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Homotopical Categories: from Model Categories to ( ,)-Categories ∞
HOMOTOPICAL CATEGORIES: FROM MODEL CATEGORIES TO ( ;1)-CATEGORIES 1 EMILY RIEHL Abstract. This chapter, written for Stable categories and structured ring spectra, edited by Andrew J. Blumberg, Teena Gerhardt, and Michael A. Hill, surveys the history of homotopical categories, from Gabriel and Zisman’s categories of frac- tions to Quillen’s model categories, through Dwyer and Kan’s simplicial localiza- tions and culminating in ( ;1)-categories, first introduced through concrete mod- 1 els and later re-conceptualized in a model-independent framework. This reader is not presumed to have prior acquaintance with any of these concepts. Suggested exercises are included to fertilize intuitions and copious references point to exter- nal sources with more details. A running theme of homotopy limits and colimits is included to explain the kinds of problems homotopical categories are designed to solve as well as technical approaches to these problems. Contents 1. The history of homotopical categories 2 2. Categories of fractions and localization 5 2.1. The Gabriel–Zisman category of fractions 5 3. Model category presentations of homotopical categories 7 3.1. Model category structures via weak factorization systems 8 3.2. On functoriality of factorizations 12 3.3. The homotopy relation on arrows 13 3.4. The homotopy category of a model category 17 3.5. Quillen’s model structure on simplicial sets 19 4. Derived functors between model categories 20 4.1. Derived functors and equivalence of homotopy theories 21 4.2. Quillen functors 24 4.3. Derived composites and derived adjunctions 25 4.4. Monoidal and enriched model categories 27 4.5. -
Hamilton's Ricci Flow
The University of Melbourne, Department of Mathematics and Statistics Hamilton's Ricci Flow Nick Sheridan Supervisor: Associate Professor Craig Hodgson Second Reader: Professor Hyam Rubinstein Honours Thesis, November 2006. Abstract The aim of this project is to introduce the basics of Hamilton's Ricci Flow. The Ricci flow is a pde for evolving the metric tensor in a Riemannian manifold to make it \rounder", in the hope that one may draw topological conclusions from the existence of such \round" metrics. Indeed, the Ricci flow has recently been used to prove two very deep theorems in topology, namely the Geometrization and Poincar´eConjectures. We begin with a brief survey of the differential geometry that is needed in the Ricci flow, then proceed to introduce its basic properties and the basic techniques used to understand it, for example, proving existence and uniqueness and bounds on derivatives of curvature under the Ricci flow using the maximum principle. We use these results to prove the \original" Ricci flow theorem { the 1982 theorem of Richard Hamilton that closed 3-manifolds which admit metrics of strictly positive Ricci curvature are diffeomorphic to quotients of the round 3-sphere by finite groups of isometries acting freely. We conclude with a qualitative discussion of the ideas behind the proof of the Geometrization Conjecture using the Ricci flow. Most of the project is based on the book by Chow and Knopf [6], the notes by Peter Topping [28] (which have recently been made into a book, see [29]), the papers of Richard Hamilton (in particular [9]) and the lecture course on Geometric Evolution Equations presented by Ben Andrews at the 2006 ICE-EM Graduate School held at the University of Queensland. -
Model Category Theory in Homological Action
New York Journal of Mathematics New York J. Math. 20 (2014) 1077{1159. Six model structures for DG-modules over DGAs: model category theory in homological action Tobias Barthel, J.P. May and Emily Riehl The authors dedicate this paper to John Moore, who pioneered this area of mathematics. He was the senior author's adviser, and his mathematical philosophy pervades this work and indeed pervades algebraic topology at its best. Abstract. In Part 1, we describe six projective-type model structures on the category of differential graded modules over a differential graded algebra A over a commutative ring R. When R is a field, the six collapse to three and are well-known, at least to folklore, but in the general case the new relative and mixed model structures