Upload
others
View
7
Download
0
Embed Size (px)
Citation preview
Geometry & Topology GGGGGGGG G GG
GGGG
T TTTTTT
TTTTTTTT
Volume 3 (1999) 369ā396Published: 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 Ļ(X) = 0, and let f : X ā S1 be a circle-valued Morse function. We define 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 defined byTuraev [28].
We proved a similar result in our previous paper [7], but the present paper re-fines this by separating closed orbits and flow lines according to their homologyclasses. (Previously we only considered their intersection numbers with a fixedlevel 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 SeibergāWitteninvariant of X . Our result, together with this conjecture, implies that theSeibergāWitten invariant equals the Turaev torsion. This was conjectured byTuraev [28] and refines the theorem of Meng and Taubes [14].
AMS Classification numbers Primary: 57R70
Secondary: 53C07, 57R19, 58F09
Keywords: MorseāNovikov complex, Reidemeister torsion, SeibergāWitteninvariants
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 different homology classes are related to the Reidemeister torsion of theunderlying manifold. For example, Fried [2] defined a ātwisted Lefschetz zetafunctionā 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 fixed 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 defined a weaker version of I and showed that itequals a form of topological Reidemeister torsion. Later we received a preprintfrom Turaev [28] defining a refined version of the latter invariant, which we callāTuraev torsionā here. Along similar lines we can refine 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 sufficient to prove this refinement, so herewe introduce a different and simpler approach. This paper is independent of [7],except that the latter paper defines certain compactifications in Morse theorywhich we use here, and also provides more background and context.
We now proceed to define our invariant I more precisely and state our maintheorem. We then describe the application to three-dimensional SeibergāWittentheory. In section 2 we give some background definitions, and in section 3 weprove the main theorem. In section 4 we give more details on the relation toSeibergāWitten theory.
1.1 Statement of results
The basic setup for this paper is as follows. Let X be a closed connectedoriented nādimensional manifold. We assume throughout that Ļ(X) = 0, sothat we can define Reidemeister torsion. Also, our result is most interestingwhen b1(X) > 0.
Michael Hutchings and Yi-Jen Lee
Geometry and Topology, Volume 3 (1999)
370
Let f : X ā 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 āf 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 Ī³ : S1 ā X with Ī³ā²(t) = āĪ»āf forsome Ī» > 0. We declare two closed orbits to be equivalent if they differ byreparametrization. The period p(Ī³) is the largest integer p such that Ī³ factorsthrough a pāfold covering S1 ā S1 . A closed orbit is nondegenerate if det(1ādĻ(x)) 6= 0, where Ļ is the pth return map at a point x ā Ī³(S1) ā X . If so,the Lefschetz sign Īµ(Ī³) is the sign of this determinant.
Notation 1.2 Let H1 := H1(X). Let Īø ā H1(X;Z) denote the pullback byf of the āupwardā generator of H1(S1;Z).
Let Ī = Nov(H1,āĪø) denote the Novikov ring [17, 5], consisting of functionsH1 ā Z that are finitely supported on the set h ā H1 | āĪø(h) ā¤ C foreach C ā R. This ring has the obvious addition, and the convolution prod-uct. We denote a function a : H1 ā Z by the (possibly infinite) formal sumā
hāH1a(h) Ā· h .
Definition 1.3 [2, 20] We count closed orbits with the zeta function, whichis a function H1 ā Z defined, in the above notation, by
Ī¶ := exp
āĪ³āO
Īµ(Ī³)p(Ī³)
[Ī³]
. (1)
Here O denotes the set of closed orbits, and [Ī³] := Ī³ā[S1] is the homology classof Ī³ in H1(X).
A compactness argument using Assumption 1.1, together with the observationthat āĪø([Ī³]) > 0 for all Ī³ , shows that Ī¶ ā ĪāQ.
We remark that there is also a product formula [3, 7, 8]
Ī¶ =āĪ³āI
(1ā (ā1)iā [Ī³])ā(ā1)i0 . (2)
Circle-valued Morse theory and Reidemeister torsion
Geometry and Topology, Volume 3 (1999)
371
Here I denotes the set of irreducible (period 1) closed orbits, and iā(Ī³) andi0(Ī³) denote the numbers of real eigenvalues of the return map in the intervals(āā,ā1) and (ā1, 1), respectively. Equation (2) shows that in fact Ī¶ ā Ī,ie, Ī¶ has integer coefficients. A third formula for the zeta function, in terms offixed 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 definitionsare 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 Cā(X) over Z[H1]. The Reidemeister torsion of this complexis an element of Q(Z[H1])/ Ā±H1 , where Q(R) denotes the total quotient ringof R. The Ā±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 anāEuler structureā. The space Eul(X) of Euler structures is a natural affinespace 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 ofā
iHi(X;Q) (see [26]). We can then define the Turaev torsion
Ļ(X; o) : Eul(X)ā Q(Z[H1]). (3)
This is an H1āequivariant map which does not depend on the triangulation. Wewrite Ļ(X):=Ā±Ļ(X, o); this is an H1āequivariant map Eul(X)ā Q(Z[H1])/Ā±1 .
Example 1.4 If X is the 3āmanifold obtained by zero surgery on a knotK ā S3 , then for a suitable Euler structure Ī¾ ,
Ļ(X)(Ī¾) =Alex(K)(1ā t)2
where Alex(K) ā Z[t, tā1]/ Ā± 1 is the Alexander polynomial of K and t is agenerator of H1(X) ' Z.
On the Morse theory side, the Novikov complex CNā is a chain complex overthe Novikov ring Ī, 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 define the Morse-theoretictorsion
Ļ(CNā) : Eul(X)ā Q(Ī)/ Ā± 1. (4)
Michael Hutchings and Yi-Jen Lee
Geometry and Topology, Volume 3 (1999)
372
Definition 1.5 Define I : Eul(X)ā Q(Ī)/Ā± 1 to be the product of the zetafunction and the Morse-theoretic torsion:
I := Ī¶ Ā· Ļ(CNā).
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 Īø .
One can prove this a priori ; see [7] for the rough idea and [6] for the details.Although this may help define 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(Ļ(X))
as maps Eul(X)ā Q(Ī)/Ā± 1.
Here i : Q(Z[H1])ā Q(Ī) is induced by the inclusion Z[H1]ā Ī.
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 fixed point theorem, cf [15, 3].
