Lectures on Symplectic Geometry


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1 Lectures on Symplectic Geometry Ana Cannas da Silva 1 revised January 2006 Published by SpringerVerlag as number 1764 of the series Lecture Notes in Mathematics. The original publication is available at 1 or
2 Foreword These notes approximately transcribe a 15week course on symplectic geometry I taught at UC Berkeley in the Fall of The course at Berkeley was greatly inspired in content and style by Victor Guillemin, whose masterly teaching of beautiful courses on topics related to symplectic geometry at MIT, I was lucky enough to experience as a graduate student. I am very thankful to him! That course also borrowed from the 1997 Park City summer courses on symplectic geometry and topology, and from many talks and discussions of the symplectic geometry group at MIT. Among the regular participants in the MIT informal symplectic seminar 9396, I would like to acknowledge the contributions of Allen Knutson, Chris Woodward, David Metzler, Eckhard Meinrenken, Elisa Prato, Eugene Lerman, Jonathan Weitsman, Lisa Jeffrey, Reyer Sjamaar, Shaun Martin, Stephanie Singer, Sue Tolman and, last but not least, Yael Karshon. Thanks to everyone sitting in Math 242 in the Fall of 1997 for all the comments they made, and especially to those who wrote notes on the basis of which I was better able to reconstruct what went on: Alexandru Scorpan, Ben Davis, David Martinez, Don Barkauskas, Ezra Miller, Henrique Bursztyn, JohnPeter Lund, Laura De Marco, Olga Radko, Peter Přibík, Pieter Collins, Sarah Packman, Stephen Bigelow, Susan Harrington, Tolga Etgü and Yi Ma. I am indebted to Chris Tuffley, Megumi Harada and Saul Schleimer who read the first draft of these notes and spotted many mistakes, and to Fernando Louro, Grisha Mikhalkin and, particularly, João Baptista who suggested several improvements and careful corrections. Of course I am fully responsible for the remaining errors and imprecisions. The interest of Alan Weinstein, Allen Knutson, Chris Woodward, Eugene Lerman, JiangHua Lu, Kai Cieliebak, Rahul Pandharipande, Viktor Ginzburg and Yael Karshon was crucial at the last stages of the preparation of this manuscript. I am grateful to them, and to Michèle Audin for her inspiring texts and lectures. Finally, many thanks to Faye Yeager and Debbie Craig who typed pages of messy notes into neat L A TEX, to João Palhoto Matos for his technical support, and to Catriona Byrne, Ina Lindemann, Ingrid März and the rest of the SpringerVerlag mathematics editorial team for their expert advice. Ana Cannas da Silva Berkeley, November 1998 and Lisbon, September 2000 v
3 CONTENTS vii Contents Foreword v Introduction 1 I Symplectic Manifolds 3 1 Symplectic Forms SkewSymmetric Bilinear Maps Symplectic Vector Spaces Symplectic Manifolds Symplectomorphisms Homework 1: Symplectic Linear Algebra 8 2 Symplectic Form on the Cotangent Bundle Cotangent Bundle Tautological and Canonical Forms in Coordinates CoordinateFree Definitions Naturality of the Tautological and Canonical Forms Homework 2: Symplectic Volume 13 II Symplectomorphisms 15 3 Lagrangian Submanifolds Submanifolds Lagrangian Submanifolds of T X Conormal Bundles Application to Symplectomorphisms Homework 3: Tautological Form and Symplectomorphisms 20 4 Generating Functions Constructing Symplectomorphisms Method of Generating Functions Application to Geodesic Flow Homework 4: Geodesic Flow 27
4 viii CONTENTS 5 Recurrence Periodic Points Billiards Poincaré Recurrence III Local Forms 35 6 Preparation for the Local Theory Isotopies and Vector Fields Tubular Neighborhood Theorem Homotopy Formula Homework 5: Tubular Neighborhoods in R n 41 7 Moser Theorems Notions of Equivalence for Symplectic Structures Moser Trick Moser Relative Theorem DarbouxMoserWeinstein Theory Darboux Theorem Lagrangian Subspaces Weinstein Lagrangian Neighborhood Theorem Homework 6: Oriented Surfaces 50 9 Weinstein Tubular Neighborhood Theorem Observation from Linear Algebra Tubular Neighborhoods Application 1: Tangent Space to the Group of Symplectomorphisms Application 2: Fixed Points of Symplectomorphisms IV Contact Manifolds Contact Forms Contact Structures Examples First Properties Homework 7: Manifolds of Contact Elements 61