offer interesting alterna- tives to the model structures in common use. The construction of some of these model structures requires two new variants of the small object argument, an enriched and an algebraic one, and we describe these more generally. In Part 2, we present a variety of theoretical and calculational cofi- brant approximations in these model categories. The classical bar con- struction gives cofibrant approximations in the relative model structure, but generally not in the usual one. In the usual model structure, there are two quite different ways to lift cofibrant approximations from the level of homology modules over homology algebras, where they are classi- cal projective resolutions, to the level of DG-modules over DG-algebras. The new theory makes model theoretic sense of earlier explicit calcu- lations based on one of these constructions. -
Complicial Sets, an Overture
COMPLICIAL SETS, AN OVERTURE EMILY RIEHL Abstract. The aim of these notes is to introduce the intuition motivating the notion of a complicial set, a simplicial set with certain marked \thin" simplices that witness a composition relation between the simplices on their boundary. By varying the marking conventions, complicial sets can be used to model (1; n)-categories for each n ≥ 0, including n = 1. For this reason, compli- cial sets present a fertile setting for thinking about weak infinite dimensional categories in varying dimensions. This overture is presented in three acts: the first introducing simplicial models of higher categories; the second defining the Street nerve, which embeds strict !-categories as strict complicial sets; and the third exploring an important saturation condition on the marked simplices in a complicial set and presenting a variety of model structures that capture their basic homotopy theory. Scattered throughout are suggested exercises for the reader who wants to engage more deeply with these notions. Contents 1. Introducing complicial sets 3 1.1. Quasi-categories as ( ; 1)-categories 3 1.2. Towards a simplicial1 model of ( ; 2)-categories 5 1.3. Stratified simplicial sets1 6 1.4. Complicial sets 7 1.5. n-trivialization and the n-core 9 2. The Street nerve of an !-category 10 2.1. !-categories 11 2.2. Orientals 13 2.3. The Street nerve as a strict complicial set 15 3. Saturated complicial sets 16 3.1. Weak complicial sets from strict !-categories 17 3.2. Saturation 19 3.3. Model categories of complicial sets 22 arXiv:1610.06801v1 [math.CT] 21 Oct 2016 References 23 As the objects that mathematicians study increase in sophistication, so do their natural habitats. -
Inverse Mean Curvature Evolution of Entire Graphs
INVERSE MEAN CURVATURE EVOLUTION OF ENTIRE GRAPHS PANAGIOTA DASKALOPOULOS AND GERHARD HUISKEN n Abstract. We study the evolution of strictly mean-convex entire graphs over R by Inverse Mean Curvature flow. First we establish the global existence of starshaped entire graphs with superlinear growth at infinity. The main result in this work concerns the critical case of asymptotically conical entire convex graphs. In this case we show that there exists a time T < +1, which depends on the growth at infinity of the initial data, such that the unique solution of the flow exists for all t < T . Moreover, as t ! T the solution converges to a flat plane. Our techniques exploit the ultra-fast diffusion character of the fully-nonlinear flow, a property that implies that the asymptotic behavior at spatial infinity of our solution plays a crucial influence on the maximal time of existence, as such behavior propagates infinitely fast towards the interior. Contents 1. Introduction 1 2. The geometric equations and preliminaries 5 3. Self-similar solutions 10 4. The super-linear case and short time existence 13 5. Lp bounds on 1=H 23 6. L1 estimates on 1=H 29 7. Long time existence Theorem 1.1 40 References 41 arXiv:1709.06665v1 [math.DG] 19 Sep 2017 1. Introduction n n+1 We consider a family of immersions Ft : M ! R of n-dimensional mean convex hypersurfaces n+1 n in R . We say that Mt := Ft(M ) moves by inverse mean curvature flow if @ (1.1) F (z; t) = H−1(z; t) ν(z; t); z 2 M n @t where H(z; t) > 0 and ν(z; t) denote the mean curvature and exterior unit normal of the surface Mt at the point F (z; t). -
The Newtonian Limit of General Relativity
