circle-valued morse theory and reidemeister...

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Volume 3 (1999) 369396Published: 26 October 1999

Circle-valued Morse theory and Reidemeister torsion

Michael Hutchings

Yi-Jen Lee

Dept of Math, Stanford University, Stanford, CA 94305, USA

Dept of Math, Princeton University, Princeton, NJ 08544, USA

Email: [email protected] and [email protected]

Abstract

Let X be a closed manifold with (X) = 0, and let f : X ! S1 be a circle-valued Morse function. We dene an invariant I which counts closed orbits ofthe gradient of f , together with flow lines between the critical points. We showthat our invariant equals a form of topological Reidemeister torsion dened byTuraev [28].

We proved a similar result in our previous paper [7], but the present paper re-nes this by separating closed orbits and flow lines according to their homologyclasses. (Previously we only considered their intersection numbers with a xedlevel set.) The proof here is independent of the proof in [7], and also simpler.

Aside from its Morse-theoretic interest, this work is motivated by the fact thatwhen X is three-dimensional and b1(X) > 0, the invariant I equals a count-ing invariant I3(X) which was conjectured in [7] to equal the SeibergWitteninvariant of X . Our result, together with this conjecture, implies that theSeibergWitten invariant equals the Turaev torsion. This was conjectured byTuraev [28] and renes the theorem of Meng and Taubes [14].

AMS Classication numbers Primary: 57R70

Secondary: 53C07, 57R19, 58F09

Keywords: MorseNovikov complex, Reidemeister torsion, SeibergWitteninvariants

Proposed: Ralph Cohen Received: 28 June 1999Seconded: Robion Kirby, Steve Ferry Accepted: 21 October 1999

ISSN 1364-0380

Copyright Geometry and Topology

369

1 Introduction

Given a flow on a manifold, it is natural to ask how many closed orbits thereare. It turns out that for some well-behaved flows, the numbers of closed orbitsin dierent homology classes are related to the Reidemeister torsion of theunderlying manifold. For example, Fried [2] dened a \twisted Lefschetz zetafunction" counting closed orbits of certain nonsingular hyperbolic flows andshowed that it equals a version of topological Reidemeister torsion, which isindependent of the flow.

In this paper, we are interested in the gradient flow of a circle-valued Morsefunction. For singular flows such as this one, the zeta function is no longerinvariant under deformation of the flow. It turns out that this lack of invariancecan be xed by considering the Novikov complex, which counts gradient flowlines between critical points. We will show that one can obtain a topologicalinvariant by multiplying the zeta function by the Reidemeister torsion of theNovikov complex. We call the resulting invariant I .

In our previous work [7], we dened a weaker version of I and showed that itequals a form of topological Reidemeister torsion. Later we received a preprintfrom Turaev [28] dening a rened version of the latter invariant, which we call\Turaev torsion" here. Along similar lines we can rene the Morse theoreticinvariant in [7] to obtain the invariant I . The main result of this paper assertsthat I equals Turaev torsion.

Our previous methods are not quite sucient to prove this renement, so herewe introduce a dierent and simpler approach. This paper is independent of [7],except that the latter paper denes certain compactications in Morse theorywhich we use here, and also provides more background and context.

We now proceed to dene our invariant I more precisely and state our maintheorem. We then describe the application to three-dimensional SeibergWittentheory. In section 2 we give some background denitions, and in section 3 weprove the main theorem. In section 4 we give more details on the relation toSeibergWitten theory.

1.1 Statement of results

The basic setup for this paper is as follows. Let X be a closed connectedoriented ndimensional manifold. We assume throughout that (X) = 0, sothat we can dene Reidemeister torsion. Also, our result is most interestingwhen b1(X) > 0.

Michael Hutchings and Yi-Jen Lee

Geometry and Topology, Volume 3 (1999)

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Let f : X ! S1 . In order to consider the gradient flow of f , we endow X witha Riemannian metric. We make the following assumptions:

Assumption 1.1 (a) f is a Morse function.

(b) The ascending and descending manifolds of the critical points of f inter-sect transversely (see section 2.1).

(c) The closed orbits of the gradient rf are nondegenerate (see below).

A standard transversality argument shows that these assumptions hold if f andthe metric are generic.

A closed orbit is a nonconstant map γ : S1 ! X with γ0(t) = −rf forsome > 0. We declare two closed orbits to be equivalent if they dier byreparametrization. The period p(γ) is the largest integer p such that γ factorsthrough a pfold covering S1 ! S1 . A closed orbit is nondegenerate if det(1−d(x)) 6= 0, where is the pth return map at a point x 2 γ(S1) X . If so,the Lefschetz sign (γ) is the sign of this determinant.

Notation 1.2 Let H1 := H1(X). Let 2 H1(X;Z) denote the pullback byf of the \upward" generator of H1(S1;Z).

Let = Nov(H1;−) denote the Novikov ring [17, 5], consisting of functionsH1 ! Z that are nitely supported on the set fh 2 H1 j −(h) Cg foreach C 2 R. This ring has the obvious addition, and the convolution prod-uct. We denote a function a : H1 ! Z by the (possibly innite) formal sumP

h2H1a(h) h .

Denition 1.3 [2, 20] We count closed orbits with the zeta function, whichis a function H1 ! Z dened, in the above notation, by

:= exp

0@Xγ2O

(γ)p(γ)

[γ]

1A : (1)

Here O denotes the set of closed orbits, and [γ] := γ[S1] is the homology classof γ in H1(X).

A compactness argument using Assumption 1.1, together with the observationthat −([γ]) > 0 for all γ , shows that 2 ⊗Q.

We remark that there is also a product formula [3, 7, 8]

=Yγ2I

(1− (−1)i− [γ])−(−1)i0 : (2)



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Here I denotes the set of irreducible (period 1) closed orbits, and i−(γ) andi0(γ) denote the numbers of real eigenvalues of the return map in the intervals(−1;−1) and (−1; 1), respectively. Equation (2) shows that in fact 2 ,ie, has integer coecients. A third formula for the zeta function, in terms ofxed points of return maps, is given in equation (13).

We now introduce a notion of topological Reidemeister torsion following Turaev[28], and an analogous notion of Morse-theoretic torsion. Detailed denitionsare given in section 2.3.