(2) The extreme case when Īø = 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 Īø ā H1(X), regarded as a map H1(X) ā Z, induces a mapĪ ā Z((t)) sending h 7ā tĪø(h) . This in turn induces a partially defined mapQ(Ī) ā 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 1āform Āµ and the Novikov ring is graded by ā[Āµ]. It seems possible toextend Theorem 1.7 to this case by approximating Āµ by closed 1āforms inrational cohomology classes, to which Theorem 1.7 applies. Another proof forclosed 1āforms is given in [6] by first proving Theorem 1.6 for closed 1āforms,and then using this to reduce to the real-valued case.
Circle-valued Morse theory and Reidemeister torsion
Geometry and Topology, Volume 3 (1999)
373
(5) Some previous papers, such as [19], studied the torsion of the Novikov com-plex (or the Whitehead torsion, which is sharper but only defined ārelativelyāunless 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 coefficient 1. This is useful for understandingthe obstructions to the existence of nonvanishing closed 1āforms [10, 4]. How-ever, the extra information in the zeta function is important for the connectionwith SeibergāWitten theory below.
(6) A homology orientation of X can apparently remove the sign ambiguityin Ļ(CNā). However we have not checked if Theorem 1.7 holds with the signambiguity removed this way.
1.2 Application to SeibergāWitten 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 SeibergāWitten invariant of X is a function
SWX,o : Spinc(X)ā Z
which counts Rāinvariant solutions to the SeibergāWitten equations on XĆR,modulo gauge equivalence. (See eg [13, 14, 18].)
Taubes [24] has shown that the SW invariant of a symplectic fourāmanifoldequals a āGromov invariantā counting pseudoholomorphic curves. In [7] weproposed that using similar analysis, one might be able to show that the SWinvariant of a 3āmanifold is equal to a Morse theory invariant
I3 : Spinc(X)ā Z.
The invariant I3 counts certain unions of closed orbits and flow lines of thegradient vector field of a Morse function f : X ā S1 with no index 0 or 3critical points. We review the definition of I3 in section 4.
Conjecture 1.9 [7] The SeibergāWitten invariant agrees with our Morse the-ory invariant:
SWX,o = Ā±I3.
(When b1(X) = 1, the SW invariant also depends on a choice of āchamberā,and in this conjecture we use the chamber determined by r ā df for r >> 0.)
Michael Hutchings and Yi-Jen Lee
Geometry and Topology, Volume 3 (1999)
374
Remark 1.10 If f has no critical points, and if we arrange for df to beharmonic, then this conjecture is a corollary of Taubesā theorem [24] appliedto the symplectic fourāmanifold (X Ć S1, df ā§ ds + āXdf). Here s denotesthe S1 coordinate. The idea is that for a suitable homology orientation, ifs ā Spinc(X), then
SWX,o(s) = SWXĆS1(Ļās) = GrXĆS1(Ļās) = I3(s).
The first equality expresses the fact that all solutions to the SW equationson X Ć S1 are S1āinvariant; see [18] for details of this equality. The secondequality is Taubesā theorem; here GrXĆS1(Ļās) counts, in the sense of [25],pseudoholomorphic curves in a certain S1āinvariant homology class in XĆS1 .An energy argument shows that for a suitable almost complex structure, everysuch curve is a union of closed orbits of āf 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 different method [22].
In another direction, Turaev [28] conjectured a combinatorial formula for theSeibergāWitten invariant as follows. If dim(X) = 3 and b1(X) > 1, then for anEuler structure Ī¾ , the torsion Ļ(X; o)(Ī¾) is actually in the group ring Z[H1].If b1 = 1, then i(Ļ(X; o)(Ī¾)) ā Ī, rather than in the quotient ring. Given ahomology orientation o, one can then define a map
T (X; o) : Eul(X) āā Z,Ī¾ 7āā i(Ļ(X; o)(Ī¾))(0).
where (0) indicates evaluation on 0 ā H1 . (This depends on the sign of [df ]when b1 = 1.) There is also a natural isomorphism ı : Spinc(X) ā Eul(X)([27], see section 4.3).
Conjecture 1.11 (Turaev [28]) The SeibergāWitten invariant agrees withthe Turaev torsion:
SWX,o = T (X; o) ı : Spinc(X)ā Z.
This statement is a refinement of the theorem of Meng and Taubes [14], whichgives an āaveragedā version of this equivalence, in which one sums over spin-cstructures that differ 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:
Circle-valued Morse theory and Reidemeister torsion
Geometry and Topology, Volume 3 (1999)
375
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 first 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 refine 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
SeibergāWitten
Acknowledgments This paper would not exist were it not for Taubesā workon SeibergāWitten 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 definition ofthe Novikov complex, which counts gradient flow lines between critical points.Section 2.2 reviews Turaevās Euler structures, which are needed for the mostrefined version of Reidemeister torsion. Section 2.3 gives the precise definitionsof 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 definitions 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
Michael Hutchings and Yi-Jen Lee
Geometry and Topology, Volume 3 (1999)
376
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 nā i respectively.
The Novikov complex (CNā, āf ) is defined as follows. On X , we can lift ourMorse function to a map f : X ā R. Let CNi denote the set of (possiblyinfinite) linear combinations Ī± of critical points of index i in X , such that foreach C ā R, the sum Ī± contains only finitely many critical points p ā X withf(p) > C . The action of H1 on the critical points by covering transformationsinduces an action of the Novikov ring Ī on CNi . In fact CNi is a free Īāmodule; one can specify a basis for CNā by choosing a lift of each critical pointin X to X .
We define āf : CNi ā CNiā1 as follows. If p, q ā X are critical points of indexi and iā 1 respectively, let ćāfp, qć denote the signed number of gradient flowlines from p to q . If p is a critical point of index i, define
āfp :=āq
ćāfp, qćq
where the sum is over all critical points q ā X of index i ā 1. We count flowlines using the sign conventions from [7]. These conventions are chosen so that(āf )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 āhalf-infiniteā chains in X :
Hā(CNā, āf ) ' Hā(Cā(X)ā Ī).
See eg [17, 19, 21, 7]. For example, if X = S1 and f : S1 ā S1 has nonzerodegree, then the homology of the Novikov complex vanishes.
2.2 Euler structures
We now discuss three different notions of āEuler structureā and how they relate.One can ignore this material at the expense of allowing an H1 ambiguity inReidemeister torsion.
Definition 2.2 (Turaev [27]) If X is a closed smooth manifold with Ļ(X) =0 and n = dim(X) > 1, a smooth Euler structure on X is a nonsingularvector field on X , where two such vector fields are declared equivalent if theirrestrictions to the complement of a ball in X are homotopic through nonsingularvector fields. We let Eul(X) denote the space of smooth Euler structures.
