nonexistence results on generalized bent functions · 2019. 8. 2. · generalized bent functions...
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Nonexistence results on Generalized BentFunctions
Ka Hin LeungNational University of Singapore
Bernhard SchmidtNanyang Technological University
Qi WangSouthern University of Science and Technology
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K.T. Arasu, J.F. Dillon, K.H. Leung and S.L. Ma,Cyclic relative difference sets with classicalparameters, Journal of Combinatorial Theory SeriesA, 94(2001), 118-126.
K.T. Arasu, K.H. Leung, S.L. Ma, A. Nabavi andD.K. Ray-Chaudhuri, Circulant weighing matrices ofweight 22t, Designs, Codes and Cryptography,41(2006), 111-123.
K.T. Arasu, K.H. Leung, S.L. Ma, A. Nabavi andD.K. Ray-Chaudhuri, Determination of all possibleorders of weight 16 circulant weighing matrices,Finite Fields and their Applications, 12(2006),498-538.
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Definition 1
A function f : Zn2 → Z2 is called a bent function if
Wf (y) :=∑x∈Zn
2
(−1)f (x)+y ·x = ±2n/2
for all y ∈ Zn2. Here y · x denotes the usual inner
product.
Wf (y) is called the Walsh transform of f .
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O. Rothaus originally defined f to be a bentfunction if |Wf (y)| is a constant and he had shownthat |Wf (y)| = 2n/2. It is clear that n must be even.
Bent functions are a highly active research field dueto their numerous applications in informationtheory, cryptography and coding theory.
There are more than 25 different generalizations ofbent functions recorded in Tokareva’s 2015monograph.
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We concentrate on two generalizations of bentfunctions.
Our objecitve is to introduce two number theorticmethods to obtain some non-existence results andwe believe our techniques can be applied to studyother related problems.
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Generalized Bent Functions (I) Kumar, Scholtz and Welch
Definition 2
Let m and q be positive integers and let ζq be aprimitive complex qth root of unity. A functionf : Zm
q → Zq is called a generalized bentfunction (GBF) if∣∣∣∣∣∣
∑x∈Zm
q
ζ f (x)−v ·xq
∣∣∣∣∣∣2
= qm for all v ∈ Zmq . (1)
Here x · v denotes the usual dot product.
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Note that we have simply replaced 2 by q inDefinition 1.
Kumar, Scholtz, and Welch showed that GBFs fromZmq to Zq exist whenever
m is even or q 6≡ 2 (mod 4).
However, not a single GBF from Zmq to Zq with m
odd and q ≡ 2 (mod 4) is known.
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Conjecture There is no GBF from Zmq to Zq if m is
odd and q ≡ 2 (mod 4).
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Standard techniques, such as using self conjugateproperty, work in some cases.
Some researchers tried to find directly the form ofX ∈ Z[ζq] such that |X |2 = qm. But suchtechniques often work in very specific cases.
In case m = 1 and q = 2pr where p is prime, theconjecture was recently solved in [LS 2019].
For m = 3, there are still a few unsolved cases.
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Bent functions in term of group ring equations
Our approach is different from other researchers,and the first step is to use group ring elements.Instead of Zm
q , we use multiplicative notation Cmq .
Let G = Cmq and let f : G → Zq be any function.
We associate f with an element Df in Z[ζq][G ] via
Df =∑x∈G
ζ f (x)q x .
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For any v ∈ G , we define
F (v) =∑x∈Zm
q
ζ f (x)−x ·vq .
and a character χv of G such that
χv(x) = ζ−v ·xq for all x ∈ G .
It well known that every complex character of G isequal to some χv for some v ∈ G .
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Note that for all v ∈ G ,
χv(Df ) =∑x∈G
ζ f (x)q χv(x) =∑x∈G
ζ f (x)−v ·xq = F (v).
(2)From (11) and (2), we see that f is a GBF if andonly if
|χ(Df )|2 = qm for all χ ∈ G . (3)
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Proposition 3
Let m and q be positive integers, let f : Cmq → Zq
be a function, and set Df =∑
x∈G ζf (x)q x . Then f is
a GBF if and only if
DfD(−1)f = qm. (4)
In view of the above equation, we may also view aGeneralized Bent function as a generalization ofweighing matrices.