Let ~X denote the universal (connected) abelian cover of X , whose automor-phism group is H1(X). A smooth triangulation of X lifts to ~X and gives rise toa chain complex C( ~X) over Z[H1]. The Reidemeister torsion of this complexis an element of Q(Z[H1])= H1 , where Q(R) denotes the total quotient ringof R. The H1 ambiguity arises because the Reidemeister torsion depends ona choice of ordered basis.

Turaev [27] showed that the H1 ambiguity can be resolved by the choice of an\Euler structure". The space Eul(X) of Euler structures is a natural anespace over H1(X), reviewed in section 2.2. One can also resolve the signambiguity by choosing a homology orientation of X , ie, an orientation o ofL

iHi(X;Q) (see [26]). We can then dene the Turaev torsion

(X; o) : Eul(X)! Q(Z[H1]): (3)

This is an H1equivariant map which does not depend on the triangulation. Wewrite (X):=(X; o); this is an H1equivariant map Eul(X)! Q(Z[H1])=1 .

Example 1.4 If X is the 3manifold obtained by zero surgery on a knotK S3 , then for a suitable Euler structure ,

(X)() =Alex(K)(1− t)2

where Alex(K) 2 Z[t; t−1]= 1 is the Alexander polynomial of K and t is agenerator of H1(X) ’ Z.

On the Morse theory side, the Novikov complex CN is a chain complex overthe Novikov ring , whose chains are generated by critical points of the pullbackof f to ~X , and whose boundary operator counts gradient flow lines betweencritical points (see section 2.1). We can similarly dene the Morse-theoretictorsion

(CN) : Eul(X)! Q()= 1: (4)



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Denition 1.5 Dene I : Eul(X)! Q()= 1 to be the product of the zetafunction and the Morse-theoretic torsion:

I := (CN):

Theorem 1.6 The Morse theory invariant I is independent of the metric anddepends only on the homotopy class of f , ie the cohomology class .

One can prove this a priori ; see [7] for the rough idea and [6] for the details.Although this may help dene related invariants in other contexts, in the presentcontext it is easier to compute I directly, which will prove Theorem 1.6 aposteriori . That is what we will do in this paper.

Theorem 1.7 (Main theorem) Our Morse theory invariant I is equal to thetopological torsion:

I = i((X))

as maps Eul(X)! Q()= 1.

Here i : Q(Z[H1])! Q() is induced by the inclusion Z[H1]! .

Remarks 1.8 (1) In the extreme case when there are no critical points, Xis a mapping torus and this theorem reduces to an equivariant version of theLefschetz xed point theorem, cf [15, 3].

(2) The extreme case when = 0, so that f lifts to a real-valued Morsefunction, is also essentially classical (cf [16]), and we treat it in section 2.4. Inthis case the Morse-theoretic torsion is a topological invariant; lack of invarianceand existence of closed orbits arise simultaneously when we pass from real-valued to circle-valued Morse theory.

(3) The class 2 H1(X), regarded as a map H1(X) ! Z, induces a map ! Z((t)) sending h 7! t(h) . This in turn induces a partially dened mapQ() ! Q((t)). One can deduce the main result of our previous paper [7] byapplying this map to Theorem 1.7.

(4) The statement of Theorem 1.7 makes sense when df is replaced by a genericclosed 1form and the Novikov ring is graded by −[]. It seems possible toextend Theorem 1.7 to this case by approximating by closed 1forms inrational cohomology classes, to which Theorem 1.7 applies. Another proof forclosed 1forms is given in [6] by rst proving Theorem 1.6 for closed 1forms,and then using this to reduce to the real-valued case.



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(5) Some previous papers, such as [19], studied the torsion of the Novikov com-plex (or the Whitehead torsion, which is sharper but only dened \relatively"unless the Novikov complex is acyclic), without considering the zeta function.In this case one can still obtain a topological invariant by modding out by unitsin the Novikov ring with leading coecient 1. This is useful for understandingthe obstructions to the existence of nonvanishing closed 1forms [10, 4]. How-ever, the extra information in the zeta function is important for the connectionwith SeibergWitten theory below.

(6) A homology orientation of X can apparently remove the sign ambiguityin (CN). However we have not checked if Theorem 1.7 holds with the signambiguity removed this way.

1.2 Application to SeibergWitten theory

We now consider the special case when dim(X) = 3 and b1(X) > 0. LetSpinc(X) denote the set of spin-c structures on X . Given a homology orienta-tion o, the SeibergWitten invariant of X is a function

SWX;o : Spinc(X)! Z

which counts Rinvariant solutions to the SeibergWitten equations on XR,modulo gauge equivalence. (See eg [13, 14, 18].)

Taubes [24] has shown that the SW invariant of a symplectic fourmanifoldequals a \Gromov invariant" counting pseudoholomorphic curves. In [7] weproposed that using similar analysis, one might be able to show that the SWinvariant of a 3manifold is equal to a Morse theory invariant

I3 : Spinc(X)! Z:

The invariant I3 counts certain unions of closed orbits and flow lines of thegradient vector eld of a Morse function f : X ! S1 with no index 0 or 3critical points. We review the denition of I3 in section 4.

Conjecture 1.9 [7] The SeibergWitten invariant agrees with our Morse the-ory invariant:

SWX;o = I3:

(When b1(X) = 1, the SW invariant also depends on a choice of \chamber",and in this conjecture we use the chamber determined by r df for r >> 0.)



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Remark 1.10 If f has no critical points, and if we arrange for df to beharmonic, then this conjecture is a corollary of Taubes’ theorem [24] appliedto the symplectic fourmanifold (X S1; df ^ ds + Xdf). Here s denotesthe S1 coordinate. The idea is that for a suitable homology orientation, ifs 2 Spinc(X), then

SWX;o(s) = SWXS1(s) = GrXS1(s) = I3(s):

The rst equality expresses the fact that all solutions to the SW equationson X S1 are S1invariant; see [18] for details of this equality. The secondequality is Taubes’ theorem; here GrXS1(s) counts, in the sense of [25],pseudoholomorphic curves in a certain S1invariant homology class in XS1 .An energy argument shows that for a suitable almost complex structure, everysuch curve is a union of closed orbits of rf crossed with S1 . This leads to thethird equality (cf [8, Thm. 0.1]), using the fact that I3 is a reparametrizationof the zeta function in this case.

Salamon has proved a statement equivalent to Conjecture 1.9 in this case usinga dierent method [22].

