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Introduction to Smooth Manifolds
Preface
Prerequisites
Exercises and Problems
About the Second Edition
Acknowledgments
Contents
Chapter 1: Smooth Manifolds
Topological Manifolds
Coordinate Charts
Examples of Topological Manifolds
Topological Properties of Manifolds
Connectivity
Local Compactness and Paracompactness
Fundamental Groups of Manifolds
Smooth Structures
Local Coordinate Representations
Examples of Smooth Manifolds
The Einstein Summation Convention
More Examples
Manifolds with Boundary
Smooth Structures on Manifolds with Boundary
Problems
Chapter 2: Smooth Maps
Smooth Functions and Smooth Maps
Smooth Functions on Manifolds
Smooth Maps Between Manifolds
Diffeomorphisms
Partitions of Unity
Applications of Partitions of Unity
Problems
Chapter 3: Tangent Vectors
Tangent Vectors
Geometric Tangent Vectors
Tangent Vectors on Manifolds
The Differential of a Smooth Map
Computations in Coordinates
The Differential in Coordinates
Change of Coordinates
The Tangent Bundle
Velocity Vectors of Curves
Alternative Definitions of the Tangent Space
Tangent Vectors as Derivations of the Space of Germs
Tangent Vectors as Equivalence Classes of Curves
Tangent Vectors as Equivalence Classes of n-Tuples
Categories and Functors
Problems
Chapter 4: Submersions, Immersions, and Embeddings
Maps of Constant Rank
Local Diffeomorphisms
The Rank Theorem
The Rank Theorem for Manifolds with Boundary
Embeddings
Submersions
Smooth Covering Maps
Problems
Chapter 5: Submanifolds
Embedded Submanifolds
Slice Charts for Embedded Submanifolds
Level Sets
Immersed Submanifolds
Restricting Maps to Submanifolds
Uniqueness of Smooth Structures on Submanifolds
Extending Functions from Submanifolds
The Tangent Space to a Submanifold
Submanifolds with Boundary
Problems
Chapter 6: Sard's Theorem
Sets of Measure Zero
Sard's Theorem
The Whitney Embedding Theorem
The Whitney Approximation Theorems
Tubular Neighborhoods
Smooth Approximation of Maps Between Manifolds
Transversality
Problems
Chapter 7: Lie Groups
Basic Definitions
Lie Group Homomorphisms
The Universal Covering Group
Lie Subgroups
Group Actions and Equivariant Maps
Equivariant Maps
Semidirect Products
Representations
Problems
Chapter 8: Vector Fields
Vector Fields on Manifolds
Local and Global Frames
Vector Fields as Derivations of Cinfty(M)
Vector Fields and Smooth Maps
Vector Fields and Submanifolds
Lie Brackets
The Lie Algebra of a Lie Group
Induced Lie Algebra Homomorphisms
The Lie Algebra of a Lie Subgroup
Problems
Chapter 9: Integral Curves and Flows
Integral Curves
Flows
The Fundamental Theorem on Flows
Complete Vector Fields
Flowouts
Regular Points and Singular Points
Flows and Flowouts on Manifolds with Boundary
Lie Derivatives
Commuting Vector Fields
Commuting Frames
Time-Dependent Vector Fields
First-Order Partial Differential Equations
Linear Equations
Quasilinear Equations
Problems
Chapter 10: Vector Bundles
Vector Bundles
Local and Global Sections of Vector Bundles
Local and Global Frames
Bundle Homomorphisms
Subbundles
Fiber Bundles
Problems
Chapter 11: The Cotangent Bundle
Covectors
Tangent Covectors on Manifolds
Covector Fields
Coframes
The Differential of a Function
Pullbacks of Covector Fields
Restricting Covector Fields to Submanifolds
Line Integrals
Conservative Covector Fields
Problems
Chapter 12: Tensors
Multilinear Algebra
Abstract Tensor Products of Vector Spaces
Covariant and Contravariant Tensors on a Vector Space
Symmetric and Alternating Tensors
Symmetric Tensors
Alternating Tensors
Tensors and Tensor Fields on Manifolds
Pullbacks of Tensor Fields
Lie Derivatives of Tensor Fields
Problems
Chapter 13: Riemannian Metrics
Riemannian Manifolds
Pullback Metrics
Riemannian Submanifolds
The Normal Bundle
The Riemannian Distance Function
The Tangent-Cotangent Isomorphism
Pseudo-Riemannian Metrics
Problems
Chapter 14: Differential Forms
The Algebra of Alternating Tensors
Elementary Alternating Tensors
The Wedge Product
Interior Multiplication
Differential Forms on Manifolds
Exterior Derivatives
Exterior Derivatives and Vector Calculus in R3
An Invariant Formula for the Exterior Derivative
Lie Derivatives of Differential Forms
Problems
Chapter 15: Orientations
Orientations of Vector Spaces
Orientations of Manifolds
Orientations of Hypersurfaces
Boundary Orientations
The Riemannian Volume Form
Hypersurfaces in Riemannian Manifolds
Orientations and Covering Maps
The Orientation Covering
Problems
Chapter 16: Integration on Manifolds
The Geometry of Volume Measurement
Integration of Differential Forms
Integration on Manifolds
Integration on Lie Groups
Stokes's Theorem
Manifolds with Corners
Integration on Riemannian Manifolds
Integration of Functions on Riemannian Manifolds
The Divergence Theorem
Surface Integrals
Densities
The Riemannian Density
Problems
Chapter 17: De Rham Cohomology
The de Rham Cohomology Groups
Elementary Computations