Circle-valued Morse theory and Reidemeister torsion
Geometry and Topology, Volume 3 (1999)
377
By obstruction theory, Eul(X) is an affine space over Hnā1(X;Ļnā1(Snā1)) =H1(X). (It is nonempty since Ļ(X) = 0.)
The following alternate definition of smooth Euler structures is useful for Morsetheory, and also works well when n = 1. If v is a vector field on X withnondegenerate zeroes, let H1(X, v) denote the set of homology classes of 1āchains Ī³ ā X with āĪ³ = vā1(0), where the points in vā1(0) are orientedin the standard way. The set H1(X, v) is a subset of the relative homologyH1(X, vā1(0)) and is an affine space over H1(X).
Definition 2.3 One can show by a cobordism argument that the spacesH1(X, v) for different v ās are canonically isomorphic to each other, and henceto a single space. We call this space Eul(X). We let iv : H1(X, v) ā Eul(X)denote the canonical isomorphism.
If n > 1, we can go from Definition 2.3 to Definition 2.2 as follows. GivenĪ³ ā H1(X, v), we can represent Ī³ by disjoint paths connecting the zeroes of vin pairs. We then construct a nonsingular vector field by cancelling the zeroes ofv in a neighborhood of Ī³ . (If v has no zeroes, we send 0 ā H1(X) = H1(X, v)to the Euler structure represented by v and extend equivariantly.)
Definition 2.4 [27] Let (X,T ) be a finite connected CWācomplex with cellsĻi. (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 Ļi and hiĻi,with hi ā H1(X) = Aut(X), are considered equivalent if
āi(ā1)dim(Ļi)hi = 0.
We let Eul(X,T ) denote the space of combinatorial Euler structures of theCWācomplex (X,T ). This is clearly an affine space over H1(X).
Note that if T is a refinement of the cell-structure T with cells Ļj, thenthere is a canonical isomorphism Eul(X,T )ā Eul(X, T ) sending Ļi to Ļj,where Ļj ā Ļi if Ļj ā Ļi .
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 ).
Michael Hutchings and Yi-Jen Lee
Geometry and Topology, Volume 3 (1999)
378
The idea is that there is a natural vector field 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 field on X . We can perturb this toa smooth vector field vT with a nondegenerate zero of sign (ā1)i in the centerof each iāsimplex. Then a smooth Euler structure Ī¾ can be represented by achain Ī³ consisting of paths connecting the zeroes in pairs, with [Ī³] = iā1
vT(Ī¾) ā
H1(X, vT ). We can lift the chain Ī³ to X , and the induced lifts of its endpointsdetermine a combinatorial Euler structure.
2.3 Reidemeister torsion
We now review the definition of Reidemeister torsion of certain chain complexes.We then use this algebra to define Reidemeister torsion for the two geometriccomplexes we are interested in.
2.3.1 Algebra
Let (Cā, ā) be a finite complex of finite dimensional vector spaces over a field F .The standard short exact sequences 0ā Zi ā Ci ā Biā1 ā 0 and 0ā Bi āZi ā Hi ā 0 induce canonical isomorphisms det(Ci) = det(Zi)ādet(Biā1) anddet(Zi) = det(Bi)ādet(Hi), where ādetā denotes top exterior power. Combiningthese isomorphisms gives an isomorphism
Ī¦:āi
det(Ci)(ā1)i āāi
det(Hi)(ā1)i . (5)
Let e be an ordered basis for Cā , ie, an ordered basis ei for each Ci . Let h bean ordered basis for Hā . Let [e] ā
āi det(Ci)(ā1)i and [h] ā
āi det(Hi)(ā1)i
denote the resulting volume forms.
Definition 2.6 We define the Reidemeister torsion
Ļ(Cā, e, h) := Ī¦([e])/[h] ā FĆ.
We also define
Ļ(Cā, e) :=Ļ(Cā, e, 1) if Hā = 0,
0 otherwise.
Usually we will be interested in Ļ rather than Ļ . In practice, we can computethe torsion Ļ in terms of an alternating product of determinants as follows.
Circle-valued Morse theory and Reidemeister torsion
Geometry and Topology, Volume 3 (1999)
379
Lemma 2.7 If Hā = 0, we can find a decomposition Cā = Aā āBā such that(i) Ai and Bi are spanned by subbases of the basis ei , and (ii) the map
ĻBiā1 ā|Ai : Ai ā Biā1
(which we abbreviate by ā : Ai ā Biā1 ) is an isomorphism. Then
Ļ(Cā, e) := Ā±āi
det(ā : Ai ā Biā1)(ā1)i .
Here the determinants are computed using the subbases of e.
We now extend the definition of torsion to complexes over certain rings whichmight not be fields.
Definition 2.8 [28] Let R be a ring, and assume that its total quotient ring(denoted by Q(R)) is a finite sum of fields, Q(R) = ājFj . Let (Cā, ā) be afinite complex of finitely generated free Rāmodules with an ordered basis e.We define
Ļ(Cā, e) :=āj
Ļ(Cā āR Fj , eā 1) āāj
Fj = Q(R).
In this case Hā(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 := hj a ābasisā forHā(C) when hj is a basis for Hā(Cā āR Fj) for each j . Given h = hj, wedefine
Ļ(Cā, e, h) :=āj
Ļ(Cā āR Fj , eā 1, hj) ā Q(R)Ć.
Example 2.9 If 0 ā C2āā C1 ā 0 is a 2āterm complex with C1 = C2 , and
if e is a basis which is identical on C1 and C2 , then
Ļ(Cā, e) = det(ā).
We are interested in the rings Z[H1] and Ī. Their quotients are finite sums offields (see eg [28]), and these decompositions are compatible with the inclusionZ[H1]ā Ī.
In section 3.2, we will need the following product formula for torsion. Let R be aring such that Q(R) is a finite sum of fields Fj . Let 0ā Cā ā C ā²ā ā C ā²ā²ā ā 0 bea short exact sequence of finite complexes of finitely generated free Rāmodules.Let e, eā², eā²ā² be bases for Cā, C ā²ā, C ā²ā²ā compatible with the exact sequence. Leth, hā², hā²ā² be bases for the homology as in Definition 2.8. Let Lā denote the longexact sequence in homology, regarded as an acyclic chain complex, and let bdenote the basis for Lā obtained by combining h, hā², hā²ā² .
Michael Hutchings and Yi-Jen Lee
Geometry and Topology, Volume 3 (1999)
380
Lemma 2.10 We have the following product formula for torsion:
Ļ(C ā², eā², hā²) = Ļ(C, e, h)Ļ (C ā²ā², eā²ā², hā²ā²)Ļ(Lā, b).