In another direction, Turaev [28] conjectured a combinatorial formula for theSeibergWitten invariant as follows. If dim(X) = 3 and b1(X) > 1, then for anEuler structure , the torsion (X; o)() is actually in the group ring Z[H1].If b1 = 1, then i((X; o)()) 2 , rather than in the quotient ring. Given ahomology orientation o, one can then dene a map

T (X; o) : Eul(X) −! Z; 7−! i((X; o)())(0):

where (0) indicates evaluation on 0 2 H1 . (This depends on the sign of [df ]when b1 = 1.) There is also a natural isomorphism : Spinc(X) ! Eul(X)([27], see section 4.3).

Conjecture 1.11 (Turaev [28]) The SeibergWitten invariant agrees withthe Turaev torsion:

SWX;o = T (X; o) : Spinc(X)! Z:

This statement is a renement of the theorem of Meng and Taubes [14], whichgives an \averaged" version of this equivalence, in which one sums over spin-cstructures that dier by torsion elements of H2(X;Z).

The invariant I3 turns out to be a reparametrization of the more general in-variant I . Thus we can apply Theorem 1.7 to prove:



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Theorem 1.12 Conjecture 1.9 is equivalent to Conjecture 1.11 (modulo signs).

The detailed proof is given in section 4.

Update (1) Three days after the rst version of this paper was posted onthe internet, a preprint by Pajitnov [20] appeared, giving a result similar toTheorem 1.7, using Whitehead torsion.

(2) Turaev [29] has shown how to rene the methods of Meng and Taubes toprove Conjecture 1.11, modulo signs. Together with our results, this indirectlyproves Conjecture 1.9. However, one might still desire a direct analytic proof.The following is a summary of the situation:

Thm. 1.7

S1 Morse theory = Turaev torsionanalytic proof? @@@@ Meng-Taubes/Turaev

SeibergWitten

Acknowledgments This paper would not exist were it not for Taubes’ workon SeibergWitten and Gromov invariants. We thank him for sharing his ideasgenerously. We also thank R Bott, R Forman, and D Salamon for helpfulconversations.

2 Background

We now give some necessary background. Section 2.1 reviews the denition ofthe Novikov complex, which counts gradient flow lines between critical points.Section 2.2 reviews Turaev’s Euler structures, which are needed for the mostrened version of Reidemeister torsion. Section 2.3 gives the precise denitionsof the versions of Reidemeister torsion that we use. Finally, section 2.4 provesTheorem 1.7 for real-valued Morse functions, as a warmup for some of thearguments in section 3.

2.1 The Novikov complex

We begin with some standard denitions from Morse theory. If p is a criticalpoint in X of f or in ~X of the pullback of f , the descending manifold D(p) isthe set of all points x such that upward gradient flow starting at x converges



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to p. Similarly the ascending manifold A(p) is the set of all points from whichdownward gradient flow converges to p. If ind(p) = i, then D(p) and A(p) areembedded open balls of dimensions i and n− i respectively.

The Novikov complex (CN; @f ) is dened as follows. On ~X , we can lift ourMorse function to a map ~f : ~X ! R. Let CNi denote the set of (possiblyinnite) linear combinations of critical points of index i in ~X , such that foreach C 2 R, the sum contains only nitely many critical points p 2 ~X with~f(p) > C . The action of H1 on the critical points by covering transformationsinduces an action of the Novikov ring on CNi . In fact CNi is a free module; one can specify a basis for CN by choosing a lift of each critical pointin X to ~X .

We dene @f : CNi ! CNi−1 as follows. If p; q 2 ~X are critical points of indexi and i− 1 respectively, let h@fp; qi denote the signed number of gradient flowlines from p to q . If p is a critical point of index i, dene

@fp :=Xq

h@fp; qiq

where the sum is over all critical points q 2 ~X of index i − 1. We count flowlines using the sign conventions from [7]. These conventions are chosen so that(@f )2 = 0 and so that equation (16) holds.

Theorem 2.1 (Novikov) The homology of the Novikov complex is naturallyisomorphic to the homology of the \half-innite" chains in ~X :

H(CN; @f ) ’ H(C( ~X)⊗ ):

See eg [17, 19, 21, 7]. For example, if X = S1 and f : S1 ! S1 has nonzerodegree, then the homology of the Novikov complex vanishes.

2.2 Euler structures

We now discuss three dierent notions of \Euler structure" and how they relate.One can ignore this material at the expense of allowing an H1 ambiguity inReidemeister torsion.

Denition 2.2 (Turaev [27]) If X is a closed smooth manifold with (X) =0 and n = dim(X) > 1, a smooth Euler structure on X is a nonsingularvector eld on X , where two such vector elds are declared equivalent if theirrestrictions to the complement of a ball in X are homotopic through nonsingularvector elds. We let Eul(X) denote the space of smooth Euler structures.



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By obstruction theory, Eul(X) is an ane space over Hn−1(X;n−1(Sn−1)) =H1(X). (It is nonempty since (X) = 0.)

The following alternate denition of smooth Euler structures is useful for Morsetheory, and also works well when n = 1. If v is a vector eld on X withnondegenerate zeroes, let H1(X; v) denote the set of homology classes of 1chains γ 2 X with @γ = v−1(0), where the points in v−1(0) are orientedin the standard way. The set H1(X; v) is a subset of the relative homologyH1(X; v−1(0)) and is an ane space over H1(X).

Denition 2.3 One can show by a cobordism argument that the spacesH1(X; v) for dierent v ’s are canonically isomorphic to each other, and henceto a single space. We call this space Eul(X). We let iv : H1(X; v) ! Eul(X)denote the canonical isomorphism.

If n > 1, we can go from Denition 2.3 to Denition 2.2 as follows. Givenγ 2 H1(X; v), we can represent γ by disjoint paths connecting the zeroes of vin pairs. We then construct a nonsingular vector eld by cancelling the zeroes ofv in a neighborhood of γ . (If v has no zeroes, we send 0 2 H1(X) = H1(X; v)to the Euler structure represented by v and extend equivariantly.)

Denition 2.4 [27] Let (X;T ) be a nite connected CWcomplex with cellsfig. (X denotes the underlying topological space; T denotes the cell struc-ture.) A combinatorial Euler structure on (X;T ) is a choice of a lift of each cellto the universal abelian cover ~X , where two such sets of lifts f ~ig and fhi ~ig,with hi 2 H1(X) = Aut( ~X), are considered equivalent if

Pi(−1)dim(i)hi = 0.