5 CONTENTS ix 11 Contact Dynamics Reeb Vector Fields Symplectization Conjectures of Seifert and Weinstein V Compatible Almost Complex Structures Almost Complex Structures Three Geometries Complex Structures on Vector Spaces Compatible Structures Homework 8: Compatible Linear Structures Compatible Triples Compatibility Triple of Structures First Consequences Homework 9: Contractibility Dolbeault Theory Splittings Forms of Type (l, m) JHolomorphic Functions Dolbeault Cohomology Homework 10: Integrability 82 VI Kähler Manifolds Complex Manifolds Complex Charts Forms on Complex Manifolds Differentials Homework 11: Complex Projective Space Kähler Forms Kähler Forms An Application Recipe to Obtain Kähler Forms Local Canonical Form for Kähler Forms Homework 12: The FubiniStudy Structure 96
6 x CONTENTS 17 Compact Kähler Manifolds Hodge Theory Immediate Topological Consequences Compact Examples and Counterexamples Main Kähler Manifolds VII Hamiltonian Mechanics Hamiltonian Vector Fields Hamiltonian and Symplectic Vector Fields Classical Mechanics Brackets Integrable Systems Homework 13: Simple Pendulum Variational Principles Equations of Motion Principle of Least Action Variational Problems Solving the EulerLagrange Equations Minimizing Properties Homework 14: Minimizing Geodesics Legendre Transform Strict Convexity Legendre Transform Application to Variational Problems Homework 15: Legendre Transform 125 VIII Moment Maps Actions OneParameter Groups of Diffeomorphisms Lie Groups Smooth Actions Symplectic and Hamiltonian Actions Adjoint and Coadjoint Representations Homework 16: Hermitian Matrices 132
7 CONTENTS xi 22 Hamiltonian Actions Moment and Comoment Maps Orbit Spaces Preview of Reduction Classical Examples Homework 17: Coadjoint Orbits 139 IX Symplectic Reduction The MarsdenWeinsteinMeyer Theorem Statement Ingredients Proof of the MarsdenWeinsteinMeyer Theorem Reduction Noether Principle Elementary Theory of Reduction Reduction for Product Groups Reduction at Other Levels Orbifolds Homework 18: Spherical Pendulum 152 X Moment Maps Revisited Moment Map in Gauge Theory Connections on a Principal Bundle Connection and Curvature Forms Symplectic Structure on the Space of Connections Action of the Gauge Group Case of Circle Bundles Homework 19: Examples of Moment Maps Existence and Uniqueness of Moment Maps Lie Algebras of Vector Fields Lie Algebra Cohomology Existence of Moment Maps Uniqueness of Moment Maps Homework 20: Examples of Reduction 168
8 xii CONTENTS 27 Convexity Convexity Theorem Effective Actions Examples Homework 21: Connectedness 174 XI Symplectic Toric Manifolds Classification of Symplectic Toric Manifolds Delzant Polytopes Delzant Theorem Sketch of Delzant Construction Delzant Construction Algebraic SetUp The ZeroLevel Conclusion of the Delzant Construction Idea Behind the Delzant Construction Homework 22: Delzant Theorem DuistermaatHeckman Theorems DuistermaatHeckman Polynomial Local Form for Reduced Spaces Variation of the Symplectic Volume Homework 23: S 1 Equivariant Cohomology 197 References 199 Index 207
9 Introduction The goal of these notes is to provide a fast introduction to symplectic geometry. A symplectic form is a closed nondegenerate 2form. A symplectic manifold is a manifold equipped with a symplectic form. Symplectic geometry is the geometry of symplectic manifolds. Symplectic manifolds are necessarily evendimensional and orientable, since nondegeneracy says that the top exterior power of a symplectic form is a volume form. The closedness condition is a natural differential equation, which forces all symplectic manifolds to being locally indistinguishable. (These assertions will be explained in Lecture 1 and Homework 2.) The list of questions on symplectic forms begins with those of existence and uniqueness on a given manifold. For specific symplectic manifolds, one would like to understand the geometry and the topology of special