Proof This follows from [16].
2.3.2 Geometry
We now define the Turaev torsion (3). In the notation of Definition 2.4, let(X,T ) be a finite connected CWācomplex with universal abelian cover X . Lif-ing the cells gives a chain complex Cā(X, T ) over Z[H1(X)]. A combinatorialEuler structure Ī¾ determines a set of lifts of each cell to X , up to equiva-lence. Choose one set of lifts; this gives a basis for Cā(X, T ). A homologyorientation o determines an orientation of this basis, via the isomorphism (5)applied to Cā(X,T ). Let e(Ī¾, o) denote the resulting ordered basis. We de-fine the combinatorial Turaev torsion Ļ(X,T ; o) to be the H1āequivariant mapEul(X,T )ā Q(Z[H1]) given by
Ļ(X,T ; o)(Ī¾) := Ļ(Cā(X, T ), e(Ī¾, o)) ā Q(Z[H1]).
Note that the right hand side of this equation does not depend on the choiceof a set of lifts. Furthermore, Ļ(X,T ; o) = Ļ(X, T ; o) under the canonicalisomorphism Eul(X,T )ā Eul(X, T ), if T is a refinement of T .
Definition 2.11 Let X be a closed connected smooth manifold with Ļ(X) =0, with a smooth Euler structure Ī¾ ā Eul(X) and a homology orientationo. Choose a smooth triangulation T of X . Let Ī¾T ā Eul(X,T ) denote thecombinatorial Euler structure equivalent to Ī¾ via Lemma 2.5. We define theTuraev torsion
Ļ(X; o)(Ī¾) := Ļ(X,T ; o)(Ī¾T ).
The results of [27] show that the Turaev torsion does not depend on the choiceof smooth triangulation T .
We now define the Morse theoretic torsion (4). A smooth Euler structure Ī¾ canbe represented by a chain Ī³ connecting the critical points of f in pairs, with[Ī³] = iā1
āf (Ī¾) ā H1(X,āf). We can lift Ī³ to X , and the induced lifts of theendpoints determine a basis e(Ī¾) for CNā .
Definition 2.12 We define the Morse theoretic torsion Ļ(CNā) : Eul(X) āQ(Ī)/ Ā± 1 by
Ļ(CNā)(Ī¾) := Ļ(CNā, e(Ī¾)).
Circle-valued Morse theory and Reidemeister torsion
Geometry and Topology, Volume 3 (1999)
381
The map Ļ(CNā) is H1āequivariant, and again does not depend on the choiceof lifting. There is a sign ambiguity because the basis e(Ī¾) is unordered. Inthe special case when f has no critical points, we define Ļ(CNā) to be the H1
equivariant map such that Ļ(CNā)(Ī¾) = 1 for the smooth Euler structure Ī¾represented by āf .
In the future, we call two bases for Cā(X, T ) or CNā equivalent if they corre-spond to the same Euler structure Ī¾ .
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 ā S1 lifts to a real-valued Morsefunction X ā R.
Proof In this case Ī¶ = 1, so we just need to check that the Morse theoreticand topological torsions agree. This is essentially classical (cf [16]), except forthe identification of the bases determined by an Euler structure.
If Ī¾ is an Euler structure, then the bifurcation analysis in [11] shows thatĻ(CNā)(Ī¾) 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 affect the Novikov complex.
Now let T be a smooth triangulation of X . We can apparently perturb thevector field VT of section 2.2 so that it is the gradient of a Morse functionF : X ā R with respect to some metric. In this case, the Novikov complex(CNā, āF ) is identical to the chain complex (Cā(X;T ), ā). Moreover the basesdetermined by Ī¾ 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 ā² which āapproximatesāX and is adapted to the vector field āf . In section 3.2 we prove a technical
Michael Hutchings and Yi-Jen Lee
Geometry and Topology, Volume 3 (1999)
382
lemma (Lemma 3.6) asserting that X and X ā² have the same Reidemeistertorsion. The heart of the proof is in section 3.3 and section 3.4, where wedetermine the torsion of X ā² by a short computation, and then interpret theanswer geometrically to recover the invariant I .
3.1 The cell complex X ā²
Assume 0 ā S1 = R/Z is a regular value of f (by composing f with a rotationif necessary). Let Ī£ := fā1(0). Let Y be the compact manifold with boundaryobtained by cutting X along Ī£. We can write āY = Ī£1 t Ī£0 , where Ī£i iscanonically isomorphic to Ī£, and āāf points inward along Ī£1 .
We give Y a cell decomposition as follows. Let T1 be a smooth triangulationof Ī£1 such that each simplex is transverse to the ascending manifolds of thecritical points in Y . If p ā Y is a critical point, let D0(p) denote the descendingmanifold of p in Y . If Ļ ā T1 is a simplex, let F(Ļ) denote the set of ally ā Y such that upward gradient flow starting at y hits Ļ . Choose a celldecomposition T0 of Ī£0 , such that the intersections with Ī£0 of D0(p) andF(Ļ) are subcomplexes, for each critical point p and each simplex Ļ ā T1 .
Lemma 3.1 The cells in T1 and T0 , together with all the cells D0(p) andF(Ļ), give a legitimate cell decomposition, T ā²Y , of Y .
Proof Recall that D0(p) and F(Ļ) have natural compactifications using bro-ken flow lines (cf [7]). It may be shown by āinduction on heightā that thesecompactifications are homeomorphic to closed balls. There are moreover natu-ral continuous maps of the compactifications to Y which send the interiors ofthe balls homeomorphically to D0(p) and F(Ļ). The transversality conditionon T1 and Assumption 1.1(b) ensure that the boundary of a cell consists oflower dimensional cells in T ā²Y .
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 Ļ : (Ī£0, T0)ā (Ī£1, T1) be a cellular approximation to the canonicalidentification Ī£0 ā Ī£1 . Consider the mapping cylinder of Ļ:
MĻ =(Ī£0 Ć [0, 1]) t Ī£1
(x, 1) ā¼ Ļ(x).
This has a cell decomposition consisting of T0 and T1 , together with the cellsā Ć (0, 1) for each ā ā T0 . There is a canonical inclusion Ī£0 ā MĻ sendingx 7ā (x, 0), and there is also a canonical inclusion Ī£1 āMĻ .
Circle-valued Morse theory and Reidemeister torsion
Geometry and Topology, Volume 3 (1999)
383
Definition 3.2 Let X ā² be the space obtained by gluing Y and MĻ alongĪ£0 tĪ£1 .