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We will only concern with the case q = 2pa where pis a prime. Write G = U · K with U = Cm
pa andK = Cm
2 .
Suppose f : G → Z2pa is a GBF. Let Df be definedas before. Then
DfD(−1)f = 2mpam.
Let χ : U → C∗ be any character of U . We extendχ to a ring homomorphism Z[ζ][G ]→ Z[ζ][K ] bylinearity and setting χ(g) = g for all g ∈ K .
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Write Dχ = χ(Df ) =∑
h∈K xhh with xh ∈ Z[ζ] andΘ =
∑p−1x=1(xp)ζxp .
For any character τ on K , we have|τ(Dχ)|2 = 2mpam and thus τ(Dχ) ≡ 0 (mod Θam),we can then conclude
xh|K | =∑τ∈K
τ(Dχ)τ(h)−1 ≡ 0 mod (Θam)
for all h ∈ K .
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As |K | = 2m and gcd(2,Θ) = 1 in Z[ζ], it followsthat xh ≡ 0 (mod Θam) for all h ∈ K .
Thus Eχ := Dχ/Θam is an element of Z[ζ][K ].
EχE(−1)χ = 2m, as |Θ|2 = p.
For any character τ on K , |τ(Eχ)|2 = 2m. Thisleads us to study X ∈ Z[ζ] with |X |2 = 2m.
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Suppose X =∑n−1
i=0 aiζin and |X |2 = k for an integer
k ∈ Z.
X is not uniquely written in the form of∑n−1
i=0 aiζin.
Our first step is to represent X in a certain way thatis ‘unique’.
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Definition 4
Let G = 〈g〉 be a cyclic group of order n. For
Z =∑n−1
i=0 aigi ∈ Z[G ], write Z (ζn) =
∑n−1i=0 aiζ
in.
We say that Z is minimal if
|supp(Z )| = min {|supp(Y )| : Y ∈ Z[G ], Y (ζn) = Z (ζn)} .
If X ∈ Z[ζn] and Z (ζn) = X , then we say Z is aalias of X .
We define the length of X to be |supp(Z )|, whereZ is a minimal alias of X and denoted it by σ(X ).
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Theorem 5
Suppose |X |2 = n in Z[ζ] and Z =∑pa−1
i=0 aigi is a
minimal alias of X in Z[〈g〉] where ◦(g) = pa. Then
n ≥ 1
p − 1
((p − σ(X ))
pa−1∑i=0
a2i + σ(X ) max{0, σ(X )− p
2}
).
(5)In particular,
n ≥ max
{pσ(X )
2(p − 1),σ(X )(p − σ(X ))
p − 1
}. (6)
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Definition 6 (M-function)
For X ∈ Z[ζn], let
M(X ) =1
ϕ(n)
∑σ∈Gal(Q(ζn)/Q)
(XX )σ,
where ϕ denotes the Euler totient function.
The notion of M-functions was introduced byCassels. Theorem 5 still holds if we replace n byM(X ).
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Note that M(X ) ≥ 1 for all nonzero X ∈ Z[ζn] bythe inequality of geometric and arithmetic means,since ∏
σ∈Gal(Q(ζn)Q)
(XX )σ ≥ 1.
The following is a consequence of Cassel’s results.
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Result 7
Let X ∈ Z[ζn], let q be a prime divisor of n, andwrite n = qn′ with (q, n′) = 1. ThenX =
∑q−1i=0 Xiζ
iq with Xi ∈ Z[ζn′] and
M(X ) =1
q − 1
q−1∑i<j
M(Xi − Xj). (7)
To make use of the inequality in Theorem 5, we alsoneed to find a bound for σ(X ).
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Result 8
Let X ∈ Z[ζn], let q be a prime divisor of n, andwrite n = qbn′ with (q, n′) = 1. If b > 1, then
X =∑qb−1−1
i=0 Xiζi with Xi ∈ Z[ζqn′] and
M(X ) =
qb−1−1∑i=0
M(Xi). (8)
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The notion of multiplier is then useful in finding abound for σ(X ) by the following:
Proposition 9
Let t be an integer with gcd(t, p) = 1 and let G bea cyclic group of order pa. Write Opa(t) = f andsuppose that f divides p − 1. Let σ be theautomorphism of Q(ζ) determined by ζσ = ζ t . IfX σ = X for X ∈ Z[ζ], then there is a minimal aliasZ ∈ Z[G ] of X with
Z (t) = Z .
