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Graduate Texts in Mathematics 171 Editorial Board S. Axler K.A. Ribet
Graduate Texts in Mathematics 1 TAKEUTI/ZARING. Introduction to Axiomatic Set Theory. 2nd ed. 2 OXTOBY. Measure and Category. 2nd ed. 3 SCHAEFER. Topological Vector Spaces. 2nd ed. 4 HILTON/STAMMBACH. A Course in Homological Algebra. 2nd ed. 5 MAC LANE. Categories for the Working Mathematician. 2nd ed. J.-P. SERRE. A Course in Arithmetic. 6 HUGHES/PIPER. Projective Planes. 7 8 TAKEUTI/ZARING. Axiomatic Set Theory. 9 HUMPHREYS. Introduction to Lie Algebras and Representation Theory. 10 COHEN. A Course in Simple Homotopy Theory. 34 SPITZER. Principles of Random Walk. 2nd ed. 35 ALEXANDER/WERMER. Several Complex Variables and Banach Algebras. 3rd ed. 36 KELLEY/NAMIOKA et al. Linear Topological Spaces. 37 MONK. Mathematical Logic. 38 GRAUERT/FRITZSCHE. Several Complex Variables. 39 ARVESON. An Invitation to C*-Algebras. 40 KEMENY/SNELL/KNAPP. Denumerable Markov Chains. 2nd ed. 41 APOSTOL. Modular Functions and Dirichlet Series in Number Theory. 2nd ed. 42 J.-P. SERRE. Linear Representations of 11 CONWAY. Functions of One Complex Finite Groups. Variable I. 2nd ed. 12 BEALS. Advanced Mathematical Analysis. 13 ANDERSON/FULLER. Rings and Categories of Modules. 2nd ed. 14 GOLUBITSKY/GUILLEMIN. Stable Mappings and Their Singularities. 15 BERBERIAN. Lectures in Functional Analysis and Operator Theory. 16 WINTER. The Structure of Fields. 17 ROSENBLATT. Random Processes. 2nd ed. 18 HALMOS. Measure Theory. 19 HALMOS. A Hilbert Space Problem Book. 2nd ed. 20 HUSEMOLLER. Fibre Bundles. 3rd ed. 21 HUMPHREYS. Linear Algebraic Groups. 22 BARNES/MACK. An Algebraic Introduction to Mathematical Logic. 23 GREUB. Linear Algebra. 4th ed. 24 HOLMES. Geometric Functional Analysis and Its Applications. 25 HEWITT/STROMBERG. Real and Abstract Analysis. 26 MANES. Algebraic Theories. 27 KELLEY. General Topology. 28 ZARISKI/SAMUEL. Commutative Algebra. Vol. I. 43 GILLMAN/JERISON. Rings of Continuous Functions. 44 KENDIG. Elementary Algebraic Geometry. 45 LOÈVE. Probability Theory I. 4th ed. 46 LOÈVE. Probability Theory II. 4th ed. 47 MOISE. Geometric Topology in Dimensions 2 and 3. 48 SACHS/WU. General Relativity for Mathematicians. 49 GRUENBERG/WEIR. Linear Geometry. 2nd ed. 50 EDWARDS. Fermat’s Last Theorem. 51 KLINGENBERG. A Course in Differential Geometry. 52 HARTSHORNE. Algebraic Geometry. 53 MANIN. A Course in Mathematical Logic. 54 GRAVER/WATKINS. Combinatorics with Emphasis on the Theory of Graphs. 55 BROWN/PEARCY. Introduction to Operator Theory I: Elements of Functional Analysis. 56 MASSEY. Algebraic Topology: An Introduction. 57 CROWELL/FOX. Introduction to Knot Theory. 29 ZARISKI/SAMUEL. Commutative Algebra. Vol. II. 30 JACOBSON. Lectures in Abstract Algebra I. Basic Concepts. 58 KOBLITZ. p-adic Numbers, p-adic Analysis, and Zeta-Functions. 2nd ed. 59 LANG. Cyclotomic Fields. 60 ARNOLD. Mathematical Methods in 31 JACOBSON. Lectures in Abstract Algebra Classical Mechanics. 2nd ed. II. Linear Algebra. 61 WHITEHEAD. Elements of Homotopy 32 JACOBSON. Lectures in Abstract Algebra Theory. III. Theory of Fields and Galois Theory. 33 HIRSCH. Differential Topology. 62 KARGAPOLOV/MERIZJAKOV. Fundamentals of the Theory of Groups. 63 BOLLOBAS. Graph Theory. (continued after index)
Peter Petersen Riemannian Geometry Second Edition
Peter Petersen Department of Mathematics University of California, Los Angeles Los Angeles, CA 90095-1555 USA petersen@math.ucla.edu Editorial Board: S. Axler Department of Mathematics San Francisco State University San Francisco, CA 94132 USA axler@sfsu.edu K.A. Ribet Department of Mathematics University of California, Berkeley Berkeley, CA 94720-3840 USA ribet@math.berkeley.edu Mathematics Subject Classification (2000): 53-01 Library of Congress Control Number: 2006923825 ISBN-10: 0-387-29246-2 ISBN-13: 978-0387-29246-5 Printed on acid-free paper. e-ISBN 0-387-29403-1 © 2006 Springer Science +Business Media, LLC All rights reserved. This work may not be translated or copied in whole or in part without the written permission of the publisher (Springer Science+Business Media, LLC, 233 Spring Street, New York, NY 10013, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights. Printed in the United States of America. (MVY) 9 8 7 6 5 4 3 2 1 springer.com
To my wife, Laura