We let Eul(X;T ) denote the space of combinatorial Euler structures of theCWcomplex (X;T ). This is clearly an ane space over H1(X).

Note that if T is a renement of the cell-structure T with cells fjg, thenthere is a canonical isomorphism Eul(X;T )! Eul(X; T ) sending f~ig to f~jg,where ~j ~i if j i .

Lemma 2.5 [27] If X is a closed smooth manifold with a smooth triangula-tion T , then there is a natural isomorphism between the spaces of smooth andcombinatorial Euler structures:

Eul(X) ’ Eul(X;T ):



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The idea is that there is a natural vector eld on each simplex with a zero at thecenter of each face and which points into the simplex near the boundary. Thesepiece together to give a continuous vector eld on X . We can perturb this toa smooth vector eld vT with a nondegenerate zero of sign (−1)i in the centerof each isimplex. Then a smooth Euler structure can be represented by achain γ consisting of paths connecting the zeroes in pairs, with [γ] = i−1

vT() 2

H1(X; vT ). We can lift the chain γ to ~X , and the induced lifts of its endpointsdetermine a combinatorial Euler structure.

2.3 Reidemeister torsion

We now review the denition of Reidemeister torsion of certain chain complexes.We then use this algebra to dene Reidemeister torsion for the two geometriccomplexes we are interested in.

2.3.1 Algebra

Let (C; @) be a nite complex of nite dimensional vector spaces over a eld F .The standard short exact sequences 0! Zi ! Ci ! Bi−1 ! 0 and 0! Bi !Zi ! Hi ! 0 induce canonical isomorphisms det(Ci) = det(Zi)⊗det(Bi−1) anddet(Zi) = det(Bi)⊗det(Hi), where ‘det’ denotes top exterior power. Combiningthese isomorphisms gives an isomorphism

:Oi

det(Ci)(−1)i !Oi

det(Hi)(−1)i : (5)

Let e be an ordered basis for C , ie, an ordered basis ei for each Ci . Let h bean ordered basis for H . Let [e] 2

Ni det(Ci)(−1)i and [h] 2

Ni det(Hi)(−1)i

denote the resulting volume forms.

Denition 2.6 We dene the Reidemeister torsion

(C; e; h) := ([e])=[h] 2 F:

We also dene

(C; e) :=(C; e; 1) if H = 0,

0 otherwise.

Usually we will be interested in rather than . In practice, we can computethe torsion in terms of an alternating product of determinants as follows.



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Lemma 2.7 If H = 0, we can nd a decomposition C = A B such that(i) Ai and Bi are spanned by subbases of the basis ei , and (ii) the map

Bi−1 @jAi : Ai ! Bi−1

(which we abbreviate by @ : Ai ! Bi−1 ) is an isomorphism. Then

(C; e) := Yi

det(@ : Ai ! Bi−1)(−1)i :

Here the determinants are computed using the subbases of e.

We now extend the denition of torsion to complexes over certain rings whichmight not be elds.

Denition 2.8 [28] Let R be a ring, and assume that its total quotient ring(denoted by Q(R)) is a nite sum of elds, Q(R) = jFj . Let (C; @) be anite complex of nitely generated free Rmodules with an ordered basis e.We dene

(C; e) :=Xj

(C ⊗R Fj ; e⊗ 1) 2Mj

Fj = Q(R):

In this case H(C) might not be free, in which case it does not have a basis inthe usual sense. However in this paper we call a set h := fhjg a \basis" forH(C) when hj is a basis for H(C ⊗R Fj) for each j . Given h = fhjg, wedene

(C; e; h) :=Xj

(C ⊗R Fj ; e⊗ 1; hj) 2 Q(R):

Example 2.9 If 0 ! C2@! C1 ! 0 is a 2term complex with C1 = C2 , and

if e is a basis which is identical on C1 and C2 , then

(C; e) = det(@):

We are interested in the rings Z[H1] and . Their quotients are nite sums ofelds (see eg [28]), and these decompositions are compatible with the inclusionZ[H1]! .

In section 3.2, we will need the following product formula for torsion. Let R be aring such that Q(R) is a nite sum of elds Fj . Let 0! C ! C 0 ! C 00 ! 0 bea short exact sequence of nite complexes of nitely generated free Rmodules.Let e; e0; e00 be bases for C; C 0; C 00 compatible with the exact sequence. Leth; h0; h00 be bases for the homology as in Denition 2.8. Let L denote the longexact sequence in homology, regarded as an acyclic chain complex, and let bdenote the basis for L obtained by combining h; h0; h00 .



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Lemma 2.10 We have the following product formula for torsion:

(C 0; e0; h0) = (C; e; h) (C 00; e00; h00)(L; b):

Proof This follows from [16].

2.3.2 Geometry

We now dene the Turaev torsion (3). In the notation of Denition 2.4, let(X;T ) be a nite connected CWcomplex with universal abelian cover ~X . Lif-ing the cells gives a chain complex C( ~X;T ) over Z[H1(X)]. A combinatorialEuler structure determines a set of lifts of each cell to ~X , up to equiva-lence. Choose one set of lifts; this gives a basis for C( ~X;T ). A homologyorientation o determines an orientation of this basis, via the isomorphism (5)applied to C(X;T ). Let e(; o) denote the resulting ordered basis. We de-ne the combinatorial Turaev torsion (X;T ; o) to be the H1equivariant mapEul(X;T )! Q(Z[H1]) given by

(X;T ; o)() := (C( ~X;T ); e(; o)) 2 Q(Z[H1]):

Note that the right hand side of this equation does not depend on the choiceof a set of lifts. Furthermore, (X;T ; o) = (X; T ; o) under the canonicalisomorphism Eul(X;T )! Eul(X; T ), if T is a renement of T .

Denition 2.11 Let X be a closed connected smooth manifold with (X) =0, with a smooth Euler structure 2 Eul(X) and a homology orientationo. Choose a smooth triangulation T of X . Let T 2 Eul(X;T ) denote thecombinatorial Euler structure equivalent to via Lemma 2.5. We dene theTuraev torsion

(X; o)() := (X;T ; o)(T ):

The results of [27] show that the Turaev torsion does not depend on the choiceof smooth triangulation T .