submanifolds, the dynamics of certain vector fields or systems of differential equations, the symmetries and extra structure, etc. Two centuries ago, symplectic geometry provided a language for classical mechanics. Through its recent huge development, it conquered an independent and rich territory, as a central branch of differential geometry and topology. To mention just a few key landmarks, one may say that symplectic geometry began to take its modern shape with the formulation of the Arnold conjectures in the 60 s and with the foundational work of Weinstein in the 70 s. A paper of Gromov [49] in the 80 s gave the subject a whole new set of tools: pseudoholomorphic curves. Gromov also first showed that important results from complex Kähler geometry remain true in the more general symplectic category, and this direction was continued rather dramatically in the 90 s in the work of Donaldson on the topology of symplectic manifolds and their symplectic submanifolds, and in the work of Taubes in the context of the SeibergWitten invariants. Symplectic geometry is significantly stimulated by important interactions with global analysis, mathematical physics, lowdimensional topology, dynamical systems, algebraic geometry, integrable systems, microlocal analysis, partial differential equations, representation theory, quantization, equivariant cohomology, geometric combinatorics, etc. As a curiosity, note that two centuries ago the name symplectic geometry did not exist. If you consult a major English dictionary, you are likely to find that symplectic is the name for a bone in a fish s head. However, as clarified in [105], the word symplectic in mathematics was coined by Weyl [110, p.165] who substituted the Latin root in complex by the corresponding Greek root, in order to label the symplectic group. Weyl thus avoided that this group connote the complex numbers, and also spared us from much confusion that would have arisen, had the name remained the former one in honor of Abel: abelian linear group. This text is essentially the set of notes of a 15week course on symplectic geometry with 2 hourandahalf lectures per week. The course targeted secondyear graduate students in mathematics, though the audience was more diverse, including advanced undergraduates, postdocs and graduate students from other departments. The present text should hence still be appropriate for a secondyear graduate course or for an independent study project. 1
10 2 INTRODUCTION There are scattered short exercises throughout the text. At the end of most lectures, some longer guided problems, called homework, were designed to complement the exposition or extend the reader s understanding. Geometry of manifolds was the basic prerequisite for the original course, so the same holds now for the notes. In particular, some familiarity with de Rham theory and classical Lie groups is expected. As for conventions: unless otherwise indicated, all vector spaces are real and finitedimensional, all maps are smooth (i.e., C ) and all manifolds are smooth, Hausdorff and second countable. Here is a brief summary of the contents of this book. Parts IIII explain classical topics, including cotangent bundles, symplectomorphisms, lagrangian submanifolds and local forms. Parts IVVI concentrate on important related areas, such as contact geometry and Kähler geometry. Classical hamiltonian theory enters in Parts VIIVIII, starting the second half of this book, which is devoted to a selection of themes from hamiltonian dynamical systems and symmetry. Parts IXXI discuss the moment map whose preponderance has been growing steadily for the past twenty years. There are by now excellent references on symplectic geometry, a subset of which is in the bibliography. However, the most efficient introduction to a subject is often a short elementary treatment, and these notes attempt to serve that purpose. The author hopes that these notes provide a taste of areas of current research, and will prepare the reader to explore recent papers and extensive books in symplectic geometry, where the pace is much faster.