The space X ā² inherits a cell decomposition, but for our computations we prefera simpler cell decomposition, obtained by fusing some cells together as follows.If ā is a cell in Y of the form D0(p) or F(Ļ), we define a corresponding cellin X ā² by
ā := ā āŖ ((āā ā© Ī£0)Ć [0, 1)).
Here (āā ā© Ī£0)Ć [0, 1) indicates a subset of MĻ .
Definition 3.3 Let T ā² be the cell decomposition of X ā² consisting of cells ofthe following types:
(a) D0(p) for p ā Y a critical point;
(b) simplices in T1 ;
(c) F(Ļ) for Ļ ā T1 .
3.2 X and X ā² have the same Reidemeister torsion
We now show that X and X ā² have the same Reidemeister torsion, if the Eulerstructures are compatible in an appropriate sense.
We begin by noting that Hā(X ā²) = Hā(X), and Hā(X ā²) = Hā(X) as Z[H1]āmodules, as one can see from the exact sequences (8), (9) below. Note that theuniversal abelian cover X ā² of X ā² is obtained from X by modifying a neighbor-hood of the inverse image of Ī£.
Notation 3.4 (1) If Z is a subset of X or X ā² , then Z will denote the inverseimage of Z in X or X ā² . 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 Ī¾ 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 Ī¾ā² on X ā² consists of an equivalence class of lifts of the cells T ā² to X ā² .
Definition 3.5 We say that Ī¾ and Ī¾ā² are compatible if, within these equiva-lence classes, the lifts can be chosen so that:
Michael Hutchings and Yi-Jen Lee
Geometry and Topology, Volume 3 (1999)
384
(a) The lift of each critical point p in Ī¾ is contained in the lift of the cellD0(p) in Ī¾ā² .
(b) For each simplex Ļ ā T1 , the lift of Ļ in Ī¾ā² agrees with the ātopā of thelift of F(Ļ) in Ī¾ā² .
The compatibility conditions in Definition 3.5 induce an isomorphism fromEul(X) to Eul(X ā², T ā²) as affine spaces over H1 .
Recall that Ļ(X) := Ā±Ļ(X, o); similarly write Ļ(X ā², T ā²) := Ā±Ļ(X ā², T ā², o).
Lemma 3.6 If the Euler structures Ī¾ ā Eul(X) and Ī¾ā² ā Eul(X ā², T ā²) arecompatible as above, then
Ļ(X)(Ī¾) = Ā±Ļ(X ā², T ā²)(Ī¾ā²).
Proof The strategy is to compute the torsion of X and X ā² 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 Ī£Ć[0, 1] consisting of cells āĆ0,āĆ (0, 1), and āĆ1, where ā is a lift of a simplex ā ā T1 . Also recall thatMĻ has a natural cell decomposition. We claim that with respect to these cellstructures,
Ļ (Cā(Ī£Ć [0, 1]), eĪ£, h) = Ā±Ļ(Cā(MĻ), eā²Ī£, hā²), (6)
provided that the bases eĪ£, h, eā²Ī£, h
ā² satisfy the following conditions:
(a) The bases h, hā² for homology agree under the isomorphism induced bythe canonical map Ī£Ć [0, 1]āMĻ .
(b) The bases eĪ£, eā²Ī£ are given by lifts of cells such that:
(i) The lifts of the cells in Ī£Ć 1 ā Ī£Ć [0, 1] and Ī£1 āMĻ agree.(ii) The lift of each cell ā in Ī£Ć 0 ā Ī£Ć [0, 1] or in Ī£0 Ć 0 āMĻ
is adjacent to the lift of the cell ā Ć (0, 1) in Ī£ Ć [0, 1] or MĻ
respectively.
To prove (6), we compute both sides by applying the product formula for torsionto the relative exact sequences
0ā Cā(Ī£Ć 1)ā Cā(Ī£Ć [0, 1])ā Cā(Ī£Ć [0, 1], Ī£ Ć 1)ā 0,
0ā Cā(Ī£1)ā Cā(MĻ)ā Cā(MĻ, Ī£1)ā 0.
Circle-valued Morse theory and Reidemeister torsion
Geometry and Topology, Volume 3 (1999)
385
The answers agree, because condition (a) implies that the Ļ(Lā) factors agree,condition (b(i)) implies that the Ļ(Cā(Ī£)) factors agree, and condition (b(ii))implies that
Ļ(Cā(Ī£Ć [0, 1], Ī£ Ć 1), eĪ£, 1) = Ā±Ļ(Cā(MĻ, Ī£1), eā²Ī£, 1) = Ā±1.
Step 2 Let TY be a smooth triangulation on Y whose restriction to eachcomponent of āY = Ī£0 tĪ£1 agrees with T1 . The smooth Euler structure Ī¾ onX determines an equivalence class of bases, eY , for Cā(Y , Ī£0;TY ), because TYglues to a smooth triangulation of X .
Let T ā²Y denote the cell decomposition of Y given by Lemma 3.1. The com-binatorial Euler structure Ī¾ā² on X ā² determines an equivalence class of baseseā²Y for Cā(Y , Ī£0;T ā²Y ), because the cells of T ā²Y in Y \ Ī£0 are in one to onecorrespondence with the cells of X ā² .
We claim that
Ļ(Cā(Y , Ī£0;T ā²Y ), eā²Y , hā²) = Ā±Ļ(Cā(Y , Ī£0;TY ), eY , h) (7)
provided that the bases hā² 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 ā R. Let CMā(f) denote the Morse complex of fon the covering Y . A direct computation, using the compatibility of Ī¾ and Ī¾ā² ,shows that
Ļ(Cā(Y , Ī£0;T ā²Y ), eā²Y , hā²) = Ā±Ļ(CMā(f), eā²ā², hā²ā²).
(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 eā²ā²
for CMā(f) is determined by the lifts of the critical points determined by Ī¾as before, and we assume that the bases hā², hā²ā² on homology agree under thestandard isomorphism Hā(Y , Ī£0) ' Hā(CMā(f)).
We also have
Ļ(CMā(f), eā²ā², hā²ā²) = Ā±Ļ(Cā(Y , Ī£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 Ī£0 and does not point outward along Ī£1 ,and (ii) wherever the gradient is tangent to Ī£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 CMā(FY ) = Cā(Y , Ī£0, TY ),we have Ļ(CMā(f), eā²ā², hā²ā²) = Ā±Ļ(CMā(FY ), ifF (eā²ā²), hā²ā²), where ifF (eā²ā²) is the
Michael Hutchings and Yi-Jen Lee
Geometry and Topology, Volume 3 (1999)
386
equivalence class of lifts (which correspond to bases of CMā ) induced from eā²ā²
via the homotopy from f to FY . But ifF (eā²ā²) = eY because the homotopy fromāf to āFY extends to a homotopy of vector fields on X = Y āŖ (Ī£Ć I), whichis homotopic to a homotopy from āf to the standard vector field 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 ā² .
