# Lecture III: Neighbourhoods

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1 Lecture III: Neighbourhoods Jonathan Evans 7th October 2010 Jonathan Evans () Lecture III: Neighbourhoods 7th October / 18

2 Jonathan Evans () Lecture III: Neighbourhoods 7th October / 18 In this lecture we will see some of the local flexibility of symplectic manifolds. We will prove that: if you deform a symplectic form in its cohomology class then you get a diffeomorphic symplectic form. a point has a neighbourhood symplectomorphic to a neighbourhood of 0 in the standard symplectic vector space. a symplectic submanifold has a neighbourhood symplectomorphic to a neighbourhood of the zero section in its symplectic normal bundle (which we ll define). a symplectic isotopy of subsets which extends to some neighbourhood can (under purely topological assumptions) be extended to a global symplectic isotopy (which we ll define).

3 We begin with some basic remarks about cohomology. Here (X, ω) is a closed symplectic manifold. Symplectic forms are closed 2-forms. Therefore they are cocycles for the de Rham complex. kth De Rham cohomology is the group of closed k-forms divided by the group of exact k-forms. Since ω n is a nonvanishing volume form it represents a nonzero class in H 2n (X ; R). In particular [ω] 0 H 2 (X ; R). This gives an elementary obstruction to a closed manifold being symplectic: it must have nonvanishing b 2 = dim H 2 (X ; R). 1 In the noncompact case this is no longer the case. Recall that T M is naturally symplectic and the symplectic form is exact ω = dλ where λ is the canonical 1-form, in particular [ω] = 0. 1 Give me an example of a compact, orientable, even-dimensional manifold which doesn t admit a symplectic form. Jonathan Evans () Lecture III: Neighbourhoods 7th October / 18

4 Jonathan Evans () Lecture III: Neighbourhoods 7th October / 18 Moser s argument Consider a smooth family of symplectic forms ω t on a fixed compact manifold X and suppose that d dt [ω t] = 0. Then d dt ω t = dσ t. Remark We can actually choose σ t to vary smoothly with t. To see this, pick a metric and recall from Hodge theory that the operator d : Ω 1 Ω 2 has an adjoint d : Ω 2 Ω 1 and that d imd : imd dω 1 is an isomorphism. We can pick σ t to be the unique antiderivative for ω in the image of d.

5 Jonathan Evans () Lecture III: Neighbourhoods 7th October / 18 Now define X t to be the vector field such that Cartan s formula implies that ι Xt ω t = σ t L Xt ω t = dι Xt ω t = dσ t = ω t so if we let φ t be the time-t flow of X t then d dt φ t ω t = φ t L Xt ω t + φ t ω t = 0

6 Jonathan Evans () Lecture III: Neighbourhoods 7th October / 18 This argument implies Theorem (Moser) If ω t is a family of symplectic forms on a compact a manifold X such that the cohomology class of ω t is constant then there is a family of diffeomorphisms φ t such that φ t ω t = ω 0. a Where did we use compactness? Consider the space Ω(X ) of all symplectic forms on X and the action of the group Diff(X ). Then the space Ω(X )/Diff(X ) is called the moduli space of symplectic forms on X. Theorem The moduli space of symplectic forms is a b 2 (X )-dimensional manifold. Proof. The map ω [ω] is a local homeomorphism: Moser s theorem implies local injectivity; local surjectivity follows because the condition for a 2-form to be symplectic is an open condition on the space of closed 2-forms.

7 Jonathan Evans () Lecture III: Neighbourhoods 7th October / 18 Although we know it s a finite-dimensional manifold, not much else is known about the topology of M(X ). Smith and Vidussi have given some examples where it is disconnected. Taubes has proved that for CP 2 the map M(X ) H 2 (CP 2 ; R) = R is globally injective and has image R \ {0}.

8 Jonathan Evans () Lecture III: Neighbourhoods 7th October / 18 Neighbourhood theorems We ll now use Moser-style arguments to prove a number of other theorems. The most general result we ll prove is: Theorem Let X be a compact manifold, Q X a compact submanifold and ω 0, ω 1 closed 2-forms on X which are equal and nondegenerate on TX Q. Then there exist neighbourhoods N 0 and N 1 of Q and a diffeomorphism ψ : N 0 N 1 which is the identity on Q and such that ψ ω 1 = ω 0.

9 Jonathan Evans () Lecture III: Neighbourhoods 7th October / 18 Proof We ll construct a 1-form σ on a neighbourhood N 0 which vanishes on TX Q and such that dσ is the 2-form τ := ω 1 ω 0. The family ω t = ω 0 + tτ is a family of symplectic forms on some neighbourhood N 0 N 0 (since nondegeneracy is an open condition). Now we can form a vector field X t such that ι Xt ω t = σ. The time-t flow of this (ψ t ) exists on some subneighbourhood N 0 N 0 and by Moser s argument ψt ω t = ω 0. Since X t = 0 on Q, the time-1 flow ψ 1 is the relevant diffeomorphism for the theorem.

10 Jonathan Evans () Lecture III: Neighbourhoods 7th October / 18 To construct the form σ, we fix a Riemannian metric on X and look at the exponential map T Q X which is a diffeomorphism onto its image when restricted to a small radius ɛ neighbourhood N 0 of the zero-section. Further, define the map φ t (exp(q, v)) = exp(q, vt) for t [0, 1]. This satisfies φ 0 τ = 0 (since φ 0 collapses everything onto Q where the forms agree) and φ 1 τ = τ (since φ 1 is the identity). For t > 0, φ t is a diffeomorphism generated by the vector field X t = φ t (φ t ) 1.