Preface This book is meant to be an introduction to Riemannian geometry. The reader is assumed to have some knowledge of standard manifold theory, including basic theory of tensors, forms, and Lie groups. At times we shall also assume familiarity with algebraic topology and de Rham cohomology. Specifically, we recommend that the reader is familiar with texts like [14], [63], or [87, vol. 1]. For the readers who have only learned a minimum of tensor analysis we have an appendix which covers Lie derivatives, forms, Stokes’ theorem, ˇCech cohomology, and de Rham cohomology. The reader should also have a nodding acquaintance with ordinary differential equations. For this, a text like [67]is more than sufficient. Most of the material usually taught in basic Riemannian geometry, as well as several more advanced topics, is presented in this text. Several theorems from chapters 7 to 11 appear for the first time in textbook form. This is particularly surprising as we have included essentially only the material students of Riemannian geometry must know. The approach we have taken sometimes deviates from the standard path. Aside from the usual variational approach (added in the second edition) we have also developed a more elementary approach that simply uses standard calculus together with some techniques from differential equations. Our motivation for this treatment has been that examples become a natural and integral part of the text rather than a separate item that is sometimes minimized. Another desirable by-product has been that one actually gets the feeling that gradients, Hessians, Laplacians, curvatures, and many other things are actually computable. We emphasize throughout the text the importance of using the correct type of coordinates depending on the theoretical situation at hand. First, we develop a substitute for the second variation formula by using adapted frames or coordinates. This is the approach mentioned above that can be used as an alternative to varia- tional calculus. These are coordinates naturally associated to a distance function. If, for example we use the function that measures the distance to a point, then the adapted coordinates are nothing but polar coordinates. Next, we have exponential coordinates, which are of fundamental importance in showing that distance func- tions are smooth. Then distance coordinates are used first to show that distance- preserving maps are smooth, and then later to give good coordinate systems in which the metric is sufficiently controlled so that one can prove, say, Cheeger’s finiteness theorem. Finally, we have harmonic coordinates. These coordinates have some magical properties. One, in particular, is that in such coordinates the Ricci curvature is essentially the Laplacian of the metric. From a more physical viewpoint, the reader will get the idea that we are also using the Hamilton-Jacobi equations instead of only relying on the Euler-Lagrange vii
viii PREFACE equations to develop Riemannian geometry (see [5]for an explanation of these mat- ters). It is simply a matter of taste which path one wishes to follow, but surprisingly, the Hamilton-Jacobi approach has never been tried systematically in Riemannian geometry. The book can be divided into five imaginary parts Part I: Tensor geometry, consisting of chapters 1-4. Part II: Classical geodesic geometry, consisting of chapters 5 and 6. Part III: Geometry `a la Bochner and Cartan, consisting of chapters 7 and 8. Part IV: Comparison geometry, consisting of chapters 9-11. Appendix: De Rham cohomology. Chapters 1-8 give a pretty complete picture of some of the most classical results in Riemannian geometry, while chapters 9-11 explain some of the more recent de- velopments in Riemannian geometry. The individual chapters contain the following material: Chapter 1: Riemannian manifolds, isometries, immersions, and submersions are defined. Homogeneous spaces and covering maps are also briefly mentioned. We have a discussion on various types of warped products, leading to an elementary account of why the Hopf fibration is also a Riemannian submersion. Chapter 2: Many of the tensor constructions one needs on Riemannian man- ifolds are developed. First the Riemannian connection is defined, and it is shown how one can use the connection to define the classical notions of Hessian, Laplacian, and divergence on Riemannian manifolds. We proceed to define all of the important curvature concepts and discuss a few simple properties. Aside from these important tensor concepts, we also develop several important formulas that relate curvature and the underlying metric. These formulas are to some extent our replacement for the second variation formula. The chapter ends with a short section where such tensor operations as contractions, type changes, and inner products are briefly discussed. Chapter 3: First, we indicate some general situations where it is possible to diagonalize the curvature operator and Ricci tensor. The rest of the chapter is devoted to calculating curvatures in several concrete situations such as: spheres, product spheres, warped products, and doubly warped products. This is used to exhibit some interesting examples that are Ricci flat and scalar flat. In particular, we explain