# A Crash Course of Floer Homology for Lagrangian Intersections

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1 A Crash Course of Floer Homology for Lagrangian Intersections Manabu AKAHO Department of Mathematics Tokyo Metropolitan University 1 Introduction There are several kinds of Floer homology: Floer homology for Lagrangian intersections [1], Floer homology for fixed points of symplectic diffeomorphisms [2], Floer homology in Yang-Mills theory [3]. Beyond them there exists Morse theory or Morse homology for infinite dimentional manifolds. In this manuscript we will explain the theory with respect to the first case. 2 Morse homology Let M be a closed manifold of finite dimension and f : M R a smooth function. We call a point p critical if and only if df p = 0. Then we can obtain a symmetric bilinear form Hf p : T p M T p M R called the Hessian. The definition of Hf p is Hf p (u, v) := n i,j=1 2 f x i x j (p)a i b j, where u = n i=1 a i ( x i ) p and v = n j=1 b j ( x j ) p. If Hf p is non-degenerate, then we call the critical point non-degenerate, and we call the number of the 1

2 2 Manabu AKAHO negative eigenvalues of ( 2 ) f (p) x i x j 1 i,j n the Morse index of f at p. We shall use µ(p) to denote the index. If all the critical points of f are non-degenerate, then we call f a Morse function. In fact, there are many Morse functions on M. Let g be a Riemannian metric on M. For f : M R there exists a unique vector field gradf on M such that g(v, gradf) = v(f). We call gradf the gradient vector field of f. For critical points p and q we consider the set M(p, q) := {x : R M ẋ = gradf, lim x(τ) = p, lim x(τ) = q}. τ τ Note that R acts on M(p, q) by (a x)(τ) := x(τ + a) for a R. We denote the quotient by ˆM(p, q). Theorem 2.1 For generic Riemannian metrics of dimension µ(p) µ(q) 1. ˆM(p, q) is a smooth manifold Moreover the following compactification theorem holds. Theorem 2.2 (1) If µ(p) µ(q) 1 = 0, then ˆM(p, q) is compact. (2) If µ(p) µ(q) 1 = 1, then we have a suitable compactification of ˆM(p, q) so that the boundary is µ(r)=µ(p) 1 ˆM(p, r) ˆM(r, q). ˆM(p, q) compatible with the compact- We can choose a suitable orientation of ification. Let C k be the Z-module whose basis elements are the critical points of Morse index k, C k := Zp. µ(p)=k We define a linear map : C k C k 1 in terms of the canonical bases by p := ˆM(p, q)q, µ(q)=k 1

3 A Crash Course of Floer Homology for Lagrangian Intersections 3 where ˆM(p, q) counts the elements of ˆM(p, q) with the orientation or sign. Then we can calculate x = ˆM(x, y)y = µ(y)=µ(x) 1 µ(y)=µ(x) 1 µ(z)=µ(y) 1 From Theorem 2.2 (2) the number is zero. µ(y)=µ(x) 1 ˆM(x, y) ˆM(y, z) ˆM(x, y) ˆM(y, z)z. Theorem = 0 and the homology is isomorphic to the singular homology of M. We call the homology Morse homology. Here is an example [4]. (Here we omit the signs. For simplicity the reader may consider the example over Z 2.) Let M be a 2-sphere and f the height function as in Figure 1. p q r s Figure 1 The indices of the critical points are µ(p) = µ(q) = 2 and µ(r) = 1 and µ(s) = 0. In this case ˆM(p, r) and ˆM(q, r) consist of one point and ˆM(r, s) consists of two points, and we can identify ˆM(p, s) with an open interval and compactify ˆM(p, s) so that the boundary is ˆM(p, r) ˆM(r, s) as in Figure 2.

4 4 Manabu AKAHO p p p r r r s s Figure 2 s We have and the homology is p = r, q = r, r = 0, s = 0, Z[p + q] Z[s] which is isomorphic to the singular homology of the sphere. 3 Symplectic preliminaries Let M be a smooth manifold and ω a 2-form on M. We call ω a symplectic form if and only if ω is closed and non-degenerate. From the non-degeneracy we can conclude that the dimension of M is even. We have the following examples of symplectic manifolds: R 2n with ω := n i=1 dx i dy i, Kähler manifolds with Kähler forms, Let X be a smooth manifold and (x 1,..., x n ) a local coordinate system. We have a local coordinate system of the cotangent bundle T X such that n i=1 y i dx i corresponds to (x 1, y 1,..., x n, y n ). Then the 2-form ω := ni=1 dx i dy i is a symplectic form on T X.