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Theorem 10
Let n be a nonsquare integer. Let G = 〈g〉 be acyclic group of order pa and let P be the subgroupof G of order p. Assume there is X =
∑aiζ
i ∈ Z[ζ]with |X |2 = n. Then if Z =
∑aig
i ∈ Z[G ] is aminimal alias of X , there is Y ∈ Z[G ] such that
ZZ (−1) = n + PY . (9)
Moreover, we have σ(X ) ≤ n and
p ≤ n2 + n + 1. (10)
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Corollary 11
Let p be a an odd prime, and let s 6= p be a prime.If a (ps, p, ps, s) relative difference set exists in anabelian group G , then p ≤ s2 + s + 1.
It can be shown that the existence of such relativedifference set implies the existence of X ∈ Z[ζp]such that |X |2 = s.
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Corollary 12
Suppose there exists X ∈ Z[ζ] with |X |2 = 2m.Write f = ordp(2). We have the following.
(a) p ≤ 22m + 2m + 1 and f < 2m+1 is odd.
(b) f < 2m or p ≤ f 2−2mf−2m .
(c) If p > 22(m−2) + 2m−2 + 1, thenX 6≡ 0 (mod 2).
(d) σ(X ) ∈ {uf , uf + 1} for some positive integer uand σ(X ) < 2m+1.
(e) p ≡ 7 (mod 8) orp ≡ 1, 9, 17, 25, 33, 41, 49, 57 (mod 64).
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In particular, it shows that if a GBF function fromCm2pa to Z2pa exists,
then p = 1 when m = 1;and p = 7, 23, 31, 73.
To remove some more cases, we need to useanother property of GBF.
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Proposition 13
Let m and q be positive integers, G = Cmq , let
f : G → Zq be a bent function, and set
Df =∑
x∈G ζf (x)q x . Then∑
τ∈G
τ(Df )τχ(Df ) = 0 for all χ ∈ G \ {χ0}.
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The key is to consider the set
W = {w ∈ Z[ζ] : ww = 2m}.
Condition:
v + w 6≡ 0 (mod 2) for all v ,w ∈ W with w 6= ±v .
If the above condition is satisfied in Z[ζ], then byusing the convolution property, it can be shown thatno such GBF exists.
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Theorem 14 (L & Schmidt 2019)
Suppose that m is odd. If a GBF from Zm2pa to Z2pa
exists, then the following hold.
m ≥ 3.
If m = 3, then p = 7.
If m = 5, then p ∈ {7, 23, 31, 73, 89}.If m = 7, then p ∈ {7, 23, 31, 47, 71, 73, 79, 89103, 223, 233, 337, 431, 601, 631, 881, 1103, 1801}.If m ≥ 7, then p ≤ 22m/9 orordp(2) ≤ (2m + 3)/5.
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Generalized Bent Functions (II)
Definition 15
Let m and n be positive integers and let ζm be aprimitive complex mth root of unity. A functionf : Zn
2 → Zm is called an (m, n) generalized bentfunction (GBF) if∣∣∣∣∣∣
∑x∈Zn
2
ζ f (x)m (−1)v ·x
∣∣∣∣∣∣2
= 2n for all v ∈ Zn2. (11)
Here x · v denotes the usual dot product.
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(m; n)-GBF exists if both m and n are even; or 4|m.It remains to study the following two cases:
(i) m is odd.
(ii) m ≡ 2 mod 4 and n is odd.
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Notation
For any subset S of G , we denote the group ringelement
∑g∈S g by S as well.
Let t be an integer coprime to q. ForX =
∑g∈G agg ∈ Z[ζq][G ], we write
X (t) =∑
aσggt
where σ is the automorphism of Q(ζq) determinedby ζσq = ζ tq.
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Instead of using just the additive group Z2 and Zm,we also use the multiplicative notation, C2 and Cm.
From now on, we write G = C n2 .
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Bent functions in terms of group ring equations
Definition 16
Let f : G (= C n2 )→ Zm be a function. We define an
element Bf in the group ring Z[ζm][G ]corresponding to f by
Bf :=∑x∈G
ζ f (x)m x .
