Relative Gromov - Witten invariants
نویسندگان
چکیده
We define relative Gromov-Witten invariants of a symplectic manifold relative to a codimension-two symplectic submanifold. These invariants are the key ingredients in the symplectic sum formula of [IP4]. The main step is the construction of a compact space of ‘V -stable’ maps. Simple special cases include the Hurwitz numbers for algebraic curves and the enumerative invariants of Caporaso and Harris. Gromov-Witten invariants are invariants of a closed symplectic manifold (X,ω). To define them, one introduces a compatible almost complex structure J and a perturbation term ν, and considers the maps f : C → X from a genus g complex curve C with n marked points which satisfy the pseudoholomorphic map equation ∂f = ν and represent a class A = [f ] ∈ H2(X). The set of such maps, together with their limits, forms the compact space of stable maps Mg,n(X,A). For each stable map, the domain determines a point in the Deligne-Mumford space Mg,n of curves, and evaluation at each marked point determines a point in X. Thus there is a map Mg,n(X,A) → Mg,n ×X . (0.1) The Gromov-Witten invariant of (X,ω) is the homology class of the image for generic (J, ν). It depends only on the isotopy class of the symplectic structure. By choosing bases of the cohomologies of Mg,n and X n, the GW invariant can be viewed as a collection of numbers that count the number of stable maps satisfying constraints. In important cases these numbers are equal to enumerative invariants defined by algebraic geometry. In this article we construct Gromov-Witten invariants for a symplectic manifold (X,ω) relative to a codimension two symplectic submanifold V . These invariants are designed for use in formulas describing how GW invariants ∗The research of both authors was partially supported by the N.S.F. The first author was also supported by a Sloan Research Fellowship. 46 ELENY-NICOLETA IONEL AND THOMAS H. PARKER behave under symplectic connect sums along V — an operation that removes V from X and replaces it with an open symplectic manifold Y with the symplectic structures matching on the overlap region. One expects the stable maps into the sum to be pairs of stable maps into the two sides which match in the middle. A sum formula thus requires a count of stable maps in X that keeps track of how the curves intersect V . Of course, before speaking of stable maps one must extend J and ν to the connect sum. To ensure that there is such an extension we require that the pair (J, ν) be ‘V -compatible’ as defined in Section 3. For such pairs, V is a J-holomorphic submanifold — something that is not true for generic (J, ν). The relative invariant gives counts of stable maps for these special V -compatible pairs. These counts are different from those associated with the absolute GW invariants. The restriction to V -compatible (J, ν) has repercussions. It means that pseudo-holomorphic maps f : C → V into V are automatically pseudo-holomorphic maps into X. Thus for V -compatible (J, ν), stable maps may have domain components whose image lies entirely in V . This creates problems because such maps are not transverse to V . Worse, the moduli spaces of such maps can have dimension larger than the dimension of Mg,n(X,A). We circumvent these difficulties by restricting attention to the stable maps which have no components mapped entirely into V . Such ‘V -regular’ maps intersect V in a finite set of points with multiplicity. After numbering these points, the space of V -regular maps separates into components labeled by vectors s = (s1, . . . , sl), where l is the number of intersection points and sk is the multiplicity of the kth intersection point. In Section 4 it is proved that each (irreducible) component Mg,n,s(X,A) of V -regular stable maps is an orbifold; its dimension depends on g, n,A and on the vector s. The next step is to construct a space that records the points where a V -regular map intersects V and records the homology class of the map. There is an obvious map from Mg,n,s(X,A) to H2(X)×V l that would seem to serve this purpose. However, to be useful for a connect sum gluing theorem, the relative invariant should record the homology class of the curve in X \V rather than in X. These are additional data: two elements of H2(X \V ) represent the same element of H2(X) if they differ by an element of the set R ⊂ H2(X \ V ) of rim tori (the name refers to the fact that each such class can be represented by a torus embedded in the boundary of a tubular neighborhood of V ). The subtlety is that this homology information is intertwined with the intersection data, and so the appropriate homology-intersection data form a covering space HV X of H2(X)× V l with fiber R. This is constructed in Section 5. We then come to the key step of showing that the space MV of V -regular maps carries a fundamental homology class. For this we construct an orbifold compactification of MV — the space of V -stable maps. Since MV is a union RELATIVE GROMOV-WITTEN INVARIANTS 47 of open components of different dimensions the appropriate compactification is obtained by taking the closure of Mg,n,s(X,A) separately for each g, n,A and s. This is exactly the procedure one uses to decompose a reducible variety into its irreducible components. However, since we are not in the algebraic category, this closure must be defined via analysis. The required analysis is carried out in Sections 6 and 7. There we study the sequences (fn) of V -regular maps using an iterated renormalization procedure. We show that each such sequence limits to a stable map f with additional structure. The basic point is that some of the components of such limit maps have images lying in V , but along each component in V there is a section ξ of the normal bundle of V satisfying an elliptic equation DNξ = 0; this ξ ‘remembers’ the direction from which the image of that component came as it approached V . The components which carry these sections are partially ordered according to the rate at which they approach V as fn → f . We call the stable maps with this additional structure ‘V -stable maps’. For each g, n,A and s the V -stable maps form a space M V g,n,s(X,A) which compactifies the space of V -regular maps by adding frontier strata of (real) codimension at least two. This last point requires that (J, ν) be V -compatible. In Section 3 we show that for V -compatible (J, ν) the operator DN commutes with J . Thus ker DN , when nonzero, has (real) dimension at least two. This ultimately leads to the proof in Section 7 that the frontier of the space of V -stable maps has codimension at least two. In contrast, for generic (J, ν) the space of V -stable maps is an orbifold with boundary and hence does not carry a fundamental homology class. The endgame is then straightforward. The space of V -stable maps comes with a map M V g,n,s(X,A) → Mg,n+l(s) ×X n ×H X (0.2) and relative invariants are defined in exactly the same way that the GW invariants are defined from (0.1). The new feature is the last factor, which allows us to control how the images of the maps intersect V . Thus the relative invariants give counts of V -stable maps with constraints on the complex structure of the domain, the images of the marked points, and the geometry of the intersection with V . Section 1 describes the space of stable pseudo-holomorphic maps into a symplectic manifold, including some needed features that are not yet in the literature. These are used in Section 2 to define the GW invariants for symplectic manifolds and the associated invariants, which we call GT invariants, that count possible disconnected curves. We then bring in the symplectic submanifold V and develop the ideas described above. Sections 3 and 4 begin 48 ELENY-NICOLETA IONEL AND THOMAS H. PARKER with the definition of V -compatible pairs and proceed to a description of the structure of the space of V -regular maps. Section 5 introduces rim tori and the homology-intersection space HV X . For clarity, the construction of the space of V -stable maps is separated into two parts. Section 6 contains the analysis required for several special cases with increasingly complicated limit maps. The proofs of these cases establish all the analytic facts needed for the general case while avoiding the notational burden of delineating all ways that sequences of maps can degenerate. The key argument is that of Proposition 6.6, which is essentially a parametrized version of the original renormalization argument of [PW]. With this analysis in hand, we define general V -stable maps in Section 7, prove the needed tranversality results and give the general dimension count showing that the frontier has sufficiently large codimension. In Section 8 the relative invariants are defined and shown to depend only on the isotopy class of the symplectic pair (X,V ). The final section presents three specific examples relating the relative invariants to some standard invariants of algebraic geometry and symplectic topology. Further applications are given in [IP4]. The results of this paper were announced in [IP3]. Related results are being developed by by Eliashberg and Hofer [E] and Li and Ruan [LR]. Eliashberg and Hofer consider symplectic manifolds with contact boundary and assume that the Reeb vector field has finitely many simple closed orbits. When our case is viewed from that perspective, the contact manifold is the unit circle bundle of the normal bundle of V and all of its circle fibers – infinitely many – are closed orbits. In their first version, Li and Ruan also began with contact manifolds, but the approach in the most recent version of [LR] is similar to that of [IP3]. The relative invariants we define in this paper are more general then those of [LR] and appear, at least a priori, to give different gluing formulas.
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