We can regard X as the union of Y and Ī£Ć [0, 1] along Ī£0tĪ£1 . Let T denotethe cell decomposition of X obtained by gluing the triangulation TY of Y tothe product cell structure on Ī£Ć [0, 1] obtained from T1 . We then have a shortexact sequence
0ā Cā(
Ī£0 t Ī£1
)ā Cā(Ī£ Ć [0, 1]) ā Cā(Y ;TY )ā Cā(X ;T )ā 0. (8)
Let T ā² denote the āunfusedā cell decomposition of X ā² from section 3.1. Wethen have a short exact sequence
0ā Cā(
Ī£0 t Ī£1
)ā Cā(MĻ)ā Cā(Y ;T ā²Y )ā Cā(X ā²;T ā²)ā 0. (9)
We can choose representatives eY , eā²Y such that they agree on Ī£ Ć 1 and
Ī£1 with eĪ£, eā²Ī£ respectively. Let e(Ī¾) denote the basis for Cā(X ;T ) obtained
by combining the bases eĪ£, eY of Step 1 and Step 2 respectively. Similarly leteā²(Ī¾ā²) denote the basis for Cā(X ā²;T ā²) obtained by combining the bases eā²Ī£, e
ā²Y
of Step 1 and Step 2. Then e(Ī¾) is a representative of the combinatorial Eu-ler structure on (X, T ) corresponding to the smooth Euler structure Ī¾ , andeā²(Ī¾ā²) is a representative of the image of Ī¾ā² under the canonical isomorphismEul(X ā², T ā²)ā Eul(X ā², T ā²).
Applying the product formula to the above exact sequences, and using equations(6) and (7), we obtain
Ļ(Cā(X;T ), e(Ī¾), h) = Ā±Ļ(Cā(X ā²;T ā²), eā²(Ī¾ā²), hā²). (10)
Here we are assuming that the bases h, hā² for homology agree under the naturalisomorphism Hā(X) ' Hā(X ā²). Also, to apply (7) in the above computation,one relates Y to the pair (Y ,Ī£0) as in Step 1.
In particular, equation (10) implies that
Ļ(Cā(X ;T ), e(Ī¾)) = Ā±Ļ(Cā(X ā²;T ā²), eā²(Ī¾ā²)).
This implies the lemma because Ļ(Cā(X ;T ), e(Ī¾)) = Ļ(X)(Ī¾), since the in-sertion of Ī£ Ć [0, 1] changes nothing, and similarly Ļ(Cā(X ā²;T ā²), eā²(Ī¾ā²)) =Ļ(X ā², T ā²)(Ī¾ā²).
Circle-valued Morse theory and Reidemeister torsion
Geometry and Topology, Volume 3 (1999)
387
3.3 Computing the torsion
We now compute the torsion of the approximating cell complex (X ā², T ā²), for acombinatorial Euler structure Ī¾ā² compatible with a smooth Euler structure Ī¾on 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
Īø 6= 0.
Without loss of generality, we may also assume that Īø is indivisible in H1(X;Z).(If Īø is divisible by k , we can lift f to a kāfold cover of S1 without changingthe invariant I .) Let V := Ker(Īø), and choose a splitting
H1(X) = V ā Z.Let t denote the generator of the Z component with Īø(t) = ā1. Then theNovikov ring can be identified with the ring of formal Laurent series in t withcoefficients in Z[V ]:
Ī = Z[V ]((t)).
Recall that Q(Ī) is a finite sum of fields. To prove Theorem 1.7, it suffices toshow that it holds after projecting to each such field. Let K be a field com-ponent of Q(Ī). By the Novikov isomorphism (Theorem 2.1), the complexesCNā āK and Cā(X)āK have isomorphic homology. So we will assume thatthese complexes are both acyclic, since otherwise they both have zero torsionĻ , and there is nothing to prove. In all of the calculations below, we implicitlytensor everything with the field K .
We can decompose
Ci(X ā²) = Di āEi ā Fiwhere the three summands are generated by the cells of types (a), (b), and (c)respectively from Definition 3.3. Let us choose a basis e(Ī¾ā²) for Ci(X ā²) as inDefinition 3.5. We can identify
Fi ' Eiā1.
The matrix for the boundary operator on Ci(X ā²) can then be written as
āi =
Di Ei Fi
Diā1 Ni 0 Wi
Eiā1 ātMi āĪ£i 1ā tĻiā1
Fiā1 0 0 āāĪ£iā1
. (11)
Michael Hutchings and Yi-Jen Lee
Geometry and Topology, Volume 3 (1999)
388
We remark that āĪ£i is the boundary operator on Cā(Ī£). Also Ļiā1 is a matrix
with entries in Z[V ], which can be interpreted as the return map of the gradientflow from Ī£ to Ī£, after perturbation by our cellular approximation Ļ. LikewiseMi sends D0(p), where p ā X is a critical point, to a perturbation of theādescending sliceā D0(p) ā© Ī£.
Continuing the calculation, due to the acyclicity assumption we may choosedecompositions Di = DA
i ā DBi such that DA
i and DBi are spanned by (cells
corresponding to) critical points, and the differential āf induces an isomor-phism DA
i ā DBiā1 . In the notation below, we denote matrices with domain or
range Dā by boldface letters, and we denote their restrictions to DAi and/or
projections to DBiā1 by plain letters.
We now apply Lemma 2.7 with Ai = DAi ā Fi and Bi = DB
i ā Ei . (We willexplain in a moment why this choice of Ai and Bi is legitimate.) We obtain
Ļ(Cā(X ā², e(Ī¾ā²)) =nāi=1
det(Ī©i)(ā1)i+1
where
Ī©i =( DA
i Fi
DBiā1 Ni Wi
Eiā1 ātMi 1ā tĻiā1
).
We note that 1ā tĻiā1 is invertible because Ļiā1 has entries in Z[V ]. It followsthat
det(Ī©i) = det(1ā tĻiā1) det(Ki)
where
Ki := Ni + tWi(1ā tĻiā1)ā1Mi : Di ā Diā1. (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 sufficiently fine and the cellularapproximation Ļ is sufficiently close to the identity. In particular, the matricesĪ©i 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 sufficiently fine and Ļ is sufficiently close to the identity,then
Ļ(Cā(X ā²))(Ī¾ā²) =nāi=1
(det(1ā tĻiā1) det(Ki)
)(ā1)i+1
.