Let g be a generator of Cm. We define an elementDf in the group ring Z[Cm][G ] by
Df :=∑x∈G
g f (x)x .
Note that for any character τ that maps g to ζm,τ(Df ) = Bf .
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It then follows that f is an (m, n)-GBF if and only if
|χ(Bf )|2 = 2n, (12)
for all character χ over G .
We now have the following characterization of(m, n)-GBFs.
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Proposition 17
Let f be a function from G = C n2 to Zm. Then f is
an (m, n)-GBF if and only if
BfB(−1)f = 2n. (13)
Furthermore, if f (G ) = Cm′ ⊂ Cm, then f can beregarded as an (m′, n)-GBF, where m = 2m′ withm′ odd.
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Observe that we may write
DfD(−1)f =
∑x∈G
∑y∈G
g f (y+x)g−f (y)x =∑x∈G
Exx ,
where Ex =∑
y∈G g f (y+x)g−f (y) ∈ Z[Cm].
Therefore, Bf is an (m, n)-GBF if for each characterτ of order m on Cm, τ(Ex) = 0 for all x 6= 1G .
Write Ex =∑
aigi . Note that all ai ’s are
non-negative. Clearly, we have∑aiτ(g i) = 0.
This leads us to study the notion of vanishing sums.
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Vanishing sums
Notation: For any group H , we denote
{∑g∈H
agg : ag ∈ Z and ag ≥ 0} by N[H].
Definition 18
Let D =∑m−1
i=0 aigi ∈ N[Cm]. We say D is a v-sum
if there exists a character τ of order m such thatτ(D) = 0. We say D is minimal ifτ(∑m−1
i=0 bigi) 6= 0 whenever 0 ≤ bi ≤ ai for all i
and bj < aj for some j .
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To study v-sum, we define the following:
Definition 19
Let S be a finite set. Suppose X =∑
i∈S aiµi whereµi ’s are distinct roots of unity and all ai ’s arenonzero positive integers. We define
(i) u is the exponent of X if u is the smallestpositive integer such that µui = 1 for all i .
(ii) k is the reduced exponent if k is the smallestpositive integer such that there exists j with(µiµ
−1j )k = 1 for all i .
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For example, if p is a prime, then the exponent of∑p−1i=1 ζ3ζ
ip is 3p, whereas the reduced exponent is p.
Definition 20
Suppose that X =∑
i∈S aiµi = 0 where µi ’s aredistinct roots of unity and all ai ’s are nonzerointegers. We say that the relation X = 0 is minimal,if for any proper subset I ( S ,
∑i∈I aiµi 6= 0.
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Proposition 21 (Conway, Jones)
Suppose that X =∑
i∈S aiµi = 0 is a minimalrelation with reduced exponent k and all ai ’snonzero. Then k is square free and
|S | ≥ 2 +∑
p∈P(k)
(p − 2).
Here P(k) denotes the set of all prime divisors of k .
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Note that if D =∑
i∈S aigi is a minimal v-sum,
then τ(D) =∑
i∈S aiτ(g)i is a minimal relation. Wethus define the reduced exponent of D as follows:
Definition 22
Suppose D =∑m−1
i=0 digi is minimal v-sum in
N[Cm]. We define the reduced exponent k of D asthe reduced exponent of the vanishing sumτ(D) =
∑m−1i=0 diτ(g)i .
Note that the reduced exponent defined above doesnot depend on the choice of the character τ .
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Lemma 23
If D ∈ N[Cm] is a minimal v-sum with reducedexponent k , then D = D ′h for some D ′ ∈ N[Ck ] andh ∈ Cm.
To deal with v-sum D ∈ N[Cm] which is notminimal, we first decompose it into sum of minimalv-sum. It is straight forward to prove the following:
Lemma 24
Let D ∈ N[Cm] be a v-sum. Then D =∑
Di whereDi ’s are minimal v-sums in N[Cm].
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Definition 25
Suppose D =∑m−1
i=0 digi is a v-sum in N[Cm]. We
define the c-exponent of D to be the smallest ksuch that there exist positive integer r , minimalv-sums D1, . . . ,Dr in N[Cm] with D =
∑ri=1Di and
k = lcm(k1, . . . , kr) where ki is the reduced exponetof Di for i = 1, . . . , r .
