and the Sphere Theorem

3IMON"RENDLE

'RADUATE3TUDIES IN-ATHEMATICS 6OLUME

!MERICAN-ATHEMATICAL3OCIETY http://dx.doi.org/10.1090/gsm/111

Ricci Flow and the Sphere Theorem

Simon Brendle

Graduate Studies in Mathematics Volume 111

American Mathematical Society Providence, Rhode Island EDITORIAL COMMITTEE David Cox (Chair) Steven G. Krantz Rafe Mazzeo Martin Scharlemann

2000 Mathematics Subject Classification. Primary 53C20, 53C21, 53C44.

For additional information and updates on this book, visit www.ams.org/bookpages/gsm-111

Library of Congress Cataloging-in-Publication Data Brendle, Simon, 1981– Ricci flow and the sphere theorem / Simon Brendle. p. cm. — (Graduate studies in mathematics ; v. 111) Includes bibliographical references and index. ISBN 978-0-8218-4938-5 (alk. paper) 1. Ricci flow. 2. Sphere. I. Title. QA377.3B74 2010 516.362—dc22 2009037261

Copying and reprinting. Individual readers of this publication, and nonprofit libraries acting for them, are permitted to make fair use of the material, such as to copy a chapter for use in teaching or research. Permission is granted to quote brief passages from this publication in reviews, provided the customary acknowledgment of the source is given. Republication, systematic copying, or multiple reproduction of any material in this publication is permitted only under license from the American Mathematical Society. Requests for such permission should be addressed to the Acquisitions Department, American Mathematical Society, 201 Charles Street, Providence, Rhode Island 02904-2294 USA. Requests can also be made by e-mail to [email protected]. c 2010 by the author. Printed in the United States of America. ∞ The paper used in this book is acid-free and falls within the guidelines established to ensure permanence and durability. Visit the AMS home page at http://www.ams.org/ 10987654321 151413121110 Contents

Preface v Chapter 1. A survey of sphere theorems in geometry 1 §1.1. background 1 §1.2. The Topological Sphere Theorem 6 §1.3. The Diameter Sphere Theorem 7 §1.4. The Sphere Theorem of Micallef and Moore 9 §1.5. Exotic Spheres and the Differentiable Sphere Theorem 13 Chapter 2. Hamilton’s Ricci flow 15 §2.1. Definition and special solutions 15 §2.2. Short-time existence and uniqueness 17 §2.3. Evolution of the Riemann curvature tensor 21 §2.4. Evolution of the Ricci and scalar curvature 28 Chapter 3. Interior estimates 31 §3.1. Estimates for the derivatives of the curvature tensor 31 §3.2. Derivative estimates for tensors 33 §3.3. Curvature blow-up at finite-time singularities 36 Chapter 4. Ricci flow on S2 37 §4.1. Gradient Ricci solitons on S2 37 §4.2. Monotonicity of Hamilton’s entropy functional 39 §4.3. Convergence to a constant curvature metric 45 Chapter 5. Pointwise curvature estimates 49 §5.1. Introduction 49

iii iv Contents

§5.2. The tangent and normal cone to a convex set 49 §5.3. Hamilton’s maximum principle for the Ricci flow 53 §5.4. Hamilton’s convergence criterion for the Ricci flow 58 Chapter 6. Curvature pinching in dimension 3 67 §6.1. Three-manifolds with positive Ricci curvature 67 §6.2. The curvature estimate of Hamilton and Ivey 70 Chapter 7. Preserved curvature conditions in higher dimensions 73 §7.1. Introduction 73 §7.2. Nonnegative isotropic curvature 74 §7.3. Proof of Proposition 7.4 77 §7.4. The cone C˜ 87 §7.5. The cone Cˆ 90 §7.6. An invariant set which lies between C˜ and Cˆ 93 §7.7. An overview of various curvature conditions 100 Chapter 8. Convergence results in higher dimensions 101 §8.1. An algebraic identity for curvature tensors 101 §8.2. Constructing a family of invariant cones 106 §8.3. Proof of the Differentiable Sphere Theorem 112 §8.4. An improved convergence theorem 117 Chapter 9. Rigidity results 121 §9.1. Introduction 121 §9.2. Berger’s classification of holonomy groups 121 §9.3. A version of the strict maximum principle 123 §9.4. Three-manifolds with nonnegative Ricci curvature 126 §9.5. Manifolds with nonnegative isotropic curvature 129 §9.6. K¨ahler-Einstein and quaternionic-K¨ahler manifolds 135 §9.7. A generalization of a theorem of Tachibana 146 §9.8. Classification results 149 Appendix A. Convergence of evolving metrics 155 Appendix B. Results from complex linear algebra 159 Problems 163 Bibliography 169 Index 175 Preface