how the Riemannian analogue of the Schwarzschild metric can be con- structed. Several different models of hyperbolic spaces are mentioned. We have a section on Lie groups. Here two important examples of left-invariant metrics are discussed as well the general formulas for the curvatures of bi-invariant metrics. Finally, we explain how submersions can be used to create new examples. We have paid detailed attention to the complex projective space. There are also some general comments on how submersions can be constructed using isometric group actions. Chapter 4: Here we concentrate on the special case where the Riemannian man- ifold is a hypersurface in Euclidean space. In this situation, one gets some special relations between curvatures. We give examples of simple Riemannian manifolds that cannot be represented as hypersurface metrics. Finally we give a brief in- troduction to the global Gauss-Bonnet theorem and its generalization to higher dimensions. Chapter 5: This chapter further develops the foundational topics for Riemann- ian manifolds. These include, the first variation formula, geodesics, Riemannian
PREFACE ix manifolds as metric spaces, exponential maps, geodesic completeness versus metric completeness, and maximal domains on which the exponential map is an embed- ding. The chapter ends with the classification of simply connected space forms and metric characterizations of Riemannian isometries and submersions. Chapter 6: We cover two more foundational techniques: parallel translation and the second variation formula. Some of the classical results we prove here are: The Hadamard-Cartan theorem, Cartan’s center of mass construction in nonpositive curvature and why it shows that the fundamental group of such spaces are torsion free, Preissmann’s theorem, Bonnet’s diameter estimate, and Synge’s lemma. We have supplied two proofs for some of the results dealing with non-positive curvature in order that people can see the difference between using the variational (or Euler- Lagrange) method and the Hamilton-Jacobi method. At the end of the chapter we explain some of the ingredients needed for the classical quarter pinched sphere theorem as well as Berger’s proof of this theorem. Sphere theorems will also be revisited in chapter 11. Chapter 7: Many of the classical and more recent results that arise from the Bochner technique are explained. We start with Killing fields and harmonic 1-forms as Bochner did, and finally, discuss some generalizations to harmonic p-forms. For the more advanced audience we have developed the language of Clifford multipli- cation for the study p-forms, as we feel that it is an important way of treating this material. The last section contains some more exotic, but important, situa- tions where the Bochner technique is applied to the curvature tensor. These last two sections can easily be skipped in a more elementary course. The Bochner tech- nique gives many nice bounds on the topology of closed manifolds with nonnegative curvature. In the spirit of comparison geometry, we show how Betti numbers of nonnegatively curved spaces are bounded by the prototypical compact flat manifold: the torus. The importance of the Bochner technique in Riemannian geometry cannot be sufficiently emphasized. It seems that time and again, when people least expect it, new important developments come out of this simple philosophy. While perhaps only marginally related to the Bochner technique we have also added a discussion on how the presence of Killing fields in positive sectional curva- ture can lead to topological restrictions. This is a rather new area in Riemannian geometry that has only been developed in the last 15 years. Chapter 8: Part of the theory of symmetric spaces and holonomy is developed. The standard representations of symmetric spaces as homogeneous spaces and via Lie algebras are explained. We prove Cartan’s existence theorem for isometries. We explain how one can compute curvatures in general and make some concrete calculations on several of the Grassmann manifolds including complex projective space. Having done this, we define holonomy for general manifolds, and discuss the de Rham decomposition theorem and several corollaries of it. The above examples are used to give an idea of how one can classify symmetric spaces. Also, we show in the same spirit why symmetric spaces of (non)compact type have (nonpositive) nonnegative curvature operator. Finally, we present a brief overview of how holo- nomy and symmetric spaces are related with the classification of holonomy groups. This is used in a grand synthesis, with all that has been learned up to this point, to give Gallot and Meyer’s classification of compact manifolds with nonnegative curvature operator.
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