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Example
For example, if m = 10 and h is a generator of C10,then we have
D =9∑
i=1
hi =4∑
i=0
(1 + h5)hi and
D =9∑
i=1
hi =1∑
i=0
(1 + h2 + h4 + h6 + h8)hi .
Note that (1 + h5)hi and (1 + h2 + h4 + h6 + h8)hj
are both minimal v-sums. However, c-exponent is 2.
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Lemma 26
Suppose D =∑m−1
i=0 aigi is a v-sum in N[Cm] with
c-exponent k . Write m =∏s
i=1 pαi
i andk =
∏ti=1 pi . Note that t ≤ s and pi ’s are distinct
primes. Then we have the followings:
(a) ||D|| ≥ 2 +∑t
i=1(pi − 2);
(b) D =∑t
i=1 PiEi , where Pi is the subgroup oforder pi and Ei ∈ Z[Cm] for all i ;
(c) Suppose that∏t
i=1 pαi
i |d and d |m. Ifφ : Z[Cm]→ Z[Cd ] is the natural projection,then χ(φ(D)) = 0 whenever o(χ) = d .
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Recall that if τ is a character of order m, τ(Ex) is avanishing sum.
Lemma 27
The c-exponent kx of Ex is a square free integerthat divides m such that
(a) k = p1 · · · pt .(b) 2n ≥ 2 +
∑ti=1(pi − 2);
(c) Ex =∑t
i=1 PiEi , where Pi is the subgroup oforder pi ; Ei ∈ Z[Cm] for all i .
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Proposition 28
Suppose that f is an (m, n)-GBF and m =∏r
i=1 pαi
i
where pi ’s are distinct prime. Let tx be thec-exponent of Ex (as defined in Definition 2.7) foreach 1G 6= x ∈ G . Set
I = {i : pi - tx ∀x ∈ G} and m′ =∏i /∈I
pαi
i .
Then there exists an (m′, n)-GBF. In particular, ifpi |m and pi > 2n, then there exists an(m/pi , n)-GBF.
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Proposition 28 allows to eliminate all prime factorsof m greater than 2n while proving nonexistenceresults on (m, n)-GBF.
Using Lemma 27, we are able to study the structureof Ex .
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Theorem 29
Suppose that m =∏s
i=1 pαi
i , where2 < p1 < p2 < · · · < ps are primes and αi ’s are allpositive integers.
(a) There is no (m, n)-GBF when s = 1.
(b) There is no (m, n)-GBF if s ≥ 2 and3p1 + p2 > 2n.
(c) There is no (m, n)-GBF if there is no(∏r
i=1 pαi
i , n)-GBF where pr+1 is the smallestprime such that p1 + pr+1 > 2n.
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Theorem 30
Let n be odd and m = 2∏s
i=1 pαi
i , wherep1 < p2 < · · · < ps are primes.
(a) Suppose s = 1. Then there is no (m, n)-GBF ifp1 > 2n−2; or p1 is not a Mersene prime andp1 > 2n−3; or p1 ≡ 3, 5 (mod 8).
(b) Suppose s ≥ 2 and r is the least integer suchthat pr+1 + p1 > 2n + 2. Then there is no(m, n)-GBF if there is no (2
∏ri=1 p
αi
i , n)-GBF.In particular, there is no (m, n)-GBF ifp1 > 2n−2 and p1 + p2 > 2n + 2.
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We now consider nonexistence results for a fixedn.
In view of Theorem 20 and Theorem 30, weconclude that there is no (m, n)-GBF if n = 1;and m odd for n = 2.
For n = 3, we need only to consider(2m; 3)-GBFs with m = 3a · 5b · 7c in view ofProposition 28.
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We now consider nonexistence results for a fixedn.
In view of Theorem 20 and Theorem 30, weconclude that there is no (m, n)-GBF if n = 1;and m odd for n = 2.
For n = 3, we need only to consider(2m; 3)-GBFs with m = 3a · 5b · 7c in view ofProposition 28.
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We now consider nonexistence results for a fixedn.
In view of Theorem 20 and Theorem 30, weconclude that there is no (m, n)-GBF if n = 1;and m odd for n = 2.
