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J-Holomorphic Curves and Quantum Cohomology J-holomorphic Curves and Quantum Cohomology by Dusa McDuff and Dietmar Salamon May 1995 Contents 1 Introduction 1 1.1 Symplectic manifolds . 1 1.2 J-holomorphic curves . 3 1.3 Moduli spaces . 4 1.4 Compactness . 5 1.5 Evaluation maps . 6 1.6 The Gromov-Witten invariants . 8 1.7 Quantum cohomology . 9 1.8 Novikov rings and Floer homology . 11 2 Local Behaviour 13 2.1 The generalised Cauchy-Riemann equation . 13 2.2 Critical points . 15 2.3 Somewhere injective curves . 18 3 Moduli Spaces and Transversality 23 3.1 The main theorems . 23 3.2 Elliptic regularity . 25 3.3 Implicit function theorem . 27 3.4 Transversality . 33 3.5 A regularity criterion . 38 4 Compactness 41 4.1 Energy . 42 4.2 Removal of Singularities . 43 4.3 Bubbling . 46 4.4 Gromov compactness . 50 4.5 Proof of Gromov compactness . 52 5 Compactification of Moduli Spaces 59 5.1 Semi-positivity . 59 5.2 The image of the evaluation map . 62 5.3 The image of the p-fold evaluation map . 65 5.4 The evaluation map for marked curves . 66 vii viii CONTENTS 6 Evaluation Maps and Transversality 71 6.1 Evaluation maps are submersions . 71 6.2 Moduli spaces of N-tuples of curves . 74 6.3 Moduli spaces of cusp-curves . 75 6.4 Evaluation maps for cusp-curves . 79 6.5 Proofs of the theorems in Sections 5.2 and 5.3 . 81 6.6 Proof of the theorem in Section 5.4 . 82 7 Gromov-Witten Invariants 89 7.1 Pseudo-cycles . 90 7.2 The invariant Φ . 93 7.3 Examples . 98 7.4 The invariant Ψ . 101 8 Quantum Cohomology 107 8.1 Witten's deformed cohomology ring . 107 8.2 Associativity and composition rules . 114 8.3 Flag manifolds . 119 8.4 Grassmannians . 123 8.5 The Gromov-Witten potential . 130 9 Novikov Rings and Calabi-Yau Manifolds 141 9.1 Multiply-covered curves . 142 9.2 Novikov rings . 144 9.3 Calabi-Yau manifolds . 148 10 Floer Homology 153 10.1 Floer's cochain complex . 153 10.2 Ring structure . 159 10.3 A comparison theorem . 160 10.4 Donaldson's quantum category . 162 10.5 Closing remark . 165 A Gluing 167 A.1 Cutoff functions . 168 A.2 Connected sums of J-holomorphic curves . 170 A.3 Weighted norms . 171 A.4 An estimate for the inverse . 173 A.5 Gluing . 176 B Elliptic Regularity 181 B.1 Sobolev spaces . 181 B.2 The Calderon-Zygmund inequality . 185 B.3 Cauchy-Riemann operators . 190 B.4 Elliptic bootstrapping . 192 Preface The theory of J-holomorphic curves has been of great importance to symplectic topologists ever since its inception in Gromov's paper [26] of 1985. Its appli- cations include many key results in symplectic topology: see, for example, Gro- mov [26], McDuff [42, 45], Lalonde{McDuff [36], and the collection of articles in Audin{Lafontaine [5]. It was also one of the main inspirations for the creation of Floer homology [18, 19, 73], and recently has caught the attention of mathe- matical physicists through the theory of quantum cohomology: see Vafa [82] and Aspinwall{Morrison [2]. Because of this increased interest on the part of the wider mathematical commu- nity, it is a good time to write an expository account of the field, which explains the main technical steps in the theory. Although all the details are available in the liter- ature in some form or other, they are rather scattered. Also, some improvements in exposition are now possible. Our account is not, of course, complete, but it is writ- ten with a fair amount of analytic detail, and should serve as a useful introduction to the subject. We develop the theory of the Gromov-Witten invariants as formu- lated by Ruan in [64] and give a detailed account of their applications to quantum cohomology. In particular, we give a new proof of Ruan{Tian's theorem [67, 68] that the quantum cup-product is associative. Many people have made useful comments which have added significantly to our understanding. In particular, we wish to thank Givental for explaining quantum cohomology, Ruan for several useful discussions and for pointing out to us the con- nection between associativity of quantum multiplication and the WDVV-equation, Taubes for his elegant contribution to Section 3.4, and especially Gang Liu for pointing out a significant gap in an earlier version of the gluing argument. We are also grateful to Lalonde for making helpful comments on a first draft of this manuscript. The first author wishes to acknowledge the hospitality of the Univer- sity of California at Berkeley, and the grant GER-9350075 under the NSF Visiting Professorship for Women program which provided partial support during some of the work on this book. Dusa