11 Part I Symplectic Manifolds A symplectic form is a 2form satisfying an algebraic condition nondegeneracy and an analytical condition closedness. In Lectures 1 and 2 we define symplectic forms, describe some of their basic properties, introduce the first examples, namely evendimensional euclidean spaces and cotangent bundles. 1 Symplectic Forms 1.1 SkewSymmetric Bilinear Maps Let V be an mdimensional vector space over R, and let Ω : V V R be a bilinear map. The map Ω is skewsymmetric if Ω(u, v) = Ω(v, u), for all u, v V. Theorem 1.1 (Standard Form for Skewsymmetric Bilinear Maps) Let Ω be a skewsymmetric bilinear map on V. Then there is a basis u 1,..., u k, e 1,..., e n, f 1,..., f n of V such that Ω(u i, v) = 0, for all i and all v V, Ω(e i, e j ) = 0 = Ω(f i, f j ), for all i, j, and Ω(e i, f j ) = δ ij, for all i, j. Remarks. 1. The basis in Theorem 1.1 is not unique, though it is traditionally also called a canonical basis. 2. In matrix notation with respect to such basis, we have Ω(u, v) = [ u ] Id 0 Id 0 v. Proof. This induction proof is a skewsymmetric version of the GramSchmidt process. Let U := {u V Ω(u, v) = 0 for all v V }. Choose a basis u 1,..., u k of U, and choose a complementary space W to U in V, V = U W. 3
12 4 1 SYMPLECTIC FORMS Take any nonzero e 1 W. Then there is f 1 W such that Ω(e 1, f 1 ) 0. Assume that Ω(e 1, f 1 ) = 1. Let W 1 = span of e 1, f 1 W1 Ω = {w W Ω(w, v) = 0 for all v W 1 }. Claim. W 1 W Ω 1 = {0}. Suppose that v = ae 1 + bf 1 W 1 W1 Ω. } 0 = Ω(v, e 1 ) = b 0 = Ω(v, f 1 ) = a = v = 0. Claim. W = W 1 W Ω 1. Suppose that v W has Ω(v, e 1 ) = c and Ω(v, f 1 ) = d. Then v = ( cf 1 + de 1 ) + (v + cf 1 de 1 ) } {{ } } {{ } W 1 W Ω 1. Go on: let e 2 W Ω 1, e 2 0. There is f 2 W Ω 1 such that Ω(e 2, f 2 ) 0. Assume that Ω(e 2, f 2 ) = 1. Let W 2 = span of e 2, f 2. Etc. This process eventually stops because dim V <. We hence obtain V = U W 1 W 2... W n where all summands are orthogonal with respect to Ω, and where W i has basis e i, f i with Ω(e i, f i ) = 1. The dimension of the subspace U = {u V Ω(u, v) = 0, for all v V } does not depend on the choice of basis. = k := dim U is an invariant of (V, Ω). Since k + 2n = m = dim V, = n is an invariant of (V, Ω); 2n is called the rank of Ω. 1.2 Symplectic Vector Spaces Let V be an mdimensional vector space over R, and let Ω : V V R be a bilinear map. Definition 1.2 The map Ω : V V is the linear map defined by Ω(v)(u) = Ω(v, u). The kernel of Ω is the subspace U above. Definition 1.3 A skewsymmetric bilinear map Ω is symplectic (or nondegenerate) if Ω is bijective, i.e., U = {0}. The map Ω is then called a linear symplectic structure on V, and (V, Ω) is called a symplectic vector space.
13 1.2 Symplectic Vector Spaces 5 The following are immediate properties of a linear symplectic structure Ω: Duality: the map Ω : V V is a bijection. By the standard form theorem, k = dim U = 0, so dim V = 2n is even. By Theorem 1.1, a symplectic vector space (V, Ω) has a basis e 1,..., e n, f 1,..., f n satisfying Ω(e i, f j ) = δ ij and Ω(e i, e j ) = 0 = Ω(f i, f j ). Such a basis is called a symplectic basis of (V, Ω). We have [ ] 0 Id Ω(u, v) = [ u ] v, Id 0 where the symbol v represents the column of coordinates of the vector v with respect to a symplectic basis e 1,..., e n, f 1,..., f n whereas [ v ] represents its transpose line. Not all subspaces W of a symplectic vector space (V, Ω) look the same: A subspace W is called symplectic if Ω W the span of e 1, f 1 is symplectic. is nondegenerate. For instance, A subspace W is called isotropic if Ω W 0. For instance, the span of e 1, e 2 is isotropic. Homework 1 describes subspaces W of (V, Ω) in terms of the relation between W and W Ω. The prototype of a symplectic vector space is (R 2n, Ω 0 ) with Ω 0 such that the basis {}}{ e 1 = (1, 0,..., 0),..., e n = (0,..., 0, 1, 0,..., 0), f 1 = (0,..., 0, }{{} 1, 0,..., 0),..., f n = (0,..., 0, 1), n+1 is a symplectic basis. The map Ω 0 on other vectors is determined by its values on a basis and bilinearity. n Definition 1.4 A symplectomorphism ϕ between symplectic vector spaces (V, Ω) and (V, Ω ) is a linear isomorphism ϕ : V V such that ϕ Ω = Ω. (By definition, (ϕ Ω )(u, v) = Ω (ϕ(u), ϕ(v)).) If a symplectomorphism exists, (V, Ω) and (V, Ω ) are said to be symplectomorphic.