11 Jonathan Evans () Lecture III: Neighbourhoods 7th October / 18 We have d dt (φ t τ) = φ t L Xt τ = d (φ t ι Xt τ) = dσ t Here, at the point exp(q, v) the 1-form σ t is given by σ t (V ) = τ( φ t, (φ t ) V ) so it s well-defined and smooth even at t = 0. Also, since φ t fixes Q for all t, σ t vanishes on Q = {exp(q, 0) : q Q}. Now τ = φ 1τ φ 0τ = 1 so σ = 1 0 σ tdt is the form we want. 0 d dt (φ t τ)dt = d 1 0 σ t dt

12 Jonathan Evans () Lecture III: Neighbourhoods 7th October / 18 Corollary (Darboux s theorem) Let x be a point in a 2n-dimensional symplectic manifold X. Then there exists an r > 0 and a neighbourhood of x diffeomorphic to the radius r ball in the standard symplectic vector space R 2n. Proof. Apply the theorem to the case Q = {x}. Notice that this theorem justifies our claim in the first lecture that a symplectic manifold can be described equivalently as a manifold with a closed nondegenerate 2-form or by an atlas of symplectic charts (the symplectic charts are just Darboux neighbourhoods).

13 Jonathan Evans () Lecture III: Neighbourhoods 7th October / 18 Symplectic submanifolds Let ι : Q X be the embedding of a symplectic submanifold, i.e. ι ω is a symplectic form on Q. The normal bundle of Q is νq := TX Q /TQ, but since ι T q Q is a symplectic subspace of T ι(q) X, T ι(q) X /ι T q Q is naturally identified with the symplectic orthogonal complement (ι T q Q) ω. In particular, it has a natural fibrewise symplectic form η = ω (ι TqQ) ω. We can therefore talk about the symplectic normal bundle of a symplectic submanifold Q.

14 Jonathan Evans () Lecture III: Neighbourhoods 7th October / 18 Corollary (Symplectic neighbourhood theorem) Let ι 1 : Q 1 X 1 and ι 2 : Q 2 X 2 be symplectic submanifolds which are symplectomorphic (φ : Q 1 Q 2 ) and which have a symplectic identification of their symplectic normal bundles Φ : νq 1 νq 2 living over φ. Then Q 1 has a neighbourhood symplectomorphic to a neighbourhood of Q 2. Proof. Fix metrics on X 1 and X 2 and consider the exponential map exp i : νq i X i. Let ω i be the symplectic form on X i. Set ϖ 0 = ω 1 and ϖ 1 = (exp1 1 ) Φ exp 2 ω 2 on neighbourhoods of Q 1 and apply the theorem.

15 Isotopies Let s now consider isotopies of submanifolds Q. For us, an isotopy is a family ι t : Q X of embeddings which is smooth in the C -topology on maps 2. An isotopy of a submanifold can be extended to a family of diffeomorphisms of the ambient manifold. Symplectically, an easy extension of the symplectic neighbourhood theorem says that an isotopy of symplectic submanifolds extends to an isotopy of a neighbourhood. The same will be true for Lagrangian submanifolds when we get to those. But when can we extend an isotopy of an open set to a global family of ambient symplectomorphisms? 2 The topology where a sequence of maps φ k converges to φ if the maps and all their derivatives converge pointwise. Jonathan Evans () Lecture III: Neighbourhoods 7th October / 18

16 Jonathan Evans () Lecture III: Neighbourhoods 7th October / 18 Theorem (Banyaga s symplectic isotopy extension theorem) Let ι t : Q X be an isotopy of subsets a of a symplectic manifold (X, ω) such that there is a symplectic isotopy b ι t : U X of neighbourhoods U Q extending ι t, H 2 (X, Q; R) = 0. Then there is a family ψ t Symp(X, ω) such that ψ t U = ι t. a We d better require these subsets to be deformation retracts of their neighbourhoods, but that s all. b d i.e. dt ι t ω = 0.

17 Before we go into details, recall that relative cohomology H (X, Q; R) can be defined as the cohomology of the subcomplex of the de Rham complex whose cochains are differential forms vanishing on a fixed neighbourhood of Q. Given a metric, for any 2-form τ vanishing on this neighbourhood of Q there is a canonical antiderivative σ (also vanishing on this neighbourhood) from relative Hodge theory. Jonathan Evans () Lecture III: Neighbourhoods 7th October / 18

18 Jonathan Evans () Lecture III: Neighbourhoods 7th October / 18 Proof. Let φ t be an extension of ι t to a family of diffeomorphisms of X. Write ω t = φ t ω. Since ι t is a symplectic isotopy, ω t vanishes in U and hence defines a relative cocycle. The cohomological condition now implies that there is a relative antiderivative σ t and by Hodge theory we can pick it varying smoothly in t. Now use the Moser argument to produce diffeomorphisms (fixing ι t (U)) which correct φ t by making it a symplectomorphism. Exercise : In fact, if Q is a symplectic submanifold, you don t need the cohomological condition: you can construct an antiderivative by hand. Prove this. If you get stuck, look up the proof in Denis Auroux s paper Asymptotically holomorphic families of symplectic submanifolds (GAFA 1997, vol.7, ), section 4.2.

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