Circle-valued Morse theory and Reidemeister torsion
Geometry and Topology, Volume 3 (1999)
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 Ī = Z[V ]((t)), or matrices withentries in Ī, which might depend on the choice of triangulation T1 and cellularapproximation Ļ. We write
x ā yif x ā y = O(tk), where k can be made arbitrarily large by choosing T1 suffi-ciently fine and Ļ sufficiently close to the identity.
Lemma 3.9 (a)ānā1i=0 det(1ā tĻi)(ā1)i ā Ī¶ .
(b) Under the natural identification Dā ' CNā , we have
Ki ā āfi .
Proof (a) Let f : X ā R be a lift of f , and let Ī£ := fā1(0). The downwardgradient flow of f induces partially defined return maps
Ļk : Ī£ā tkĪ£.
The definition (1) of Ī¶ is equivalent to
Ī¶ = exp
āk>0, gāV
Fix(Ļk tākgā1)gtk
k
ā Z[V ]((t)) = Ī. (13)
Here Fix(s) counts fixed points of the equivariant map s modulo coveringtransformations, with their Lefschetz signs.
Suppose to begin that Ļ = id. By the machinery used to prove the Lefschetzfixed point theorem in [1], for each k we haveā
gāVFix(Ļk tākgā1) Ā· g =
nā1āi=0
(ā1)i Tr(Ļki ) ā Z[V ]. (14)
In this case we have
Ī¶ =nā1āi=0
det(1ā tĻi)(ā1)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).
Michael Hutchings and Yi-Jen Lee
Geometry and Topology, Volume 3 (1999)
390
In general, let H : Ī£Ć[0, 1]ā Ī£ be the homotopy from id to Ļ. In [7] we defineda natural compactification Ī ā Ī£ Ć Ī£ of the graph of Ļ. Using this one candefine a compactified graph Īit of (H(Ā·, t)Ļ)i) in a similar manner. Now thereexists a positive integer N such that if the cells in T1 are all contained in ballsof radius Īµ, then the homotopy H can be chosen so that dist(H(t, x), x) < NĪµfor all t ā [0, 1] and x ā Ī£. (Cf the construction of H in [23]; by carefullycontrolling each intermediate step in the homotopy, the above claim may beachieved.) Also, the set of fixed points of Ļi lies in the interior of Īi under thediagonal map Ī£ā Ī£ĆĪ£ by definition of Īi , and is compact as a consequenceof Assumption 1.1. It follows that for any positive integer k we can choose Īµ sothat for all i ā¤ k and all t ā [0, 1], the compactified graph Īit does not cross thediagonal in Ī£ Ć Ī£. 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
ā[D(p)] = [D(āfp)] (16)
where the brackets indicate the fundamental class of the compactification of thedescending manifold [7]. Now suppose again that Ļ = 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 iā 1. (We will henceforth omit thehatsĖon D0 or F when Ļ = id, since in this case hatted and unhatted versionscan be identified.) In fact,
Ki(D0(p)) = D0(āf (p)).
To see this, note that from the definition of Mi in equation (11), we have
[D(p)] = D0(p) +āāk=0
tk+1F(ĻkMi(D0(p))).
(Here the initial descending manifolds D0 and initial downward flow F aredefined as in section 3.1, but using X and Ī£ instead of X and Ī£.) Applyingequation (11) to this gives
ā[D(p)] = Ki(D0(p)) + (terms without initial descending manifolds). (17)
Equations (16) and (17) imply that āfi = Ki when Dā (the domain/range ofKā ) is identified with CNā .
The case Ļ 6= id can be handled similarly to part (a).
Circle-valued Morse theory and Reidemeister torsion
Geometry and Topology, Volume 3 (1999)
391
We can now complete the proof of Theorem 1.7. Lemmas 3.9(b) and 2.7 implythat
nāi=1
det(Ki))(ā1)i+1 ā Ļ(CNā)(Ī¾).
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 3ādimensional case and SeibergāWitten the-
ory
We now review from [7] the definition of the Morse-theoretic invariant I3 , andthe background for Conjecture 1.9 relating this invariant to SeibergāWittentheory. We will then prove Theorem 1.12, relating this invariant to Turaevtorsion.
4.1 Motivation from SeibergāWitten theory
Let X be a closed connected oriented smooth 3āmanifold with b1(X) > 0. Lets be a spin-c structure on X . This determines a U(2)ābundle S ā X with aClifford action of TX on S . A section Ļ of S and a connection A on det(S)satisfy the SeibergāWitten equations with perturbation Ļ if, in the notation of[9],
6 āAĻ = 0,Ļ(FA) = iĻ(Ļ,Ļ) + iĻ(Ļ).
The SeibergāWitten invariant SW(s) counts solutions to these equations mod-ulo gauge equivalence. (For more on 3ādimensional SeibergāWitten invariantssee eg [13, 14, 18].)
Let us choose the perturbation to be Ļ = rādf , where f : X ā S1 is harmonic,ā 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 Clifford multiplication by df ,
S = E ā (E āKā1), (18)
where Kā1 := Ker(df : TX ā R).
Michael Hutchings and Yi-Jen Lee
Geometry and Topology, Volume 3 (1999)
392
Taking r āā, one expects that for a SeibergāWitten solution the zero set ofthe E component of Ļ to become parallel to āf . (The energy of the SeibergāWitten 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 āf 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 Ļ(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 = āāf .As in Remark 1.10, the counting of closed orbits is related to Taubesā countingof pseudoholomorphic tori in symplectic 4āmanifolds [25], which indicates thatwe should allow closed orbits to be multiply covered when they are elliptic, butnot when they are hyperbolic.
4.2 The definition of I3
We now want to define 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.Define the path matrix P by
P ij :=āĪ³āPij
Īµ(Ī³)[Ī³].
Here [Ī³] ā H1(X, vā1(0)) is the homology class of Ī³ (oriented downward), andĪµ(Ī³) is the sign of Ī³ as in section 2.1. The entries of P live in the Novikovring of the relative homology group H1(X, vā1(0)) with grading given by minusintersection number with Ī£.
Note that det(P ), regarded as a Zāvalued function on H1(X, vā1(0)), is sup-ported on H1(X, v). Also, the subset of Nov(H1(X, vā1(0))) consisting of func-tions supported on H1(X, v) is a Īāsubmodule. A generating function countingunions of closed orbits and flow lines of the type we want is now given by
Iv3 := Ī¶ Ā· det(P ) ā Nov(H1(X, v)).
In the above equation, āĀ·ā denotes the Ī action. The closed orbits are countedcorrectly as a result of the product formula for the zeta function (2).
