no 11 pp 2780ndash2799 2008
[16] A J Guirao and O Kozhushkina ldquoThe Bishop-Phelps-Bollobasproperty for numerical radius in ℓ
1(119862)rdquo Studia Mathematica
vol 218 no 1 pp 41ndash54 2013[17] J Falco ldquoThe Bishop-Phelps-Bollobas property for numerical
radius on 1198711rdquo Journal of Mathematical Analysis and Applica-
tions vol 414 no 1 pp 125ndash133 2014[18] A Aviles A J Guirao and J Rodriguez ldquoOn the Bishop-
Phelps-Bollobas property for numerical radius in119862(119870)-spacesrdquoPreprint
[19] RM Aron Y S Choi S K Kim H J Lee andMMartin ldquoTheBishop-Phelps-Bollobas version of Lindenstrauss properties Aand Brdquo Preprint
[20] V Kadets M Martın and R Paya ldquoRecent progress and openquestions on the numerical index of Banach spacesrdquo Revista dela Real Academia de Ciencias Exactas Fısicas y Naturales A vol100 no 1-2 pp 155ndash182 2006
[21] M Martın J Merı and M Popov ldquoOn the numerical index ofreal 119871
119901(120583)-spacesrdquo Israel Journal of Mathematics vol 184 pp
183ndash192 2011[22] A Iwanik ldquoNorm attaining operators on Lebesgue spacesrdquo
Pacific Journal of Mathematics vol 83 no 2 pp 381ndash386 1979[23] HH SchaeferBanach Lattices and Positive Operators Springer
New York NY USA 1974[24] J Duncan C M McGregor J D Pryce and A J White ldquoThe
numerical index of a normed spacerdquo Journal of the LondonMathematical Society Second Series vol 2 pp 481ndash488 1970
[25] Y S Choi S K Kim H J Lee and M Martin ldquoThe Bishop-Phelps-Bollobas theorem for operators on 119871
1(120583)rdquo Preprint
[26] H E Lacey The Isometric Theory of Classical Banach SpacesSpringer New York NY USA 1974
[27] G Godefroy ldquoUniversal spaces for strictly convex Banachspacesrdquo Revista de la Real Academia de Ciencias Exactas Fısicasy Naturales A vol 100 no 1-2 pp 137ndash146 2006
[28] C Finet M Martın and R Paya ldquoNumerical index andrenormingrdquo Proceedings of the American Mathematical Societyvol 131 no 3 pp 871ndash877 2003
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical Problems in Engineering
Hindawi Publishing Corporationhttpwwwhindawicom
Differential EquationsInternational Journal of
Volume 2014
Applied MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Probability and StatisticsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical PhysicsAdvances in
Complex AnalysisJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
OptimizationJournal of
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CombinatoricsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
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Operations ResearchAdvances in
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Function Spaces
Abstract and Applied AnalysisHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of Mathematics and Mathematical Sciences
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
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Algebra
Discrete Dynamics in Nature and Society
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Decision SciencesAdvances in
Discrete MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom
Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of
14 Abstract and Applied Analysis
=1003817100381710038171003817(1198631
(1198921 0) 119863
2(119892
1 0)) minus (0 119879
1198601198920)1003817100381710038171003817
=10038171003817100381710038171198782 (1198921 0) minus 1198780 (1198920 0)
1003817100381710038171003817 lt 3120576
(123)
and 1198632(119892
1 0) gt 119879
1198601198920 minus 3120576 ⩾ 1 minus 120578(120576
0) minus 3120576 gt 0
Finally define the operator 1198783by
1198783(119891 119909) = (0119863
2(119891 0) + 119892
lowast
1(119863
1(119891 0))
1198632(119892
1 0)
119909lowast
1119863
2(119892
1 0))
(124)
for (119891 119909) isin 1198711(120583|
119860)oplus
1119883 It is clear that
100381710038171003817100381711987831003817100381710038171003817 ⩽ sup