We now dene the Morse theoretic torsion (4). A smooth Euler structure canbe represented by a chain γ connecting the critical points of f in pairs, with[γ] = i−1

rf () 2 H1(X;rf). We can lift γ to ~X , and the induced lifts of theendpoints determine a basis e() for CN .

Denition 2.12 We dene the Morse theoretic torsion (CN) : Eul(X) !Q()= 1 by

(CN)() := (CN; e()):



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The map (CN) is H1equivariant, and again does not depend on the choiceof lifting. There is a sign ambiguity because the basis e() is unordered. Inthe special case when f has no critical points, we dene (CN) to be the H1

equivariant map such that (CN)() = 1 for the smooth Euler structure represented by rf .

In the future, we call two bases for C( ~X;T ) or CN equivalent if they corre-spond to the same Euler structure .

2.4 The real-valued case

Before proceeding more deeply into circle-valued Morse theory, it will be usefulto prove the main theorem for real-valued Morse functions.

Lemma 2.13 Theorem 1.7 holds when f : X ! S1 lifts to a real-valued Morsefunction X ! R.

Proof In this case = 1, so we just need to check that the Morse theoreticand topological torsions agree. This is essentially classical (cf [16]), except forthe identication of the bases determined by an Euler structure.

If is an Euler structure, then the bifurcation analysis in [11] shows that(CN)() is independent of the real-valued Morse function and the metric. Itis not hard to check that the Euler structures work out at each stage. In [11] itis assumed that the metric has a standard form near the critical points, but thiscan be arranged by a perturbation which does not aect the Novikov complex.

Now let T be a smooth triangulation of X . We can apparently perturb thevector eld VT of section 2.2 so that it is the gradient of a Morse functionF : X ! R with respect to some metric. In this case, the Novikov complex(CN; @F ) is identical to the chain complex (C( ~X;T ); @). Moreover the basesdetermined by agree. Thus the Morse-theoretic and topological torsion areequal.

3 Proof of the main theorem

We will now prove the main theorem as follows. In section 3.1 we prepare for thecomputation of torsion by constructing a cell complex X 0 which \approximates"X and is adapted to the vector eld rf . In section 3.2 we prove a technical



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lemma (Lemma 3.6) asserting that X and X 0 have the same Reidemeistertorsion. The heart of the proof is in section 3.3 and section 3.4, where wedetermine the torsion of X 0 by a short computation, and then interpret theanswer geometrically to recover the invariant I .

3.1 The cell complex X 0

Assume 0 2 S1 = R=Z is a regular value of f (by composing f with a rotationif necessary). Let := f−1(0). Let Y be the compact manifold with boundaryobtained by cutting X along . We can write @Y = 1 t 0 , where i iscanonically isomorphic to , and −rf points inward along 1 .

We give Y a cell decomposition as follows. Let T1 be a smooth triangulationof 1 such that each simplex is transverse to the ascending manifolds of thecritical points in Y . If p 2 Y is a critical point, let D0(p) denote the descendingmanifold of p in Y . If 2 T1 is a simplex, let F() denote the set of ally 2 Y such that upward gradient flow starting at y hits . Choose a celldecomposition T0 of 0 , such that the intersections with 0 of D0(p) andF() are subcomplexes, for each critical point p and each simplex 2 T1 .

Lemma 3.1 The cells in T1 and T0 , together with all the cells D0(p) andF(), give a legitimate cell decomposition, T 0Y , of Y .

Proof Recall that D0(p) and F() have natural compactications using bro-ken flow lines (cf [7]). It may be shown by \induction on height" that thesecompactications are homeomorphic to closed balls. There are moreover natu-ral continuous maps of the compactications to Y which send the interiors ofthe balls homeomorphically to D0(p) and F(). The transversality conditionon T1 and Assumption 1.1(b) ensure that the boundary of a cell consists oflower dimensional cells in T 0Y .

We would like to glue the boundary components of Y back together to obtain anice cell decomposition of X , but usually T0 will not agree with T1 . To correctfor this, let : (0; T0)! (1; T1) be a cellular approximation to the canonicalidentication 0 ! 1 . Consider the mapping cylinder of :

M =(0 [0; 1]) t 1

(x; 1) (x):

This has a cell decomposition consisting of T0 and T1 , together with the cells (0; 1) for each 2 T0 . There is a canonical inclusion 0 ! M sendingx 7! (x; 0), and there is also a canonical inclusion 1 !M .



383

Denition 3.2 Let X 0 be the space obtained by gluing Y and M along0 t1 .

The space X 0 inherits a cell decomposition, but for our computations we prefera simpler cell decomposition, obtained by fusing some cells together as follows.If is a cell in Y of the form D0(p) or F(), we dene a corresponding cellin X 0 by

:= [ ((@ \ 0) [0; 1)):

Here (@ \ 0) [0; 1) indicates a subset of M .

Denition 3.3 Let T 0 be the cell decomposition of X 0 consisting of cells ofthe following types:

(a) D0(p) for p 2 Y a critical point;

(b) simplices in T1 ;

(c) F() for 2 T1 .

3.2 X and X 0 have the same Reidemeister torsion

We now show that X and X 0 have the same Reidemeister torsion, if the Eulerstructures are compatible in an appropriate sense.

We begin by noting that H(X 0) = H(X), and H( ~X 0) = H( ~X) as Z[H1]modules, as one can see from the exact sequences (8), (9) below. Note that theuniversal abelian cover ~X 0 of X 0 is obtained from ~X by modifying a neighbor-hood of the inverse image of .

Notation 3.4 (1) If Z is a subset of X or X 0 , then ~Z will denote the inverseimage of Z in ~X or ~X 0 . So ~Z is usually not the universal abelian cover of Z .

(2) We omit the cell structures from the notation when they are clear fromcontext.

A smooth Euler structure on X corresponds to an equivalence class of liftsof the critical points of f to ~X , as in section 2.3.2. A combinatorial Eulerstructure 0 on X 0 consists of an equivalence class of lifts of the cells T 0 to ~X 0 .

Denition 3.5 We say that and 0 are compatible if, within these equiva-lence classes, the lifts can be chosen so that:



384

(a) The lift of each critical point p in is contained in the lift of the cellD0(p) in 0 .

(b) For each simplex 2 T1 , the lift of in 0 agrees with the \top" of thelift of F() in 0 .