Homotopy Invariance
Computations Using Homotopy Invariance
The Mayer-Vietoris Theorem
Computations Using the Mayer-Vietoris Theorem
Degree Theory
Proof of the Mayer-Vietoris Theorem
Problems
Chapter 18: The de Rham Theorem
Singular Homology
Singular Cohomology
Smooth Singular Homology
The de Rham Theorem
Problems
Chapter 19: Distributions and Foliations
Distributions and Involutivity
Integral Manifolds and Involutivity
Involutivity and Differential Forms
The Frobenius Theorem
Foliations
Lie Subalgebras and Lie Subgroups
Overdetermined Systems of Partial Differential Equations
Problems
Chapter 20: The Exponential Map
One-Parameter Subgroups and the Exponential Map
One-Parameter Subgroups
The Exponential Map
The Closed Subgroup Theorem
Infinitesimal Generators of Group Actions
Left Actions
The Lie Correspondence
Lie's Fundamental Theorems
Normal Subgroups
The Adjoint Representation
Ideals and Normal Subgroups
Problems
Chapter 21: Quotient Manifolds
Quotients of Manifolds by Group Actions
The Quotient Manifold Theorem
Covering Manifolds
Homogeneous Spaces
Sets with Transitive Group Actions
Applications to Lie Theory
Quotient Groups
Connectivity of Lie Groups
Problems
Chapter 22: Symplectic Manifolds
Symplectic Tensors
Symplectic Structures on Manifolds
The Canonical Symplectic Form on the Cotangent Bundle
The Darboux Theorem
Hamiltonian Vector Fields
Poisson Brackets
Hamiltonian Flowouts
Contact Structures
Nonlinear First-Order PDEs
General Nonlinear Equations
Problems
Appendix A: Review of Topology
Topological Spaces
Hausdorff Spaces
Bases and Countability
Subspaces, Products, Disjoint Unions, and Quotients
Subspaces
Product Spaces
Disjoint Union Spaces
Quotient Spaces and Quotient Maps
Open and Closed Maps
Connectedness and Compactness
Locally Compact Hausdorff Spaces
Homotopy and the Fundamental Group
Covering Maps
Appendix B: Review of Linear Algebra
Vector Spaces
Bases and Dimension
Linear Maps
Change of Basis
The Determinant
Inner Products and Norms
Norms
Direct Products and Direct Sums
Appendix C: Review of Calculus
Total and Partial Derivatives
Partial Derivatives
Multiple Integrals
Integrals of Vector-Valued Functions
Sequences and Series of Functions
The Inverse and Implicit Function Theorems
Appendix D: Review of Differential Equations
Existence, Uniqueness, and Smoothness
Nonautonomous Systems
Simple Solution Techniques
Separable Equations
2x2 Constant-Coefficient Linear Systems
Partially Uncoupled Systems
References
Notation Index
Subject Index
Graduate Texts in Mathematics 218
Graduate Texts in Mathematics Series Editors: Sheldon Axler San Francisco State University, San Francisco, CA, USA Kenneth Ribet University of California, Berkeley, CA, USA Advisory Board: Colin Adams, Williams College, Williamstown, MA, USA Alejandro Adem, University of British Columbia, Vancouver, BC, Canada Ruth Charney, Brandeis University, Waltham, MA, USA Irene M. Gamba, The University of Texas at Austin, Austin, TX, USA Roger E. Howe, Yale University, New Haven, CT, USA David Jerison, Massachusetts Institute of Technology, Cambridge, MA, USA Jeffrey C. Lagarias, University of Michigan, Ann Arbor, MI, USA Jill Pipher, Brown University, Providence, RI, USA Fadil Santosa, University of Minnesota, Minneapolis, MN, USA Amie Wilkinson, University of Chicago, Chicago, IL, USA Graduate Texts in Mathematics bridge the gap between passive study and creative understanding, offering graduate-level introductions to advanced topics in mathe- matics. The volumes are carefully written as teaching aids and highlight character- istic features of the theory. Although these books are frequently used as textbooks in graduate courses, they are also suitable for individual study. For further volumes: www.springer.com/series/136
John M. Lee Introduction to Smooth Manifolds Second Edition
John M. Lee Department of Mathematics University of Washington Seattle, WA, USA ISSN 0072-5285 ISBN 978-1-4419-9981-8 DOI 10.1007/978-1-4419-9982-5 Springer New York Heidelberg Dordrecht London ISBN 978-1-4419-9982-5 (eBook) Library of Congress Control Number: 2012945172 Mathematics Subject Classification: 53-01, 58-01, 57-01 © Springer Science+Business Media New York 2003, 2013 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. Exempted from this legal reservation are brief excerpts in connection with reviews or scholarly analysis or material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. Duplication of this publication or parts thereof is permitted only under the provisions of the Copyright Law of the Publisher’s location, in its current version, and permission for use must always be obtained from Springer. Permissions for use may be obtained through RightsLink at the Copyright Clearance Center. Violations are liable to prosecution under the respective Copyright Law. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. While the advice and information in this book are believed to be true and accurate at the date of pub- lication, neither the authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty, express or implied, with respect to the material contained herein. Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com)