119891isin1198781198711(120583|119860)
(10038171003817100381710038171198632(119891 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119891 0)
1003816100381610038161003816) (125)
Notice also that
10038171003817100381710038171198631(119891 0)
1003817100381710038171003817
⩽10038171003817100381710038171198631(119891 0)
1003817100381710038171003817 +10038171003817100381710038171198632(119891 0) minus 119879
1198601198911003817100381710038171003817
=1003817100381710038171003817(1198631
(119891 0) 1198632(119891 0)) minus (0 119879
119860119891)1003817100381710038171003817
=10038171003817100381710038171198782 (119891 119909) minus 1198780 (119891 119909)
1003817100381710038171003817
(126)
for all (119891 119909) isin 1198711(120583|
119860)oplus
1119883 Hence we have
10038171003817100381710038171198783 minus 11987821003817100381710038171003817
= 2 sup119891isin1198781198711(120583|119860)
10038171003817100381710038171198631(119891 0)
1003817100381710038171003817 ⩽ 210038171003817100381710038171198782 minus 1198780
1003817100381710038171003817 lt 4120576(127)
On the other hand let 119866 1198711(120583|
119860) rarr 119871
1(120583|
119860) be defined by
119866(119891) = 119892lowast
1119891 for every 119891 isin 119871
1(120583|
119860) Then we have
V (1198782)
= sup 1003816100381610038161003816119911lowast11987821199111003816100381610038161003816 (119911 119911
lowast) isin Π (119871
1(120583|
119860) oplus
1119883)
⩾ sup10038161003816100381610038161003816100381610038161003816
119909lowast119863
2(119866(
1
120583 (119862)120594119862) 0)
+119892lowast
1119863
1(119866(
1
120583 (119862)120594119862) 0)
10038161003816100381610038161003816100381610038161003816
119909lowastisin 119878
119883lowast 119862 isin Σ
119860 120583 (119862) gt 0
= sup10038171003817100381710038171003817100381710038171003817
1198632(119866(
1
120583 (119862)120594119862) 0)
10038171003817100381710038171003817100381710038171003817
+
10038161003816100381610038161003816100381610038161003816
119892lowast
1119863
1(119866(
1
120583 (119862)120594119862) 0)
10038161003816100381610038161003816100381610038161003816
119862 isin Σ119860 120583 (119862) gt 0
(128)
where Σ119860is the family of measurable subsets of 119860
Hence for any simple function 119904 = sum119899
119894=1((120572
119894120583(119860
119894))120594
119860119894
) isin
1198781198711(120583|119860) where 119860
119894119894is a family of disjoint measurable subsets
with strictly positive measure we have
V (1198782)
⩾
119899
sum
119894=1
10038161003816100381610038161205721198941003816100381610038161003816 (
100381710038171003817100381710038171003817100381710038171003817
1198632(119866(
1
120583 (119860119894)120594119860119894
) 0)
100381710038171003817100381710038171003817100381710038171003817
+
100381610038161003816100381610038161003816100381610038161003816
119892lowast
1119863
1(119866(
1
120583 (119860119894)120594119860119894
) 0)
100381610038161003816100381610038161003816100381610038161003816
)
⩾10038171003817100381710038171198632(119866 (119904) 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119866 (119904) 0)
1003816100381610038161003816
(129)
Since |119892lowast1| = 1119866 is an isometric isomorphism so for each119891 isin
1198781198711(120583|119860)there exists a sequence of norm-one simple functions
(119904119896) such that 119866(119904
119896) converges to 119891 Therefore
V (1198782) ⩾ sup
119891isin1198781198711(120583|119860)
(10038171003817100381710038171198632(119891 0)
1003817100381710038171003817 +1003816100381610038161003816119892
lowast
1119863
1(119891 0)
1003816100381610038161003816) ⩾100381710038171003817100381711987831003817100381710038171003817
(130)
On the other hand we see that100381710038171003817100381711987831003817100381710038171003817 ⩾1003816100381610038161003816(119892
lowast
1 119909
lowast
1) 119878
3(119892
1 0)1003816100381610038161003816
=1003816100381610038161003816119909
lowast