For n = 3, we need only to consider(2m; 3)-GBFs with m = 3a · 5b · 7c in view ofProposition 28.
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Theorem 31
For any odd integer m′, there is no(2m′, 3)-generalized bent function.
We need to determine what Ex can be, giventhat the c-exponent kx divides 210.
To show 7 - kx , we need the following result in[Lam & L]
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Theorem 31
For any odd integer m′, there is no(2m′, 3)-generalized bent function.
We need to determine what Ex can be, giventhat the c-exponent kx divides 210.
To show 7 - kx , we need the following result in[Lam & L]
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Proposition 32
Let D ∈ N[Cm] be minimal v-sum with reducedexponent k . Then we have the followings:
(a) If k = p is prime and P is the subgroup of orderp, then D = Ph for some h ∈ Cm.
(b) If k =∏t
i=1 pi with t ≥ 2 andp1 < p2 < · · · < pt are prime, then t ≥ 3 and
||D|| ≥ (p1 − 1)(p2 − 1) + (p3 − 1).
Moreover, equality holds only ifD = (P∗1P
∗2 + P∗3 )h for some h ∈ Cm. Here
P∗i = Pi − {e}, and Pi is the subgroup of orderpi .
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To determine Ex , we first express it as∑
Di
where each Di ∈ N[Cm] is a minimal v-sum.
Observe that for each minimal v-sum Di , thereduced exponent is either a prime or a productof 3 primes.
Since ||Ex || = 8, we can apply Proposition 31 toeliminate 7.
Then, we determine what each Di can be.
Finally, using some ad hoc calcuations, it can beshown that no much GBF exists.
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To determine Ex , we first express it as∑
Di
where each Di ∈ N[Cm] is a minimal v-sum.
Observe that for each minimal v-sum Di , thereduced exponent is either a prime or a productof 3 primes.
Since ||Ex || = 8, we can apply Proposition 31 toeliminate 7.
Then, we determine what each Di can be.
Finally, using some ad hoc calcuations, it can beshown that no much GBF exists.
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To determine Ex , we first express it as∑
Di
where each Di ∈ N[Cm] is a minimal v-sum.
Observe that for each minimal v-sum Di , thereduced exponent is either a prime or a productof 3 primes.
Since ||Ex || = 8, we can apply Proposition 31 toeliminate 7.
Then, we determine what each Di can be.
Finally, using some ad hoc calcuations, it can beshown that no much GBF exists.
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To determine Ex , we first express it as∑
Di
where each Di ∈ N[Cm] is a minimal v-sum.
Observe that for each minimal v-sum Di , thereduced exponent is either a prime or a productof 3 primes.
Since ||Ex || = 8, we can apply Proposition 31 toeliminate 7.
Then, we determine what each Di can be.
Finally, using some ad hoc calcuations, it can beshown that no much GBF exists.
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To determine Ex , we first express it as∑
Di
where each Di ∈ N[Cm] is a minimal v-sum.
Observe that for each minimal v-sum Di , thereduced exponent is either a prime or a productof 3 primes.
Since ||Ex || = 8, we can apply Proposition 31 toeliminate 7.
Then, we determine what each Di can be.
Finally, using some ad hoc calcuations, it can beshown that no much GBF exists.
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Some open problems
(i) To study the case n = 4 and odd m.
(ii) To study the case when m = 2a · 3 · 5.
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Reference
J. H. Conway and A. J. Jones. TrigonometricDiophantine equations (On vanishing sums ofroots of unity). Acta Arith., 30(3):229–240,1976.
P. V. Kumar, R. A. Scholtz, and L. R. Welch,Generalized bent functions and their properties.J. Combin. Theory Ser. A, 40(1):90–107, 1985.
Y. Jiang, Y. Deng: New results on nonexistenceof generalized bent functions. Des. CodesCryptogr. 75 (2015), 375–385.
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P. V. Kumar, R. A. Scholtz, L. R. Welch:Generalized bent functions and their properties.J. Combin. Theory Ser. A 40 (1985), 90–107.
T. Y. Lam and K. H. Leung, On vanishing sumsof roots of unity. J. Algebra, 224(1):91–109,2000.
H. Liu, K. Feng, R. Feng: Nonexistence ofgeneralized bent functions from Zn
2 to Zm. Des.Codes Cryptogr. 82 (2017), 647–662.