Circle-valued Morse theory and Reidemeister torsion
Geometry and Topology, Volume 3 (1999)
393
Let jv : Spinc(X) ā H1(X, v) denote the map that sends a spin-c structureto the PoincareāLefschetz dual of c1(E), where E is the line bundle defined in(18). The map jv is an H1āequivariant isomorphism.
Definition 4.1 [7] Regarding Iv3 as a function H1(X, v)ā Z, we define
I3 := Iv3 jv : Spinc(X)ā Z.
This definition makes sense for any Morse function f : X ā 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 ofĪø ā 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) āEul(X) does not depend on v . It follows that there is a canonical isomorphismĪ¹ : Spinc(X)ā Eul(X). (This isomorphism was first defined by Turaev [27] ina different but equivalent way. The inverse map sends a smooth Euler structurerepresented by a nonsingular vector field u to the spin-c structure whose spinbundle is Cuā uā„ with a standard Clifford action.) In summary, we have thefollowing commutative triangle:
H1(X, v) ivāā Eul(X)jv Ī¹
Spinc(X)
Proof of Theorem 1.12 It is enough to show that
Iv3 iā1v = Ā±T (X; o) : Eul(X)ā Z.
By Theorem 1.7 and the definition of T , this is equivalent to asserting that forsome orientation choice, we have
det(P )(iā1v (Ī¾)) = Ļ(CNā)(Ī¾)(0)
for all Euler structures Ī¾ . If Ī¾0 is a reference Euler structure and Ī³ := iā1v (Ī¾0) ā
H1(X, v), then this equation is equivalent to
det(P )(Ā·+ Ī³) = Ļ(CNā)(Ī¾0)(Ā·) : H1 ā Z.
This last equation follows from the definition of Ļ(CNā) and Example 2.9.
Michael Hutchings and Yi-Jen Lee
Geometry and Topology, Volume 3 (1999)
394
References
[1] R F Brown, The Lefschetz fixed point theorem, Scott, Foresman and Co. (1971)
[2] D Fried, Homological identities for closed orbits, Invent. Math. 71 (1983) 419ā442
[3] D Fried, Lefschetz formulas for flows, from: āThe Lefschetz centennial con-ference, part IIIā, Contemp. Math. 58, III, Amer. Math. Soc. Providence, R.I.(1987) 19ā69
[4] K Fukaya, The symplectic sācobordism conjecture: a summary, from: āGeom-etry and Physics (Aarhus, 1995)ā, Lecture Notes in Pure and Appl. Math. 184,Dekker (1997) 209ā219
[5] H Hofer, D Salamon, Floer homology and Novikov rings, from: āThe Floermemorial volumeā, Progr. Math. 133, Birkhauser, Basel (1995) 483ā524
[6] M Hutchings, Reidemeister torsion in generalized Morse theory, e-printmath.DG/9907066
[7] M Hutchings, Y-J Lee, Circle-valued Morse theory, Reidemeister torsion,and SeibergāWitten invariants of threeāmanifolds, Topology, 38 (1999) 861ā888
[8] E Ionel, T Parker, Gromov invariants and symplectic maps, e-printdg-ga/9703013
[9] P B Kronheimer, T S Mrowka, The genus of embedded surfaces in the pro-jective plane, Math. Res. Lett. (1994) 797ā806
[10] F Latour, Existence de 1āformes fermees non singulieres dans une classe decohomologie de de Rham, Inst. Hautes Etudes Sci. Publ. Math. No. 80 (1994)135ā194
[11] F Laudenbach, On the ThomāSmale complex, appendix to J-M Bismut &W Zhang, An extension of a theorem by Cheeger and Muller, Asterisque, 205(1992)
[12] Y-J Lee, Morse theory and SeibergāWitten monopoles on 3āmanifolds, HarvardUniversity thesis (1997)
[13] Y Lim, SeibergāWitten theory of 3āmanifolds, preprint (1996)
[14] G Meng, C H Taubes, SW = Milnor torsion, Math. Res. Lett. 3 (1996)661ā674
[15] J Milnor, Infinite cyclic coverings, Collected works vol. 2, Publish or Perish(1996)
[16] J Milnor, Whitehead torsion, Bull. Amer. Math. Soc. 72 (1966) 358ā426
[17] S P Novikov, Multivalued functions and functionals. An analogue of the Morsetheory, Soviet Math. Dokl. 24 (1981) 222ā226
[18] C Okonek, A Teleman, 3ādimensional SeibergāWitten invariants and non-Kahlerian geometry, Math. Ann. 312 (1998) 261ā288
Circle-valued Morse theory and Reidemeister torsion
Geometry and Topology, Volume 3 (1999)
395
[19] A Pazhitnov, On the Novikov complex for rational Morse forms, Ann. Fac.Sci. Toulouse Math. (6) 4 (1995) 297ā338
[20] A Pajitnov, Simple homotopy type of Novikov complex for closed 1āforms andLefschetz zeta functions of the gradient flow, eprint dg-ga/9706014
[21] M Pozniak, Floer homology, Novikov rings, and clean intersections, Univ. ofWarwick thesis (1994)
[22] D Salamon, SeibergāWitten invariants of mapping tori, symplectic fixed points,and Lefschetz numbers, from: āProceedings of Gokova GeometryāTopology Con-ference 1998ā, (S Akbulut, T Onder and R Stern, editors), International Press(1999) 117ā143
[23] E Spanier, Algebraic topology, SpringerāVerlag corrected reprint (1989)
[24] C H Taubes, The SeibergāWitten and Gromov invariants, Math. Res. Lett. 2(1995) 221ā238
[25] C H Taubes, Counting pseudo-holomorphic submanifolds in dimension 4, J.Differential Geom. 44 (1996) 818ā893
[26] V Turaev, Reidemeister torsion in knot theory, Russian Math. Surveys 41(1986) 119ā182
[27] V Turaev, Euler structures, nonsingular vector fields, and torsions of Reide-meister type, Math. USSR-Izv. 34 (1990) 627ā662
[28] V Turaev, Torsion invariants of Spinc structures on 3āmanifolds, Math. Res.Lett. 4 (1997) 679ā695
[29] V Turaev, A combinatorial formulation for the SeibergāWitten invariants of3āmanifolds, Math. Res. Lett. 5 (1998) 583ā598
[30] A Weil, Numbers of solutions of equations in finite fields, Bull. Amer. Math.Soc. 55 (1949) 497ā508
Michael Hutchings and Yi-Jen Lee
Geometry and Topology, Volume 3 (1999)
396