1119863
2(119892
1 0) + 119892
lowast
1119863
1(119892
1 0)1003816100381610038161003816 = V (119878
2) = 1
(131)
Therefore 1 = 1198783 = 119878
3(119892
1 0) which proves Claim 2
Finally set 1198783= (0 ) for a suitable 119871
1(120583|
119860)oplus
1119883 rarr
119883 and define the operator 1198791 119871
1(120583) rarr 119883 by
1198791(119891) = 119879
0(119891120594
119860119888) + (119875
119860119891 0) (132)
for every 119891 isin 1198711(120583) Then we have
10038171003817100381710038171198791 (119891)1003817100381710038171003817 ⩽100381710038171003817100381711987901003817100381710038171003817
10038171003817100381710038171198911205941198601198881003817100381710038171003817 +1003817100381710038171003817100381710038171003817100381710038171003817
10038171003817100381710038171198911205941198601003817100381710038171003817 =10038171003817100381710038171198911003817100381710038171003817 (133)
for every 119891 isin 1198711(120583) so 119879
1 ⩽ 1 Also
10038171003817100381710038171198791 (1198691198601198921)1003817100381710038171003817 =10038171003817100381710038171198783 (1198921 0)
1003817100381710038171003817 =100381710038171003817100381711987831003817100381710038171003817 = 1 (134)
so 1198791attains its norm at 119869
1198601198921isin 119871
1(120583) and
10038171003817100381710038171198691198601198921 minus 11989101003817100381710038171003817 =10038171003817100381710038171198921 minus 1198920
1003817100381710038171003817 lt 120576 (135)
We also have that for any 119891 isin 1198781198711(120583)
10038171003817100381710038171198790 (119891) minus 1198791 (119891)1003817100381710038171003817 =100381710038171003817100381710038171198790(119891120594
119860) minus (119875
119860119891 0)10038171003817100381710038171003817
⩽10038171003817100381710038171198790 (119869119860119875119860119891) minus 119879119860 (119875119860119891)
1003817100381710038171003817
+10038171003817100381710038171003817119879119860(119875
119860119891) minus (119875
119860119891 0)10038171003817100381710038171003817
⩽10038171003817100381710038171198790119869119860 minus 119879119860
1003817100381710038171003817 +10038171003817100381710038171198780 minus 1198783
1003817100381710038171003817
lt 120578 (1205760) + 6120576
(136)
Hence 1198790minus 119879
1 ⩽ 120578(120576
0) + 6120576 lt 120576
0
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Abstract and Applied Analysis 15
Acknowledgments
The authors thank Gilles Godefroy and Rafael Paya forfruitful conversations about the content of this paper andVladimir Kadets for providing Lemma 18 They also appre-ciate anonymous referees for careful reading and fruitfulsuggestions about revision The second author was sup-ported by Basic Science Research Program through theNational Research Foundation of Korea (NRF) fundedby the Ministry of Education Science and Technology(NRF-2012R1A1A1006869) The third author was partiallysupported by Spanish MICINN and FEDER project noMTM2012-31755 and by Junta de Andalucıa and FEDERGrants FQM-185 and P09-FQM-4911
References
[1] F F Bonsall and J Duncan Numerical Ranges of Operators onNormed Spaces and of Elements of Normed Algebras vol 2 ofLondon Mathematical Society Lecture Note Series 2 CambridgeUniversity Press London UK 1971
[2] F F Bonsall and J Duncan Numerical Ranges II CambridgeUniversity Press London UK 1973 London MathematicalSociety Lecture Notes Series 10
[3] B Sims On numerical range and its applications to Banachalgebras [PhD thesis] University of Newcastle New SouthWales Australia 1972
[4] I D Berg and B Sims ldquoDenseness of operators which attaintheir numerical radiusrdquoAustralian Mathematical Society A vol36 no 1 pp 130ndash133 1984
[5] C S Cardassi ldquoDensity of numerical radius attaining operatorson some reflexive spacesrdquo Bulletin of the Australian Mathemat-ical Society vol 31 no 1 pp 1ndash3 1985