5 A Crash Course of Floer Homology for Lagrangian Intersections 5 If a diffeomorphism f : (M 1, ω 1 ) (M 2, ω 2 ) satisfies f ω 2 = ω 1, then we call f a symplectomorphism. For a smooth function H t : M [0, 1] R there exists a unique vector field X t such that ω(, X t ) = dh t. We call X t a Hamiltonian vector field. Let {φ t } 0 t 1 be the isotopy such that { d φ dt t = X t φ t, φ 0 = id. We call such an isotopy a Hamiltonian isotopy. From the definition we can conclude that φ t : M M is a symplectomorphism. Let L be an n-dimensional submanifold of M 2n. If ω T L = 0, then we call L a Lagrangian submanifold. We have the following examples of Lagrangian submanifolds: 1-dimensional submanifolds of Riemann surfaces, The zero-section 0 X of T X, Let {φ t } 0 t 1 be a Hamiltonian isotopy and L a Lagrangian submanifold. Then φ t (L) is also a Lagrangian submanifold. We will use the following theorem to calculate Floer homology. Theorem 3.1 If L is a Lagrangian submanifold, then we can choose a neighborhood N(L) of L which is symplectomorphic to a neighborhood N(0 L ) of the zero-section of T L, where L is identified with the zero-section. 4 Floer homology for Lagrangian intersections Let f : X R be a smooth function. We will consider H := f π : T X R, where π : T X X is the projection. For the (t-independent) Hamiltonian isotopy {φ t } 0 t 1 associated to H, φ 1 (0 X ) is the graph of df in T X, hence we can identify the intersection points of 0 X and φ 1 (0 X ) with the critical points of f. Moreover, if f is a Morse function, then they intersect transversally. From the Morse inequalities we can conclude

6 6 Manabu AKAHO Theorem 4.1 Let X be a closed manifold and f : X R a Morse function. Then {0 X φ 1 (0 X )} dim X i=0 rankh i (X). We shall denote by 0 X the deformation of 0 X under an isotopy. In comparison with Theorem 4.1, if 0 X and 0 X intersect transversally, then we have the following estimate, which is the best possible. {0 X 0 X} χ(x), where χ(x) is the Euler number of X. Floer proved the following theorem conjectured by Arnold, which is an extension of Theorem 4.1. Theorem 4.2 Let M be a compact symplectic manifold and L a Lagrangian submanifold. We assume D 2 u ω = 0 for u : D 2 X such that u( D 2 ) L. If L and φ 1 (L) intersect transversally, then {L φ 1 (L)} dim L i=0 rankh (L, Z 2 ). We shall denote by L the deformation of L under an isotopy. If L and L intersect transversally, then we have the following estimate, which is the best possible. {L L } χ(l), where χ(l) is the Euler number of L. Let M be a closed symplectic manifold and L a Lagrangian submanifold. For a Hamiltonian isotopy {φ t } 0 t 1 we define Ω := {l : [0, 1] M l(0) L, l(1) φ 1 (L), l is homotopic to φ t (x 0 )}, where x 0 L is a fixed point, and we denote the universal covering space of Ω by Ω. Ω := {u : [0, 1] [0, 1] M u(τ, 0) L, u(τ, 1) φ 1 (L), u(0, t) = φ t (x 0 )}/homotopy. We introduce a function F : Ω R F (u) := 1 0 dτ 1 0 dt ω ( u t, u ). τ

7 A Crash Course of Floer Homology for Lagrangian Intersections 7 Lemma 4.3 Assume D 2 u ω = 0 for u : D 2 X such that u( D 2 ) L. If u 0 (1, t) = u 1 (1, t), then F (u 0 ) = F (u 1 ). From this lemma we can regard F as a function on Ω, which is our Morse function on an infinite dimensional manifold. The tangent space T l Ω at l Ω is T l Ω = {ξ(t) l T M ξ(0) T l(0) L, ξ(1) T l(1) φ 1 (L)}. (Strictly speaking, we need Sobolev spaces to define infinite dimensional manifolds.) Before calculating the gradient vector field of F, we note the following fact. Lemma 4.4 There exist Riemannian metrics g and almost complex structures J such that g(u, v) = ω(u, Jv), g(u, v) = g(ju, Jv). We will use t-dependent g t and J t satisfying the above conditions. We define a metric on Ω by (ξ 1, ξ 2 ) := Then we can calculate 1 0 (df ) l (ξ) = g t (ξ 1 (t), ξ 2 (t))dt, ξ 1, ξ 2 T l Ω. = = = ( ) 1 dl ω 0 dt, ξ dt ( ) 1 dl ω 0 dt, J t( J t ξ) dt ( ) dl dt, J tξ ( ) dl J t dt, ξ. dl Hence, gradf = J t. (Strictly speaking J dt t dl is not an element of T dt lω because of the boundary conditions.) Moreover, we can conclude that (df ) l = 0 if and only if dl = 0, which implies that l is a constant map to L φ dt 1(L). Then we will consider the following sets, for p and q L φ 1 (L): { } u M(p, q) := u : R Ω = gradf, lim u(τ, [0, 1]) = p, lim τ u(τ, [0, 1]) = q. τ τ