The compatibility conditions in Denition 3.5 induce an isomorphism fromEul(X) to Eul(X 0; T 0) as ane spaces over H1 .

Recall that (X) := (X; o); similarly write (X 0; T 0) := (X 0; T 0; o).

Lemma 3.6 If the Euler structures 2 Eul(X) and 0 2 Eul(X 0; T 0) arecompatible as above, then

(X)() = (X 0; T 0)(0):

Proof The strategy is to compute the torsion of X and X 0 by cutting theminto pieces and using the product formula (Lemma 2.10) applied to variousexact sequences, and see that we obtain the same answer. We proceed in threesteps.

Step 1 Consider the cell decomposition on ~[0; 1] consisting of cells ~f0g,~ (0; 1), and ~f1g, where ~ is a lift of a simplex 2 T1 . Also recall that~M has a natural cell decomposition. We claim that with respect to these cell

structures,

(C(~ [0; 1]); e; h) = (C( ~M); e0; h0); (6)

provided that the bases e; h; e0; h

0 satisfy the following conditions:

(a) The bases h; h0 for homology agree under the isomorphism induced bythe canonical map [0; 1]!M .

(b) The bases e; e0 are given by lifts of cells such that:

(i) The lifts of the cells in f1g [0; 1] and 1 M agree.(ii) The lift of each cell in f0g [0; 1] or in 0 f0g M

is adjacent to the lift of the cell (0; 1) in [0; 1] or M

respectively.

To prove (6), we compute both sides by applying the product formula for torsionto the relative exact sequences

0! C(~ f1g)! C(~ [0; 1])! C(~ [0; 1]; ~ f1g)! 0;

0! C( ~1)! C( ~M)! C( ~M; ~1)! 0:



385

The answers agree, because condition (a) implies that the (L) factors agree,condition (b(i)) implies that the (C(~)) factors agree, and condition (b(ii))implies that

(C(~ [0; 1]; ~ f1g); e; 1) = (C( ~M; ~1); e0; 1) = 1:

Step 2 Let TY be a smooth triangulation on Y whose restriction to eachcomponent of @Y = 0 t1 agrees with T1 . The smooth Euler structure onX determines an equivalence class of bases, eY , for C( ~Y ; ~0;TY ), because TYglues to a smooth triangulation of X .

Let T 0Y denote the cell decomposition of Y given by Lemma 3.1. The com-binatorial Euler structure 0 on X 0 determines an equivalence class of basese0Y for C( ~Y ; ~0;T 0Y ), because the cells of T 0Y in Y n 0 are in one to onecorrespondence with the cells of X 0 .

We claim that

(C( ~Y ; ~0;T 0Y ); e0Y ; h0) = (C( ~Y ; ~0;TY ); eY ; h) (7)

provided that the bases h0 and h on homology agree.

To prove (7), note that the pullback of the Morse function f to Y lifts to areal-valued function f : Y ! R. Let CM(f) denote the Morse complex of fon the covering ~Y . A direct computation, using the compatibility of and 0 ,shows that

(C( ~Y ; ~0;T 0Y ); e0Y ; h0) = (CM(f); e00; h00):

(For similar calculations see section 3.3 and section 3.4; the result here corre-sponds essentially to setting t = 0 in (11) and Lemma 3.7.) Here the basis e00

for CM(f) is determined by the lifts of the critical points determined by as before, and we assume that the bases h0; h00 on homology agree under thestandard isomorphism H( ~Y ; ~0) ’ H(CM(f)).

We also have

(CM(f); e00; h00) = (C( ~Y ; ~0;TY ); eY ; h):

The idea of the proof is to vary f in the space of Morse functions on Y such that(i) the gradient points outward along 0 and does not point outward along 1 ,and (ii) wherever the gradient is tangent to 1 , the inward covariant derivativeof the gradient points inward. As in section 2.4, one can show that the resultingtorsion is independent of the Morse function. Deforming f to a Morse functionFY adapted to the triangulation TY , such that CM(FY ) = C( ~Y ; ~0; TY ),we have (CM(f); e00; h00) = (CM(FY ); ifF (e00); h00), where ifF (e00) is the



386

equivalence class of lifts (which correspond to bases of CM ) induced from e00

via the homotopy from f to FY . But ifF (e00) = eY because the homotopy fromrf to rFY extends to a homotopy of vector elds on X = Y [ ( I), whichis homotopic to a homotopy from rf to the standard vector eld associated tothe triangulation of X (cf end of section 2.2).

The above two equations prove (7).

Step 3 We now use (6) and (7) to compute the torsion of X and X 0 .

We can regard X as the union of Y and [0; 1] along 0t1 . Let T denotethe cell decomposition of X obtained by gluing the triangulation TY of Y tothe product cell structure on [0; 1] obtained from T1 . We then have a shortexact sequence

0! C

~0 t ~1

! C(~ [0; 1]) C( ~Y ;TY )! C( ~X ;T )! 0: (8)

Let T 0 denote the \unfused" cell decomposition of X 0 from section 3.1. Wethen have a short exact sequence

0! C

~0 t ~1

! C( ~M) C( ~Y ;T 0Y )! C( ~X 0;T 0)! 0: (9)

We can choose representatives eY ; e0Y such that they agree on f1g and

1 with e; e0 respectively. Let e() denote the basis for C( ~X ;T ) obtained

by combining the bases e; eY of Step 1 and Step 2 respectively. Similarly lete0(0) denote the basis for C( ~X 0;T 0) obtained by combining the bases e0; e

0Y

of Step 1 and Step 2. Then e() is a representative of the combinatorial Eu-ler structure on (X; T ) corresponding to the smooth Euler structure , ande0(0) is a representative of the image of 0 under the canonical isomorphismEul(X 0; T 0)! Eul(X 0; T 0).

Applying the product formula to the above exact sequences, and using equations(6) and (7), we obtain

(C( ~X;T ); e(); h) = (C( ~X 0;T 0); e0(0); h0): (10)

Here we are assuming that the bases h; h0 for homology agree under the naturalisomorphism H( ~X) ’ H( ~X 0). Also, to apply (7) in the above computation,one relates ~Y to the pair ( ~Y ;0) as in Step 1.

In particular, equation (10) implies that

(C( ~X ;T ); e()) = (C( ~X 0;T 0); e0(0)):

This implies the lemma because (C( ~X ;T ); e()) = (X)(), since the in-sertion of [0; 1] changes nothing, and similarly (C( ~X 0;T 0); e0(0)) =(X 0; T 0)(0).