[6] C S Cardassi ldquoNumerical radius attaining operatorsrdquo inBanach Spaces (Columbia Mo 1984) vol 1166 of Lecture Notesin Mathematics pp 11ndash14 Springer Berlin Germany 1985
[7] C S Cardassi ldquoNumerical radius-attaining operators on119862(119870)rdquoProceedings of the AmericanMathematical Society vol 95 no 4pp 537ndash543 1985
[8] MDAcostaOperadores que alcanzan su radio numerico [PhDthesis] Universidad de Granada Granada Spain 1990
[9] M D Acosta and R Paya ldquoNumerical radius attaining oper-ators and the Radon-Nikodym propertyrdquo The Bulletin of theLondon Mathematical Society vol 25 no 1 pp 67ndash73 1993
[10] R Paya ldquoA counterexample on numerical radius attainingoperatorsrdquo Israel Journal of Mathematics vol 79 no 1 pp 83ndash101 1992
[11] Y S Choi and S G Kim ldquoNorm or numerical radius attainingmultilinear mappings and polynomialsrdquo Journal of the LondonMathematical Society vol 54 no 1 pp 135ndash147 1996
[12] M D Acosta J Becerra Guerrero and M Ruiz GalanldquoNumerical-radius-attaining polynomialsrdquo The Quarterly Jour-nal of Mathematics vol 54 no 1 pp 1ndash10 2003
[13] M D Acosta and S G Kim ldquoDenseness of holomorphicfunctions attaining their numerical radiirdquo Israel Journal ofMathematics vol 161 pp 373ndash386 2007
[14] S G Kim and H J Lee ldquoNumerical peak holomorphicfunctions on Banach spacesrdquo Journal of Mathematical Analysisand Applications vol 364 no 2 pp 437ndash452 2010
[15] M D Acosta R M Aron D Garcıa and M MaestreldquoThe Bishop-Phelps-Bollobas theorem for operatorsrdquo Journal ofFunctional Analysis vol 254 no 11 pp 2780ndash2799 2008
[16] A J Guirao and O Kozhushkina ldquoThe Bishop-Phelps-Bollobasproperty for numerical radius in ℓ
1(119862)rdquo Studia Mathematica
vol 218 no 1 pp 41ndash54 2013[17] J Falco ldquoThe Bishop-Phelps-Bollobas property for numerical
radius on 1198711rdquo Journal of Mathematical Analysis and Applica-
tions vol 414 no 1 pp 125ndash133 2014[18] A Aviles A J Guirao and J Rodriguez ldquoOn the Bishop-
Phelps-Bollobas property for numerical radius in119862(119870)-spacesrdquoPreprint
[19] RM Aron Y S Choi S K Kim H J Lee andMMartin ldquoTheBishop-Phelps-Bollobas version of Lindenstrauss properties Aand Brdquo Preprint
[20] V Kadets M Martın and R Paya ldquoRecent progress and openquestions on the numerical index of Banach spacesrdquo Revista dela Real Academia de Ciencias Exactas Fısicas y Naturales A vol100 no 1-2 pp 155ndash182 2006
[21] M Martın J Merı and M Popov ldquoOn the numerical index ofreal 119871
119901(120583)-spacesrdquo Israel Journal of Mathematics vol 184 pp
183ndash192 2011[22] A Iwanik ldquoNorm attaining operators on Lebesgue spacesrdquo
Pacific Journal of Mathematics vol 83 no 2 pp 381ndash386 1979[23] HH SchaeferBanach Lattices and Positive Operators Springer
New York NY USA 1974[24] J Duncan C M McGregor J D Pryce and A J White ldquoThe
numerical index of a normed spacerdquo Journal of the LondonMathematical Society Second Series vol 2 pp 481ndash488 1970
[25] Y S Choi S K Kim H J Lee and M Martin ldquoThe Bishop-Phelps-Bollobas theorem for operators on 119871
1(120583)rdquo Preprint
[26] H E Lacey The Isometric Theory of Classical Banach SpacesSpringer New York NY USA 1974
[27] G Godefroy ldquoUniversal spaces for strictly convex Banachspacesrdquo Revista de la Real Academia de Ciencias Exactas Fısicasy Naturales A vol 100 no 1-2 pp 137ndash146 2006
[28] C Finet M Martın and R Paya ldquoNumerical index andrenormingrdquo Proceedings of the American Mathematical Societyvol 131 no 3 pp 871ndash877 2003
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical Problems in Engineering
Hindawi Publishing Corporationhttpwwwhindawicom
Differential EquationsInternational Journal of
Volume 2014
Applied MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Probability and StatisticsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical PhysicsAdvances in
Complex AnalysisJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
OptimizationJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CombinatoricsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Operations ResearchAdvances in
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Function Spaces
Abstract and Applied AnalysisHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of Mathematics and Mathematical Sciences
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Algebra
Discrete Dynamics in Nature and Society
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Decision SciencesAdvances in
Discrete MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom
Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of
Abstract and Applied Analysis 15
Acknowledgments
The authors thank Gilles Godefroy and Rafael Paya forfruitful conversations about the content of this paper andVladimir Kadets for providing Lemma 18 They also appre-ciate anonymous referees for careful reading and fruitfulsuggestions about revision The second author was sup-ported by Basic Science Research Program through theNational Research Foundation of Korea (NRF) fundedby the Ministry of Education Science and Technology(NRF-2012R1A1A1006869) The third author was partiallysupported by Spanish MICINN and FEDER project noMTM2012-31755 and by Junta de Andalucıa and FEDERGrants FQM-185 and P09-FQM-4911
References
[1] F F Bonsall and J Duncan Numerical Ranges of Operators onNormed Spaces and of Elements of Normed Algebras vol 2 ofLondon Mathematical Society Lecture Note Series 2 CambridgeUniversity Press London UK 1971
[2] F F Bonsall and J Duncan Numerical Ranges II CambridgeUniversity Press London UK 1973 London MathematicalSociety Lecture Notes Series 10
[3] B Sims On numerical range and its applications to Banachalgebras [PhD thesis] University of Newcastle New SouthWales Australia 1972
[4] I D Berg and B Sims ldquoDenseness of operators which attaintheir numerical radiusrdquoAustralian Mathematical Society A vol36 no 1 pp 130ndash133 1984
[5] C S Cardassi ldquoDensity of numerical radius attaining operatorson some reflexive spacesrdquo Bulletin of the Australian Mathemat-ical Society vol 31 no 1 pp 1ndash3 1985
[6] C S Cardassi ldquoNumerical radius attaining operatorsrdquo inBanach Spaces (Columbia Mo 1984) vol 1166 of Lecture Notesin Mathematics pp 11ndash14 Springer Berlin Germany 1985
[7] C S Cardassi ldquoNumerical radius-attaining operators on119862(119870)rdquoProceedings of the AmericanMathematical Society vol 95 no 4pp 537ndash543 1985
[8] MDAcostaOperadores que alcanzan su radio numerico [PhDthesis] Universidad de Granada Granada Spain 1990
[9] M D Acosta and R Paya ldquoNumerical radius attaining oper-ators and the Radon-Nikodym propertyrdquo The Bulletin of theLondon Mathematical Society vol 25 no 1 pp 67ndash73 1993
[10] R Paya ldquoA counterexample on numerical radius attainingoperatorsrdquo Israel Journal of Mathematics vol 79 no 1 pp 83ndash101 1992
[11] Y S Choi and S G Kim ldquoNorm or numerical radius attainingmultilinear mappings and polynomialsrdquo Journal of the LondonMathematical Society vol 54 no 1 pp 135ndash147 1996
[12] M D Acosta J Becerra Guerrero and M Ruiz GalanldquoNumerical-radius-attaining polynomialsrdquo The Quarterly Jour-nal of Mathematics vol 54 no 1 pp 1ndash10 2003
[13] M D Acosta and S G Kim ldquoDenseness of holomorphicfunctions attaining their numerical radiirdquo Israel Journal ofMathematics vol 161 pp 373ndash386 2007
[14] S G Kim and H J Lee ldquoNumerical peak holomorphicfunctions on Banach spacesrdquo Journal of Mathematical Analysisand Applications vol 364 no 2 pp 437ndash452 2010
[15] M D Acosta R M Aron D Garcıa and M MaestreldquoThe Bishop-Phelps-Bollobas theorem for operatorsrdquo Journal ofFunctional Analysis vol 254 no 11 pp 2780ndash2799 2008
[16] A J Guirao and O Kozhushkina ldquoThe Bishop-Phelps-Bollobasproperty for numerical radius in ℓ
1(119862)rdquo Studia Mathematica