Preface Manifolds crop up everywhere in mathematics. These generalizations of curves and surfaces to arbitrarily many dimensions provide the mathematical context for un- derstanding “space” in all of its manifestations. Today, the tools of manifold theory are indispensable in most major subfields of pure mathematics, and are becoming increasingly important in such diverse fields as genetics, robotics, econometrics, statistics, computer graphics, biomedical imaging, and, of course, the undisputed leader among consumers (and inspirers) of mathematics—theoretical physics. No longer the province of differential geometers alone, smooth manifold technology is now a basic skill that all mathematics students should acquire as early as possible. Over the past century or two, mathematicians have developed a wondrous collec- tion of conceptual machines that enable us to peer ever more deeply into the invisi- ble world of geometry in higher dimensions. Once their operation is mastered, these powerful machines enable us to think geometrically about the 6-dimensional solu- tion set of a polynomial equation in four complex variables, or the 10-dimensional manifold of 5 5 orthogonal matrices, as easily as we think about the familiar 2-dimensional sphere in R3. The price we pay for this power, however, is that the machines are assembled from layer upon layer of abstract structure. Starting with the familiar raw materials of Euclidean spaces, linear algebra, multivariable calculus, and differential equations, one must progress through topological spaces, smooth at- lases, tangent bundles, immersed and embedded submanifolds, vector fields, flows, cotangent bundles, tensors, Riemannian metrics, differential forms, foliations, Lie derivatives, Lie groups, Lie algebras, and more—just to get to the point where one can even think about studying specialized applications of manifold theory such as comparison theory, gauge theory, symplectic topology, or Ricci flow. This book is designed as a first-year graduate text on manifold theory, for stu- dents who already have a solid acquaintance with undergraduate linear algebra, real analysis, and topology. I have tried to focus on the portions of manifold theory that will be needed by most people who go on to use manifolds in mathematical or sci- entific research. I introduce and use all of the standard tools of the subject, and prove most of its fundamental theorems, while avoiding unnecessary generalization v
vi Preface or specialization. I try to keep the approach as concrete as possible, with pictures and intuitive discussions of how one should think geometrically about the abstract concepts, but without shying away from the powerful tools that modern mathemat- ics has to offer. To fit in all of the basics and still maintain a reasonably sane pace, I have had to omit or barely touch on a number of important topics, such as complex manifolds, infinite-dimensional manifolds, connections, geodesics, curvature, fiber bundles, sheaves, characteristic classes, and Hodge theory. Think of them as dessert, to be savored after completing this book as the main course. To convey the book’s compass, it is easiest to describe where it starts and where it ends. The starting line is drawn just after topology: I assume that the reader has had a rigorous introduction to general topology, including the fundamental group and covering spaces. One convenient source for this material is my Introduction to Topological Manifolds [LeeTM], which I wrote partly with the aim of providing the topological background needed for this book. There are other books that cover sim- ilar material well; I am especially fond of the second edition of Munkres’s Topology [Mun00]. The finish line is drawn just after a broad and solid background has been established, but before getting into the more specialized aspects of any particular subject. In particular, I introduce Riemannian metrics, but I do not go into connec- tions, geodesics, or curvature. There are many Riemannian geometry books for the interested student to take up next, including one that I wrote [LeeRM] with the goal of moving expediently in a one-quarter course from basic smooth manifold theory to nontrivial geometric theorems about curvature and topology. Similar material is covered in the last two chapters of the recent book by Jeffrey Lee (no relation) [LeeJeff09], and do Carmo [dC92] covers a bit more. For more ambitious readers, I recommend the beautiful books by Petersen [Pet06], Sharpe [Sha97], and Chavel [Cha06]. This subject is often called “differential geometry.” I have deliberately avoided using that term to describe what this