387

3.3 Computing the torsion

We now compute the torsion of the approximating cell complex (X 0; T 0), for acombinatorial Euler structure 0 compatible with a smooth Euler structure on X as in the previous section.

Since Lemma 2.13 proves the theorem for real-valued Morse functions, we as-sume from now on that

6= 0:

Without loss of generality, we may also assume that is indivisible in H1(X;Z).(If is divisible by k , we can lift f to a kfold cover of S1 without changingthe invariant I .) Let V := Ker(), and choose a splitting

H1(X) = V Z:Let t denote the generator of the Z component with (t) = −1. Then theNovikov ring can be identied with the ring of formal Laurent series in t withcoecients in Z[V ]:

= Z[V ]((t)):

Recall that Q() is a nite sum of elds. To prove Theorem 1.7, it suces toshow that it holds after projecting to each such eld. Let K be a eld com-ponent of Q(). By the Novikov isomorphism (Theorem 2.1), the complexesCN ⊗K and C( ~X)⊗K have isomorphic homology. So we will assume thatthese complexes are both acyclic, since otherwise they both have zero torsion , and there is nothing to prove. In all of the calculations below, we implicitlytensor everything with the eld K .

We can decompose

Ci( ~X 0) = Di Ei Fiwhere the three summands are generated by the cells of types (a), (b), and (c)respectively from Denition 3.3. Let us choose a basis e(0) for Ci( ~X 0) as inDenition 3.5. We can identify

Fi ’ Ei−1:

The matrix for the boundary operator on Ci( ~X 0) can then be written as

@i =

0B@Di Ei Fi

Di−1 Ni 0 Wi

Ei−1 −tMi @i 1− ti−1

Fi−1 0 0 −@i−1

1CA: (11)



388

We remark that @i is the boundary operator on C(~). Also i−1 is a matrix

with entries in Z[V ], which can be interpreted as the return map of the gradientflow from ~ to ~, after perturbation by our cellular approximation . LikewiseMi sends D0(p), where p 2 ~X is a critical point, to a perturbation of the\descending slice" D0(p) \ ~.

Continuing the calculation, due to the acyclicity assumption we may choosedecompositions Di = DA

i DBi such that DA

i and DBi are spanned by (cells

corresponding to) critical points, and the dierential @f induces an isomor-phism DA

i ! DBi−1 . In the notation below, we denote matrices with domain or

range D by boldface letters, and we denote their restrictions to DAi and/or

projections to DBi−1 by plain letters.

We now apply Lemma 2.7 with Ai = DAi Fi and Bi = DB

i Ei . (We willexplain in a moment why this choice of Ai and Bi is legitimate.) We obtain

(C( ~X 0; e(0)) =nYi=1

det(Ωi)(−1)i+1

where

Ωi = DA

i Fi

DBi−1 Ni Wi

Ei−1 −tMi 1− ti−1

:

We note that 1− ti−1 is invertible because i−1 has entries in Z[V ]. It followsthat

det(Ωi) = det(1− ti−1) det(Ki)

where

Ki := Ni + tWi(1− ti−1)−1Mi : Di ! Di−1: (12)

It will follow from Lemma 3.9(b) and the choice of DAi ;D

Bi that Ki is non-

singular, provided that the triangulation T1 is suciently ne and the cellularapproximation is suciently close to the identity. In particular, the matricesΩi are then nonsingular, so that Lemma 2.7 legitimately applies to the Ai andBi chosen above.

In conclusion, the above calculations imply the following lemma.

Lemma 3.7 If T1 is suciently ne and is suciently close to the identity,then

(C( ~X 0))(0) =nYi=1

det(1− ti−1) det(Ki)

(−1)i+1

:



389

3.4 Geometric interpretation

We will now interpret the factors on the right side of Lemma 3.7 in terms ofMorse theory.

Notation 3.8 Suppose x; y are elements of = Z[V ]((t)), or matrices withentries in , which might depend on the choice of triangulation T1 and cellularapproximation . We write

x yif x − y = O(tk), where k can be made arbitrarily large by choosing T1 su-ciently ne and suciently close to the identity.

Lemma 3.9 (a)Qn−1i=0 det(1− ti)(−1)i .

(b) Under the natural identication D ’ CN , we have

Ki @fi :

Proof (a) Let f : ~X ! R be a lift of f , and let ~ := f−1(0). The downwardgradient flow of f induces partially dened return maps

’k : ~! tk ~:

The denition (1) of is equivalent to

= exp

0@ Xk>0; g2V

Fix(’k t−kg−1)gtk

k

1A 2 Z[V ]((t)) = : (13)

Here Fix(s) counts xed points of the equivariant map s modulo coveringtransformations, with their Lefschetz signs.

Suppose to begin that = id. By the machinery used to prove the Lefschetzxed point theorem in [1], for each k we haveX

g2VFix(’k t−kg−1) g =

n−1Xi=0

(−1)i Tr(ki ) 2 Z[V ]: (14)

In this case we have

=n−1Yi=0

det(1− ti)(−1)i : (15)

To see this, it is enough to check that the logarithmic derivatives of both sidesare equal, which follows from equations (13) and (14).



390

In general, let H : [0; 1]! be the homotopy from id to . In [7] we deneda natural compactication Γ of the graph of ’. Using this one candene a compactied graph Γit of (H(; t)’)i) in a similar manner. Now thereexists a positive integer N such that if the cells in T1 are all contained in ballsof radius , then the homotopy H can be chosen so that dist(H(t; x); x) < Nfor all t 2 [0; 1] and x 2 . (Cf the construction of H in [23]; by carefullycontrolling each intermediate step in the homotopy, the above claim may beachieved.) Also, the set of xed points of ’i lies in the interior of Γi under thediagonal map ! by denition of Γi , and is compact as a consequenceof Assumption 1.1. It follows that for any positive integer k we can choose sothat for all i k and all t 2 [0; 1], the compactied graph Γit does not cross thediagonal in . Then equation (14) will hold up to order k , and thereforeso will equation (15).