vol 218 no 1 pp 41ndash54 2013[17] J Falco ldquoThe Bishop-Phelps-Bollobas property for numerical
radius on 1198711rdquo Journal of Mathematical Analysis and Applica-
tions vol 414 no 1 pp 125ndash133 2014[18] A Aviles A J Guirao and J Rodriguez ldquoOn the Bishop-
Phelps-Bollobas property for numerical radius in119862(119870)-spacesrdquoPreprint
[19] RM Aron Y S Choi S K Kim H J Lee andMMartin ldquoTheBishop-Phelps-Bollobas version of Lindenstrauss properties Aand Brdquo Preprint
[20] V Kadets M Martın and R Paya ldquoRecent progress and openquestions on the numerical index of Banach spacesrdquo Revista dela Real Academia de Ciencias Exactas Fısicas y Naturales A vol100 no 1-2 pp 155ndash182 2006
[21] M Martın J Merı and M Popov ldquoOn the numerical index ofreal 119871
119901(120583)-spacesrdquo Israel Journal of Mathematics vol 184 pp
183ndash192 2011[22] A Iwanik ldquoNorm attaining operators on Lebesgue spacesrdquo
Pacific Journal of Mathematics vol 83 no 2 pp 381ndash386 1979[23] HH SchaeferBanach Lattices and Positive Operators Springer
New York NY USA 1974[24] J Duncan C M McGregor J D Pryce and A J White ldquoThe
numerical index of a normed spacerdquo Journal of the LondonMathematical Society Second Series vol 2 pp 481ndash488 1970
[25] Y S Choi S K Kim H J Lee and M Martin ldquoThe Bishop-Phelps-Bollobas theorem for operators on 119871
1(120583)rdquo Preprint
[26] H E Lacey The Isometric Theory of Classical Banach SpacesSpringer New York NY USA 1974
[27] G Godefroy ldquoUniversal spaces for strictly convex Banachspacesrdquo Revista de la Real Academia de Ciencias Exactas Fısicasy Naturales A vol 100 no 1-2 pp 137ndash146 2006
[28] C Finet M Martın and R Paya ldquoNumerical index andrenormingrdquo Proceedings of the American Mathematical Societyvol 131 no 3 pp 871ndash877 2003
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical Problems in Engineering
Hindawi Publishing Corporationhttpwwwhindawicom
Differential EquationsInternational Journal of
Volume 2014
Applied MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Probability and StatisticsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical PhysicsAdvances in
Complex AnalysisJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
OptimizationJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CombinatoricsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Operations ResearchAdvances in
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Function Spaces
Abstract and Applied AnalysisHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of Mathematics and Mathematical Sciences
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Algebra
Discrete Dynamics in Nature and Society
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Decision SciencesAdvances in
Discrete MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom
Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical Problems in Engineering
Hindawi Publishing Corporationhttpwwwhindawicom
Differential EquationsInternational Journal of
Volume 2014
Applied MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Probability and StatisticsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical PhysicsAdvances in
Complex AnalysisJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
OptimizationJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CombinatoricsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Operations ResearchAdvances in
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Function Spaces
Abstract and Applied AnalysisHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of Mathematics and Mathematical Sciences
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Algebra
Discrete Dynamics in Nature and Society
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Decision SciencesAdvances in
Discrete MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom
Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of