In this book, we study the evolution of Riemannian metrics under the Ricci flow. This evolution equation was introduced in a seminal paper by R. Hamilton [44], following earlier work of Eells and Sampson [33]onthe harmonic map heat flow. Using the Ricci flow, Hamilton proved that ev- ery compact three-manifold with positive Ricci curvature is diffeomorphic to a spherical space form. The Ricci flow has since been used to resolve longstanding open questions in Riemannian geometry and three-dimensional topology. In this text, we focus on the convergence theory for the Ricci flow in higher dimensions and its application to the Differentiable Sphere Theo- rem. The results we describe have all appeared in research articles. How- ever, we have made an effort to simplify various arguments and streamline the exposition. In Chapter 1, we give a survey of various sphere theorems in Riemannian geometry (see also [22]). We first describe the Topological Sphere Theorem of Berger and Klingenberg. We then discuss various generalizations of that theorem, such as the Diameter Sphere Theorem of Grove and Shiohama [42] and the Sphere Theorem of Micallef and Moore [60]. These results rely on the variational theory for geodesics and harmonic maps, respectively. We will discuss the main ideas involved in the proof; however, this material will not be used in later chapters. Finally, we state the Differentiable Sphere Theorem obtained by the author and R. Schoen [20]. In Chapter 2, we state the definition of the Ricci flow and describe the short-time existence and uniqueness theory. We then study how the Riemann curvature tensor changes when the metric evolves under the Ricci flow. This evolution equation will be the basis for all the aprioriestimates established in later chapters.

v vi Preface

In Chapter 3, we describe Shi’s estimates for the covariant derivatives of the curvature tensor. As an application, we show that the Ricci flow cannot develop a singularity in finite time unless the curvature is unbounded. Moreover, we establish interior estimates for solutions of linear parabolic equations. These estimates play an important role in Sections 4.3 and 5.4. In Chapter 4, we consider the Ricci flow on S2. In Section 4.1, we show that any gradient Ricci soliton on S2 has constant curvature. We then study solutions to the Ricci flow on S2 with positive scalar curvature. A theorem of Hamilton [46] asserts that such a solution converges to a constant curvature metric after rescaling. A key ingredient in the proof is the monotonicity of Hamilton’s entropy functional. This monotonicity formula will be discussed in Section 4.2. Alternative proofs of this theorem can be found in [4], [6], [48], or [82]. The arguments in [4] and [48] are based on a careful study of the isoperimetric profile, while the proofs in [6] and [82]employPDE techniques. In Chapter 5, we describe Hamilton’s maximum principle for the Ricci flow and discuss the notion of a pinching set. We then describe a general convergence criterion for the Ricci flow. This criterion was discovered by Hamilton [45] and plays an important role in the study of Ricci flow. In Chapter 6, we explain how Hamilton’s classification of three-manifolds with positive Ricci curvature follows from the general theory developed in Chapter 5. We then describe an important curvature estimate, due to Hamil- ton and Ivey. This inequality holds for any solution to the Ricci flow in dimension 3. In Chapter 7, we describe various curvature conditions which are pre- served by the Ricci flow. We first prove that nonnegative isotropic curvature is preserved by the Ricci flow in all dimensions. This curvature condition originated in Micallef and Moore’s work on the Morse index of harmonic two-spheres and plays a central role in this book. We then consider the condition that M ×R has nonnegative isotropic curvature. This condition is stronger than nonnegative isotropic curvature, and is also preserved by the Ricci flow. Continuing in this fashion, we consider the condition that M ×R2 has nonnegative isotropic curvature, and the condition that M × S2(1) has nonnegative isotropic curvature. (Here, S2(1) denotes a two-dimensional sphere of constant curvature 1.) We show that these conditions are pre- served by the Ricci flow as well. In Chapter 8, we present the proof of the Differentiable Sphere Theorem. More generally, we show that every compact Riemannian manifold M with the property that M × R has positive isotropic curvature is diffeomorphic to a spherical space form. This theorem is the main result of Chapter 8. It Preface vii can be viewed as a generalization of Hamilton’s work in dimension 3 and was originally proved in [17]. In Chapter 9, we prove various rigidity theorems. In particular, we clas- sify all compact Riemannian manifolds M with the property that M × R has nonnegative isotropic curvature. Moreover, we show that any Einstein manifold with nonnegative isotropic curvature is necessarily locally symmet- ric. This generalizes classical results due to Berger [10], [11] and Tachibana [84]. In order to handle the borderline case, we employ a variant of Bony’s strict maximum principle for degenerate elliptic equations. The material presented in Chapters 2–9 is largely, though not fully, self- contained. In Section 2.2, we employ the existence and uniqueness theory for parabolic systems. In Section 4.2, we use the convergence theory for Rie- mannian manifolds developed by Cheeger and Gromov. Finally, in Chapter 9, we use Berger’s classification of holonomy groups, as well as some basic facts about K¨ahler and quaternionic-K¨ahler manifolds. There are some important aspects of Ricci flow which are not mentioned in this book. For example, we do not discuss Hamilton’s differential Harnack inequality (cf. [47], [49]) or Perelman’s crucial monotonicity formulae (see [68], [69]). A detailed exposition of Perelman’s entropy functional can be found in [63]or[85]. A generalization of Hamilton’s Harnack inequality is described in [18](seealso[24]). This book grew out of a Nachdiplom course given at ETH Z¨urich. It is a pleasure to thank the Department of Mathematics at ETH Z¨urich for its hospitality. I am especially grateful to Professor Michael Struwe and Professor Tristan Rivi`ere for many inspiring discussions. Without their en- couragement, this book would never have been written. Finally, I thank Professor Camillo De Lellis for valuable comments on an earlier version of this manuscript.