book is about, however, because the term ap- plies more properly to the study of smooth manifolds endowed with some extra structure—such as Lie groups, Riemannian manifolds, symplectic manifolds, vec- tor bundles, foliations—and of their properties that are invariant under structure- preserving maps. Although I do give all of these geometric structures their due (after all, smooth manifold theory is pretty sterile without some geometric applications), I felt that it was more honest not to suggest that the book is primarily about one or all of these geometries. Instead, it is about developing the general tools for working with smooth manifolds, so that the reader can go on to work in whatever field of differential geometry or its cousins he or she feels drawn to. There is no canonical linear path through this material. I have chosen an order- ing of topics designed to establish a good technical foundation in the first half of the book, so that I can discuss interesting applications in the second half. Once the first twelve chapters have been completed, there is some flexibility in ordering the remaining chapters. For example, Chapter 13 (Riemannian Metrics) can be post- poned if desired, although some sections of Chapters 15 and 16 would have to be postponed as well. On the other hand, Chapters 19–21 (Distributions and Foliations, The Exponential Map, and Quotient Manifolds, respectively) could in principle be
Preface vii inserted any time after Chapter 14, and much of the material can be covered even earlier if you are willing to skip over the references to differential forms. And the final chapter (Symplectic Manifolds) would make sense any time after Chapter 17, or even after Chapter 14 if you skip the references to de Rham cohomology. As you might have guessed from the size of the book, and will quickly confirm when you start reading it, my style tends toward more detailed explanations and proofs than one typically finds in graduate textbooks. I realize this is not to every instructor’s taste, but in my experience most students appreciate having the details spelled out when they are first learning the subject. The detailed proofs in the book provide students with useful models of rigor, and can free up class time for dis- cussion of the meanings and motivations behind the definitions as well as the “big ideas” underlying some of the more difficult proofs. There are plenty of opportuni- ties in the exercises and problems for students to provide arguments of their own. I should say something about my choices of conventions and notations. The old joke that “differential geometry is the study of properties that are invariant under change of notation” is funny primarily because it is alarmingly close to the truth. Every geometer has his or her favorite system of notation, and while the systems are all in some sense formally isomorphic, the transformations required to get from one to another are often not at all obvious to students. Because one of my central goals is to prepare students to read advanced texts and research articles in differential geometry, I have tried to choose notations and conventions that are as close to the mainstream as I can make them without sacrificing too much internal consistency. (One difference between this edition and the previous one is that I have changed a number of my notational conventions to make them more consistent with main- stream mathematical usage.) When there are multiple conventions in common use (such as for the wedge product or the Laplace operator), I explain what the alterna- tives are and alert the student to be aware of which convention is in use by any given writer. Striving for too much consistency in this subject can be a mistake, however, and I have eschewed absolute consistency whenever I felt it would get in the way of ease of understanding. I have also introduced some common shortcuts at an early stage, such as the Einstein summation convention and the systematic confounding of maps with their coordinate representations, both of which tend to drive students crazy at first, but pay off enormously in efficiency later. Prerequisites This subject draws on most of the topics that are covered in a typical undergraduate mathematics education. The appendices (which most readers should read, or at least skim, first) contain a cursory summary of prerequisite material on topology, linear algebra, calculus, and differential equations. Although students who have not seen this material before will not learn it from reading the appendices, I hope readers will appreciate having all of the background material collected in one place. Besides giving me a convenient way to refer to results that I want to assume as known, it also gives the reader a splendid opportunity to brush up on topics that were once (hopefully) understood but may have faded.
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