14 6 1 SYMPLECTIC FORMS The relation of being symplectomorphic is clearly an equivalence relation in the set of all evendimensional vector spaces. Furthermore, by Theorem 1.1, every 2ndimensional symplectic vector space (V, Ω) is symplectomorphic to the prototype (R 2n, Ω 0 ); a choice of a symplectic basis for (V, Ω) yields a symplectomorphism to (R 2n, Ω 0 ). Hence, nonnegative even integers classify equivalence classes for the relation of being symplectomorphic. 1.3 Symplectic Manifolds Let ω be a de Rham 2form on a manifold M, that is, for each p M, the map ω p : T p M T p M R is skewsymmetric bilinear on the tangent space to M at p, and ω p varies smoothly in p. We say that ω is closed if it satisfies the differential equation dω = 0, where d is the de Rham differential (i.e., exterior derivative). Definition 1.5 The 2form ω is symplectic if ω is closed and ω p is symplectic for all p M. If ω is symplectic, then dim T p M = dim M must be even. Definition 1.6 A symplectic manifold is a pair (M, ω) where M is a manifold and ω is a symplectic form. Example. Let M = R 2n with linear coordinates x 1,..., x n, y 1,..., y n. The form n ω 0 = dx i dy i is symplectic as can be easily checked, and the set { ( ) ( ) ( ) ( ) },...,,,..., x 1 p x n p y 1 p y n p is a symplectic basis of T p M. Example. Let M = C n with linear coordinates z 1,..., z n. The form ω 0 = i n dz k d z k 2 i=1 k=1 is symplectic. In fact, this form equals that of the previous example under the identification C n R 2n, z k = x k + iy k. Example. Let M = S 2 regarded as the set of unit vectors in R 3. Tangent vectors to S 2 at p may then be identified with vectors orthogonal to p. The standard symplectic form on S 2 is induced by the inner and exterior products: ω p (u, v) := p, u v, for u, v T p S 2 = {p}. This form is closed because it is of top degree; it is nondegenerate because p, u v 0 when u 0 and we take, for instance, v = u p.
15 1.4 Symplectomorphisms Symplectomorphisms Definition 1.7 Let (M 1, ω 1 ) and (M 2, ω 2 ) be 2ndimensional symplectic manifolds, and let ϕ : M 1 M 2 be a diffeomorphism. Then ϕ is a symplectomorphism if ϕ ω 2 = ω 1. 1 We would like to classify symplectic manifolds up to symplectomorphism. The Darboux theorem (proved in Lecture 8 and stated below) takes care of this classification locally: the dimension is the only local invariant of symplectic manifolds up to symplectomorphisms. Just as any ndimensional manifold looks locally like R n, any 2ndimensional symplectic manifold looks locally like (R 2n, ω 0 ). More precisely, any symplectic manifold (M 2n, ω) is locally symplectomorphic to (R 2n, ω 0 ). Theorem 8.1 (Darboux) Let (M, ω) be a 2ndimensional symplectic manifold, and let p be any point in M. Then there is a coordinate chart (U, x 1,..., x n, y 1,..., y n ) centered at p such that on U n ω = dx i dy i. i=1 A chart (U, x 1,..., x n, y 1,..., y n ) as in Theorem 8.1 is called a Darboux chart. By Theorem 8.1, the prototype of a local piece of a 2ndimensional symplectic manifold is M = R 2n, with linear coordinates (x 1,..., x n, y 1,..., y n ), and with symplectic form n ω 0 = dx i dy i. i=1 1 Recall that, by definition of pullback, at tangent vectors u, v T pm 1, we have (ϕ ω 2 ) p(u, v) = (ω 2 ) ϕ(p) (dϕ p(u), dϕ p(v)).