(b) If p is a critical point of index i, then

@[D(p)] = [D(@fp)] (16)

where the brackets indicate the fundamental class of the compactication of thedescending manifold [7]. Now suppose again that = id. Recall from equation(12) that the matrix Ki sends D0(p) to a linear combination of cells of the formD0(q), where q is a critical point of index i− 1. (We will henceforth omit thehats^on D0 or F when = id, since in this case hatted and unhatted versionscan be identied.) In fact,

Ki(D0(p)) = D0(@f (p)):

To see this, note that from the denition of Mi in equation (11), we have

[D(p)] = D0(p) +1Xk=0

tk+1F(kMi(D0(p))):

(Here the initial descending manifolds D0 and initial downward flow F aredened as in section 3.1, but using ~X and ~ instead of X and .) Applyingequation (11) to this gives

@[D(p)] = Ki(D0(p)) + (terms without initial descending manifolds). (17)

Equations (16) and (17) imply that @fi = Ki when D (the domain/range ofK ) is identied with CN .

The case 6= id can be handled similarly to part (a).



391

We can now complete the proof of Theorem 1.7. Lemmas 3.9(b) and 2.7 implythat

nYi=1

det(Ki))(−1)i+1 (CN)():

Together with Lemmas 3.6, 3.7, and 3.9(a), this implies that Theorem 1.7 holdsup to order k for all k .

4 The 3dimensional case and SeibergWitten the-

ory

We now review from [7] the denition of the Morse-theoretic invariant I3 , andthe background for Conjecture 1.9 relating this invariant to SeibergWittentheory. We will then prove Theorem 1.12, relating this invariant to Turaevtorsion.

4.1 Motivation from SeibergWitten theory

Let X be a closed connected oriented smooth 3manifold with b1(X) > 0. Lets be a spin-c structure on X . This determines a U(2)bundle S ! X with aCliord action of TX on S . A section of S and a connection A on det(S)satisfy the SeibergWitten equations with perturbation ! if, in the notation of[9],

6 @A = 0;(FA) = i( ; ) + i(!):

The SeibergWitten invariant SW(s) counts solutions to these equations mod-ulo gauge equivalence. (For more on 3dimensional SeibergWitten invariantssee eg [13, 14, 18].)

Let us choose the perturbation to be ! = rdf , where f : X ! S1 is harmonic, denotes the Hodge star, and r is a real number. By perturbing the metric,we may arrange that f is a Morse function. Away from the critical points, thespinor bundle S splits into eigenspaces of Cliord multiplication by df ,

S = E (E ⊗K−1); (18)

where K−1 := Ker(df : TX ! R).



392

Taking r !1, one expects that for a SeibergWitten solution the zero set ofthe E component of to become parallel to rf . (The energy of the SeibergWitten solution will be concentrated along this zero set. For detailed analysissee [24] and its sequels.) This suggests that SW(s) counts unions of closedorbits and flow lines of rf starting and ending at critical points, whose totalhomology class is Poincare dual to c1(E).

The above homological condition implies that in our union of closed orbits andflow lines there is precisely one flow line starting at each index 2 critical pointand ending at each index 1 critical point. (See [7]. There are no index 0 or 3critical points because f is harmonic, and there are equally many index 1 andindex 2 points because (X) = 0.) In other words, in the notation of section2.2, our union of closed orbits and flow lines lives in H1(X; v), where v = −rf .As in Remark 1.10, the counting of closed orbits is related to Taubes’ countingof pseudoholomorphic tori in symplectic 4manifolds [25], which indicates thatwe should allow closed orbits to be multiply covered when they are elliptic, butnot when they are hyperbolic.

4.2 The denition of I3

We now want to dene I3(s) to be a signed count of such unions of closedorbits and flow lines. A convenient way to do so is to use generating functionsas follows. Choose orderings of the index 1 and index 2 critical points. Let Pijdenote the set of flow lines from the ith index 2 point to the jth index 1 point.Dene the path matrix P by

P ij :=Xγ2Pij

(γ)[γ]:

Here [γ] 2 H1(X; v−1(0)) is the homology class of γ (oriented downward), and(γ) is the sign of γ as in section 2.1. The entries of P live in the Novikovring of the relative homology group H1(X; v−1(0)) with grading given by minusintersection number with .

Note that det(P ), regarded as a Zvalued function on H1(X; v−1(0)), is sup-ported on H1(X; v). Also, the subset of Nov(H1(X; v−1(0))) consisting of func-tions supported on H1(X; v) is a submodule. A generating function countingunions of closed orbits and flow lines of the type we want is now given by

Iv3 := det(P ) 2 Nov(H1(X; v)):

In the above equation, ‘’ denotes the action. The closed orbits are countedcorrectly as a result of the product formula for the zeta function (2).



393

Let jv : Spinc(X) ! H1(X; v) denote the map that sends a spin-c structureto the PoincareLefschetz dual of c1(E), where E is the line bundle dened in(18). The map jv is an H1equivariant isomorphism.

Denition 4.1 [7] Regarding Iv3 as a function H1(X; v)! Z, we dene

I3 := Iv3 jv : Spinc(X)! Z:

This denition makes sense for any Morse function f : X ! S1 with no index 0or 3 critical points, even if f is not harmonic. The calculation below will showthat I3 does not depend on f , except that there is a global sign ambiguity inI3 due to the orientation choices we made. (Also I3 depends on the sign of 2 H1(X;Z) when b1(X) = 1.)

4.3 Relation with Turaev torsion

To relate I3 to torsion, we note that the isomorphism iv jv : Spinc(X) !Eul(X) does not depend on v . It follows that there is a canonical isomorphism : Spinc(X)! Eul(X). (This isomorphism was rst dened by Turaev [27] ina dierent but equivalent way. The inverse map sends a smooth Euler structurerepresented by a nonsingular vector eld u to the spin-c structure whose spinbundle is Cu u? with a standard Cliord action.) In summary, we have thefollowing commutative triangle:

H1(X; v) iv−! Eul(X)jv- %

Spinc(X)

Proof of Theorem 1.12 It is enough to show that

Iv3 i−1v = T (X; o) : Eul(X)! Z:

By Theorem 1.7 and the denition of T , this is equivalent to asserting that forsome orientation choice, we have

det(P )(i−1v ()) = (CN)()(0)

for all Euler structures . If 0 is a reference Euler structure and γ := i−1v (0) 2

H1(X; v), then this equation is equivalent to

det(P )(+ γ) = (CN)(0)() : H1 ! Z:

This last equation follows from the denition of (CN) and Example 2.9.



394

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