8 8 Manabu AKAHO Note that R acts on M(p, q) by (a u)(τ, t) := u(τ +a, t) for a R. We denote the quotient by ˆM(p, q). We can express u = gradf as τ u τ + J(u(τ, t)) u t = 0, which is a non-linear elliptic partial differential equation. Theorem 4.5 Assume that L and φ 1 (L) intersect transversally. We can assign a number µ(p) to each p L φ 1 (L), and for generic t-dependent almost complex structures the ˆM(p, q) are smooth manifolds of dimension µ(p) µ(q) 1. The transversality of L φ 1 (L) corresponds to the non-degeneracy of critical points of Morse functions. Moreover the following compactification theorem holds. Theorem 4.6 Assume D 2 u ω = 0 for u : D 2 X such that u( D 2 ) L. (1) If µ(p) µ(q) 1 = 0, then ˆM(p, q) is compact. (2) If µ(p) µ(q) 1 = 1, then we have a suitable compactification of ˆM(p, q) so that the boundary is ˆM(p, r) ˆM(r, q). µ(r)=µ(p) 1 We will construct an analogue of Morse homology. Here we will use coefficients in Z 2. (To construct the chain complex over Z we need suitable orientations of ˆM(p, q).) Let C k be the Z 2 -vector space over the intersection points of L and φ 1 (L) of µ(p) = k, C k := Z 2 p. µ(p)=k We define a linear map : C k C k 1 in terms of the canonical bases by p := ˆM(p, q)q, µ(q)=k 1 where ˆM(p, q) counts the elements of ˆM(p, q) modulo 2. Then we can calculate x = ˆM(x, y)y = µ(y)=µ(x) 1 µ(y)=µ(x) 1 µ(z)=µ(y) 1 ˆM(x, y) ˆM(y, z)z.

9 A Crash Course of Floer Homology for Lagrangian Intersections 9 From Theorem 4.6 (2) µ(y)=µ(x) 1 ˆM(x, y) ˆM(y, z) is zero modulo 2. Theorem = 0. We call the homology Floer homology for Lagrangian submanifolds L and φ 1 (L). Although we used J t to construct Floer homology, the following theorem holds. Theorem 4.8 There is an isomorphism of vector spaces between Floer homologies for generic J t and J t. Moreover the Floer homologies for (L, φ 1 (L)) and (L, φ 1(L)) are isomorphic. To calculate Floer homology we can choose convenient Hamiltonian isotopies from Theorem 4.8. If H t is small enough, then φ 1 (L) is in N(L), where N(L) is the neighborhood as in Theorem 3.1. Moreover we choose H t so that we can identify φ 1 (L) with the graph of dh in N(0 L ), where h is a Morse function. We shall use f to denote h in the following lemma. Lemma 4.9 We consider a metric on X which, on N(L), is induced by a metric g on L, and also almost complex structures J on X which, on N(L), maps the vertical tangent vectors to horizontal tangent vectors with respect to the Levi-Civita connection of g. If x : R L satisfies then dx dτ = gradf, x τ + J t(x(τ, t)) x t = 0, where x(τ, t) := φ t (x(τ)) and J t := φ t Jφ 1 t. We can identify L φ 1 (L) with the set of critical points of f and the boundary operator of Floer homology with the one of the Morse homology over Z 2. Theorem 4.10 The Floer homology for L and φ 1 (L) is isomorphic to the singular homology of L over Z 2.

10 10 Manabu AKAHO Hence {L φ 1 (L)} = the number of the generators of the Floer s chain complex for L and φ 1 (L) the rank of the Floer homology for L and φ 1 (L) = the rank of the Morse homology of L over Z 2 = the rank of the singular homology of L over Z 2. This completes the proof of Theorem 4.1. References [1] A. Floer, Morse theory for Lagrangian intersections, J. Diff. Geom. 28, No. 3, , (1988). [2] A. Floer, Symplectic fixed points and holomorphic spheres, Comm. Math. Phys. 120, , (1989). [3] A. Floer, An instanton invariant for three manifolds, Comm. Math. Phys. 118, , (1989). [4] K. Fukaya, Gauge Theory and Topology, (in Japanese) Springer-Verlag Tokyo, (1995).

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