16 Homework 1: Symplectic Linear Algebra Given a linear subspace Y of a symplectic vector space (V, Ω), its symplectic orthogonal Y Ω is the linear subspace defined by Y Ω := {v V Ω(v, u) = 0 for all u Y }. 1. Show that dim Y + dim Y Ω = dim V. Hint: What is the kernel and image of the map 2. Show that (Y Ω ) Ω = Y. V Y = Hom(Y, R)? v Ω(v, ) Y 3. Show that, if Y and W are subspaces, then Y W W Ω Y Ω. 4. Show that: Y is symplectic (i.e., Ω Y Y V = Y Y Ω. is nondegenerate) Y Y Ω = {0} 5. We call Y isotropic when Y Y Ω (i.e., Ω Y Y 0). Show that, if Y is isotropic, then dim Y 1 2 dim V. 6. We call Y coisotropic when Y Ω Y. Check that every codimension 1 subspace Y is coisotropic. 7. An isotropic subspace Y of (V, Ω) is called lagrangian when dim Y = 1 2 dim V. Check that: Y is lagrangian Y is isotropic and coisotropic Y = Y Ω. 8. Show that, if Y is a lagrangian subspace of (V, Ω), then any basis e 1,..., e n of Y can be extended to a symplectic basis e 1,..., e n, f 1,..., f n of (V, Ω). Hint: Choose f 1 in W Ω, where W is the linear span of {e 2,..., e n}. 9. Show that, if Y is a lagrangian subspace, (V, Ω) is symplectomorphic to the space (Y Y, Ω 0 ), where Ω 0 is determined by the formula Ω 0 (u α, v β) = β(u) α(v). In fact, for any vector space E, the direct sum V = E E has a canonical symplectic structure determined by the formula above. If e 1,..., e n is a basis of E, and f 1,..., f n is the dual basis, then e 1 0,..., e n 0, 0 f 1,..., 0 f n is a symplectic basis for V. 8
17 2 Symplectic Form on the Cotangent Bundle 2.1 Cotangent Bundle Let X be any ndimensional manifold and M = T X its cotangent bundle. If the manifold structure on X is described by coordinate charts (U, x 1,..., x n ) with x i : U R, then at any x U, the differentials (dx 1 ) x,... (dx n ) x form a basis of Tx X. Namely, if ξ Tx X, then ξ = n i=1 ξ i(dx i ) x for some real coefficients ξ 1,..., ξ n. This induces a map T U R 2n (x, ξ) (x 1,..., x n, ξ 1,..., ξ n ). The chart (T U, x 1,..., x n, ξ 1,..., ξ n ) is a coordinate chart for T X; the coordinates x 1,..., x n, ξ 1,..., ξ n are the cotangent coordinates associated to the coordinates x 1,..., x n on U. The transition functions on the overlaps are smooth: given two charts (U, x 1,..., x n ), (U, x 1,..., x n), and x U U, if ξ Tx X, then n ξ = ξ i (dx i ) x = ( ) x i n ξ i x (dx j) x = ξ j (dx j) x i=1 i,j j j=1 where ξ j = ( ) i ξ i is smooth. Hence, T X is a 2ndimensional manifold. x i x j We will now construct a major class of examples of symplectic forms. The canonical forms on cotangent bundles are relevant for several branches, including analysis of differential operators, dynamical systems and classical mechanics. 2.2 Tautological and Canonical Forms in Coordinates Let (U, x 1,..., x n ) be a coordinate chart for X, with associated cotangent coordinates (T U, x 1,..., x n, ξ 1,..., ξ n ). Define a 2form ω on T U by ω = n dx i dξ i. i=1 In order to check that this definition is coordinateindependent, consider the 1form on T U n α = ξ i dx i. Clearly, ω = dα. i=1 Claim. The form α is intrinsically defined (and hence the form ω is also intrinsically defined). 9
18 10 2 SYMPLECTIC FORM ON THE COTANGENT BUNDLE Proof. Let (U, x 1,..., x n, ξ 1,..., ξ n ) and (U, x 1,..., x n, ξ 1,..., ξ n) be two cotangent coordinate charts. On U U, the two sets of coordinates are related by ξ j = ( ) i ξ i. Since dx j = ( x ) j i x i dx i, we have x i x j α = i ξ i dx i = j ξ jdx j = α. The 1form α is the tautological form or Liouville 1form and the 2form ω is the canonical symplectic form. The following section provides an alternative proof of the intrinsic character of these forms. 2.3 CoordinateFree Definitions Let M = T X p = (x, ξ) ξ Tx X π X x be the natural projection. The tautological 1form α may be defined pointwise by α p = (dπ p ) ξ T p M, where (dπ p ) is the transpose of dπ p, that is, (dπ p ) ξ = ξ dπ p : p = (x, ξ) T p M T p M π dπ p (dπ p ) x T x X T x X Equivalently, ( ) α p (v) = ξ (dπ p )v, for v T p M. Exercise. Let (U, x 1,..., x n ) be a chart on X with associated cotangent coordinates x 1,..., x n, ξ 1,..., ξ n. Show that on T U, α = n ξ i dx i. i=1 The canonical symplectic 2form ω on T X is defined as Locally, ω = n i=1 dx i dξ i. ω = dα. Exercise. Show that the tautological 1form α is uniquely characterized by the property that, for every 1form µ : X T X, µ α = µ. (See Lecture 3.)