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O. S. Rothaus: On ’bent’ functions. J. Combin.Theory Ser. A 20 (1976), 300–305.
K.-U. Schmidt: Quaternary constant-amplitudecodes for multicode CDMA. IEEE Trans. Inf.Theory 55 (2009), 1824–1832.
N. Tokareva (2015): Bent functions: results andapplications to cryptography, Academic Press
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Some recent results on Hadamard matrices
Theorem 1 (L and Momihara 2019)
Let Φ1 = {q2 : q ≡ 1 ( mod 4) is a prime power},Φ2 = {n4 ∈ N : n ≡ 1 ( mod 2)} ∪ {9n4 ∈ N : n ≡ 1 (mod 2)}, Φ3 = {5} and Φ4 = {13, 37}. Then, the followinghold:
(1) There exists a Hadamard matrix of order 4(2v + 1) forv ∈ Φ1 ∪ Φ2 ∪ Φ3 ∪ Φ4.
(2) There exists a Hadamard matrix of order 4(3v + 1) forv ∈ Φ1 ∪ Φ2 ∪ Φ3.
(3) There exists a Hadamard matrix of order 4(5v + 1) forv ∈ Φ1 ∪ Φ2 ∪ Φ3.
(4) There exists a Hadamard matrix of order 8(uv + 1) foru ∈ Φ1 ∪ Φ2 and v ∈ Φ1 ∪ Φ2 ∪ Φ3.
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Using the above result, we obtain some Hadamard matrices oforder 4p, where p is an odd prime, such matrices are known tobe difficult to be constructed. The number of odd n < 1000such that 2n4 + 1 is a prime is 32, and such n < 100 are
1, 3, 21, 45, 63, 81, 105, 153, 177, 201, 219, 225, 249, 279, 297.
Furthermore, the number of odd n < 1000 such that2 · 9n4 + 1 is a prime is 74, and such n < 100 are
1, 3, 5, 31, 45, 55, 57, 71, 79, 89, 107, 109, 119, 123, 137,
141, 159, 167, 173, 181, 197, 217, 255, 275, 285, 295.
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Let Bi , i = 1, 2, . . . , `, be ki -subsets of G andB = {Bi : i = 1, 2, . . . , `}.A family B is said to be a difference family withparameters (v ; k1, k2, . . . , k`;λ) in G if
∑i=1
BiB(−1)i = λG +
(∑i=1
ki − λ)· 0G .
For ` = 2, 4, 8, a difference family B is said to be oftype H∗` if
∑`i=1 ki − `(|G |+ 1)/4 = λ.
For ` = 4, B is said to be of type H if∑4i=1 ki − |G | = λ.
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It is well known that if there is a difference family oftype H in G , then we have a Hadamard matrix oforder 4|G | by plugging the circulant (−1, 1)matrices obtained from its blocks into theGoethals-Seidel array.For any difference family of type H∗` in G for ` = 2or 4, we construct a Hadamard matrix of order`(|G |+ 1) by plugging the circulant (−1, 1)matrices obtained from its blocks into the Szekeresarray or the Wallis-Whiteman array, respectively.
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Theorem 2 (L, Momihara and Xiang 2020)
Let q be a prime power of the formq = 12c2 + 4c + 3 with c an arbitrary integer, andlet n = q2. Then there exists a Hadamard matrix oforder 4(2n + 1).
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There are 386 prime powers of the formq = 12c2 + 4c + 3 < 107 while there are 166181 prime powersq < 107 such that q ≡ 3 ( mod 8). The first 58 prime powersof the form q = 12c2 + 4c + 3 < 105 are listed below:
3, 11, 19, 43, 59, 179, 211, 283, 563, 619, 739, 1163, 1499, 1979, 2083,
2411, 3011, 3539, 4259, 4723, 7603, 8011, 8219, 10211, 11411,
12163, 14011, 14563, 14843, 17483, 20011, 23059, 25579, 26699,
28619, 29803, 30203, 33923, 36083, 36523, 41539, 49411, 54139,
55219, 55763, 59083, 60779, 63659, 65419, 69011, 70843, 75211, ,
80363, 81019, 82339, 83003, 88411, 93283.