McDuff and Dietmar Salamon, June 1994. ix x CONTENTS 2007 Commentary on J-holomorphic curves and Quantum Cohomology We have decided to make our first set of lecture notes on J-holomorphic curves available on the Web since, despite having a few mistakes and many omissions, it is still a readable introduction to the field. It was originally published by the AMS in 1994. The version here is the revised edition first published in May 1995. It is now out of print, replaced by the enlarged version J-holomorphic curves and Symplectic Topology (AMS 2004) that we will refer to as JHOL. Main mistakes and omissions 1. The proof the Calderon{Zygmund inequality in Appendix B.2 was simply wrong. A correct proof is given in JHOL Appendix B. 2. The proof of Gromov compactness was short, but somewhat careless. Claim (ii) on p 56 in the proof of the Gromov compactness theorem 4.4.3 is wrong; the limiting curve v might be constant. We used the claim that v is nonconstant to show that the limiting process converges after finitely many steps. To correct this one needs to take more care in the rescaling argument, namely the center of the rescaling must be chosen at a point where the function jduν j2 attains a local maximum (Step 1 in the proof of Proposition 4.7.1, p. 101 in JHOL). In this way one obtains two additional bubbling points at the center and on the boundary of the unit disc for the rescaled sequence, when the limit is constant (see (iv) in Proposition 4.7.1 and Step 4 in the proof, p. 104 in JHOL). The refined rescaling argument gives as a limit a stable map, a crucial missing ingredient in the old version. One then uses JHOL Exercise 5.1.2 to establish a bound on the number of components of the limiting stable map. 3. An omission in the Gluing proof in Appendix A. The proof of the existence of the gluing map fR of Theorem A.5.2 is basically all right, but somewhat sketchy. In particular we did not prove in detail that it is a local diffeomorphism. We claim in the middle of p. 175 that this follows from the uniqueness result in Proposition 3.3.5. While this is true, several intermediate steps are needed for a complete proof. Full details are given in JHOL Chapter 10. 4. An omission in the transversality argument. In Chapter 2 we established basic results on the structure of simple J-holomorphic curves only in the case of C1-smooth J. However, in the application in Chapter 3 we need these results for J of class C` with ` < 1. This was somewhat concealed: the statement of Proposition 3.4.1 does not make clear that the elements in the universal moduli space M`(A; J) are assumed to be simple although we used that assumption in the proof. Additions in JHOL Many of the discussions in this book are quite sketchy. In JHOL they are all fleshed out, and there are many more examples given. Here are other main additions: CONTENTS xi • The existence of Gromov{Witten invariants is established in more generality (though the argument still needs some version of the \semipositive" hypothesis). Chapter 7 discusses the axioms they satisfy and also has a completely self-contained section on the very special and important example of rational curves in projective space; • a treatment of genus zero stable curves and maps (in Appendix D and Chapter 5); • a chapter on geometric applications of genus zero J-holomorphic curves without using gluing (Chapter 9); • a discussion of Hamiltonian Floer homology and applications such as spectral in- variants, explaining the set up but without doing the basic analytic proofs (Chapter 12); • much more detailed appendices on the analysis. Also an appendix giving full details of a new proof (due to Lazzarini) on positivity of intersections. Dusa McDuff and Dietmar Salamon, October 2007. xii CONTENTS Chapter 1 Introduction The theory of J-holomorphic curves is one of the new techniques which have recently revolutionized the study of symplectic geometry, making it possible to study the global structure of symplectic manifolds. The methods are also of interest in the study of K¨ahler manifolds, since often when one studies properties of holomorphic curves in such manifolds it is necessary to perturb the complex structure to be generic. The effect of this is to ensure that one is looking at persistent rather than accidental features of these curves. However, the perturbed structure may no longer be integrable, and so again one is led to the study of curves which are holomorphic with respect to some non-integrable almost complex structure J. The present book has two aims.
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