19 2.4 Naturality of the Tautological and Canonical Forms Naturality of the Tautological and Canonical Forms Let X 1 and X 2 be ndimensional manifolds with cotangent bundles M 1 = T X 1 and M 2 = T X 2, and tautological 1forms α 1 and α 2. Suppose that f : X 1 X 2 is a diffeomorphism. Then there is a natural diffeomorphism f : M 1 M 2 which lifts f; namely, if p 1 = (x 1, ξ 1 ) M 1 for x 1 X 1 and ξ 1 Tx 1 X 1, then we define { x2 = f(x f (p 1 ) = p 2 = (x 2, ξ 2 ), with 1 ) X 2 and ξ 1 = (df x1 ) ξ 2, where (df x1 ) : T x 2 X 2 T x1 X 1, so f T x1 is the inverse map of (df x1 ). Exercise. Check that f is a diffeomorphism. Here are some hints: 1. f commutes; M 1 M2 π 1 π 2 f X 2 X 1 2. f : M 1 M 2 is bijective; 3. f and f 1 are smooth. Proposition 2.1 The lift f of a diffeomorphism f : X 1 X 2 pulls the tautological form on T X 2 back to the tautological form on T X 1, i.e., (f ) α 2 = α 1. Proof. At p 1 = (x 1, ξ 1 ) M 1, this identity says where p 2 = f (p 1 ). Using the following facts, Definition of f : (df ) p 1 (α 2 ) p2 = (α 1 ) p1 ( ) p 2 = f (p 1 ) p 2 = (x 2, ξ 2 ) where x 2 = f(x 1 ) and (df x1 ) ξ 2 = ξ 1. Definition of tautological 1form: (α 1 ) p1 = (dπ 1 ) p 1 ξ 1 and (α 2 ) p2 = (dπ 2 ) p 2 ξ 2. The diagram f commutes. M 1 M2 π 1 π 2 f X 2 X 1
20 12 2 SYMPLECTIC FORM ON THE COTANGENT BUNDLE the proof of ( ) is: (df ) p 1 (α 2 ) p2 = (df ) p 1 (dπ 2 ) p 2 ξ 2 = (d(π 2 f )) p 1 ξ 2 = (d(f π 1 )) p 1 ξ 2 = (dπ 1 ) p 1 (df) x 1 ξ 2 = (dπ 1 ) p 1 ξ 1 = (α 1 ) p1. Corollary 2.2 The lift f of a diffeomorphism f : X 1 X 2 is a symplectomorphism, i.e., (f ) ω 2 = ω 1, where ω 1, ω 2 are the canonical symplectic forms. In summary, a diffeomorphism of manifolds induces a canonical symplectomorphism of cotangent bundles: f : T X 1 T X 2 f : X 1 X 2 Example. Let X 1 = X 2 = S 1. Then T S 1 is an infinite cylinder S 1 R. The canonical 2form ω is the area form ω = dθ dξ. If f : S 1 S 1 is any diffeomorphism, then f : S 1 R S 1 R is a symplectomorphism, i.e., is an areapreserving diffeomorphism of the cylinder. If f : X 1 X 2 and g : X 2 X 3 are diffeomorphisms, then (g f) = g f. In terms of the group Diff(X) of diffeomorphisms of X and the group Sympl(M, ω) of symplectomorphisms of (M, ω), we say that the map Diff(X) Sympl(M, ω) f f is a group homomorphism. This map is clearly injective. Is it surjective? Do all symplectomorphisms T X T X come from diffeomorphisms X X? No: for instance, translation along cotangent fibers is not induced by a diffeomorphism of the base manifold. A criterion for which symplectomorphisms arise as lifts of diffeomorphisms is discussed in Homework 3.
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