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Index

1/4-pinching, 6, 14, 100, 116, 117, 154 of the scalar curvature, 29 , 13 Berger’s inequality, 5, 116, 153 Bianchi identities, 2, 3 Goldberg-Kobayashi theorem, 137 Bochner formula, 41, 163 Hamilton ODE, 54 Cigar soliton, 16 Hamilton’s convergence criterion, 65 Convergence theorems for Ricci flow Hamilton’s maximum principle, 56 on S2,47 Hamilton-Ivey estimate, 72 on manifolds with curvature in the Harmonic map interior of C˜, 119 from S2 into a Riemannian manifold, 9 on manifolds with strictly 1/4-pinched Laplacian, 17 curvature, 117 Holonomy group of a Riemannian manifold on three-manifolds with positive Ricci Berger’s classification theorem, 122 curvature, 70 definition, 121 Covariant derivative, 1, 26 Curvature operator on two-forms, 3 Invariant sets for ODE, 51 Curvature pinching global, 4 K¨ahler manifold, 135 pointwise, 5 Kulkarni-Nomizu product, 101 Curvature tensor algebraic, 53 of a Riemannian manifold, 1 Lie derivative, 15 Locally irreducible manifold, 122 Locally symmetric manifold, 122 Diameter Sphere Theorem, 8 Differentiable Sphere Theorem, 14, 116, 117, 119 Nonnegative curvature operator, 3, 100, 165 Nonnegative isotropic curvature, 12, 74, 90, 93, 96, 100, 132, 149 Einstein manifolds Normal cone to a convex set, 49 definition, 15 with nonnegative isotropic curvature, 148 Entropy functional on S2, 40, 41 Orthonormal frame bundle, 126 Evolution equation of the Levi-Civita connection, 21 Pinching set of the Ricci tensor, 28 definition, 58 of the Riemann curvature tensor, 27 examples, 70, 115, 118

175 176 Index

Quadratic term in the evolution of curvature, 23, 53 Quaternionic-K¨ahler manifold, 140

Ricci flow curvature blow-up, 30, 36 definition, 15 on orbifolds, 48, 66 short-time existence and uniqueness, 20 Ricci solitons definition, 15 on S2,38 on compact three-manifolds, 166 Ricci tensor, 3 Ricci-DeTurck flow, 17 Rigidity results for manifolds with curvature in C˜, 152 for manifolds with nonnegative isotropic curvature, 149 for manifolds with weakly 1/4-pinched curvature, 154 for three-manifolds with nonnegative Ricci curvature, 128 Rosenau solution, 16

Scalar curvature, 3 Schur’s lemma, 4 Sectional curvature, 3 Shi’s derivative estimates, 32 Strict maximum principle for degenerate elliptic equations, 126 for solutions to the Ricci flow, 29, 132, 150

Tangent cone to a convex set, 49 Topological Sphere Theorem of Berger and Klingenberg, 6 of Grove and Shiohama, 8 of Micallef and Moore, 12

Uhlenbeck’s trick, 27 In 1982, R. Hamilton introduced a nonlinear evolution equation for Riemannian metrics with the aim of finding canonical metrics on manifolds. This evolution equation is known as the Ricci flow, and it has since been used widely and with great success, most notably in Perelman’s solution of the Poincaré conjecture. Furthermore, various convergence theorems have been established. This book provides a concise introduction to the subject as well as a comprehensive account of the convergence theory for the Ricci flow. The proofs rely mostly on maximum principle arguments. Special emphasis is placed on preserved curvature conditions, such as positive isotropic curvature. One of the major consequences of this theory is the Differentiable Sphere Theorem: a compact Riemannian manifold, whose sectional curvatures all lie in the interval (1,4], is diffeomorphic to a spherical space form. This question has a long history, dating back to a seminal paper by H. E. Rauch in 1951, and it was resolved in 2007 by the author and . This text originated from graduate courses given at ETH Zürich and , and it is directed at graduate students and researchers. The reader is assumed to be familiar with basic Riemannian geometry, but no previous knowledge of Ricci flow is required.

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