The Newtonian Limit of General Relativity Dissertation zur Erlangung des Doktorgrades der Naturwissenschaften an der Fakult¨atMathematik und Physik der Eberhard-Karls-Universit¨atT¨ubingen vorgelegt von Maren Reimold aus N¨ordlingen Unterst¨utztdurch das Evangelische Studienwerk Villigst e.V. T¨ubingen,September 3, 2010 Contents Deutsche Zusammenfassung 3 Introduction 5 0.1 Transitions from tangent and cotangent space and induced connections 8 0.2 Concepts of curvature . 9 0.3 Newton's theory of gravitation and Einstein's theory of relativity . 13 1 Frame theory 19 1.1 The structure of the frame theory . 19 1.2 Linear Algebra . 23 1.3 Transfer to the frame theory . 30 1.4 The case λ =0 ............................. 34 2 The Newtonian limit: definition and existence 63 2.1 Definition of the Newtonian limit . 63 2.2 Extension of spacetimes . 66 2.3 Examples . 66 2.4 Existence of a limit . 75 2.5 Static and spherically symmetric spacetimes . 82 3 Existence of genuine Newtonian limits 95 3.1 Transformation of coordinates . 96 3.2 Conditions for the curvature tensor . 103 3.3 Asymptotically flat spacetimes . 106 A Appendix 121 A.1 The Schwarzschild spacetime . 121 A.2 The Kerr spacetime . 128 Index 151 Bibliography 153 1 Deutsche Zusammenfassung So lange es die Allgemeine Relativit¨atstheoriegibt, so lange gibt es auch die zugeh¨origeFrage, inwieweit man die Newtonsche Gravitationstheorie als einen Spezialfall oder doch wenigstens als eine Grenzlage der Allgemeinen Relativit¨ats- theorie auffassen kann. Schon am 18. November 1915, eine Woche bevor -
Geometric Analysis
IAS/PARK CITY MATHEMATICS SERIES Volume 22 Geometric Analysis Hubert L. Bray Greg Galloway Rafe Mazzeo Natasa Sesum Editors American Mathematical Society Institute for Advanced Study Geometric Analysis https://doi.org/10.1090//pcms/022 IAS/PARK CITY MATHEMATICS SERIES Volume 22 Geometric Analysis Hubert L. Bray Greg Galloway Rafe Mazzeo Natasa Sesum Editors American Mathematical Society Institute for Advanced Study Hubert Lewis Bray, Gregory J. Galloway, Rafe Mazzeo, and Natasa Sesum, Volume Editors IAS/Park City Mathematics Institute runs mathematics education programs that bring together high school mathematics teachers, researchers in mathematics and mathematics education, undergraduate mathematics faculty, graduate students, and undergraduates to participate in distinct but overlapping programs of research and education. This volume contains the lecture notes from the Graduate Summer School program 2010 Mathematics Subject Classification. Primary 53-06, 35-06, 83-06. Library of Congress Cataloging-in-Publication Data Geometric analysis / Hubert L. Bray, editor [and three others]. pages cm. — (IAS/Park City mathematics series ; volume 22) Includes bibliographical references. ISBN 978-1-4704-2313-1 (alk. paper) 1. Geometric analysis. 2. Mathematical analysis. I. Bray, Hubert L., editor. QA360.G455 2015 515.1—dc23 2015031562 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 select pages 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. -
The 2-Category Theory of Quasi-Categories
Advances in Mathematics 280 (2015) 549–642 Contents lists available at ScienceDirect Advances in Mathematics www.elsevier.com/locate/aim The 2-category theory of quasi-categories Emily Riehl a,∗, Dominic Verity b a Department of Mathematics, Harvard University, Cambridge, MA 02138, USA b Centre of Australian Category Theory, Macquarie University, NSW 2109, Australia a r t i c l e i n f o a b s t r a c t Article history: In this paper we re-develop the foundations of the category Received 11 December 2013 theory of quasi-categories (also called ∞-categories) using Received in revised form 13 April 2-category theory. We show that Joyal’s strict 2-category of 2015 quasi-categories admits certain weak 2-limits, among them Accepted 22 April 2015 weak comma objects. We use these comma quasi-categories Available online xxxx Communicated by the Managing to encode universal properties relevant to limits, colimits, Editors of AIM and adjunctions and prove the expected theorems relating these notions. These universal properties have an alternate MSC: form as absolute lifting diagrams in the 2-category, which primary 18G55, 55U35, 55U40 we show are determined pointwise by the existence of certain secondary 18A05, 18D20, 18G30, initial or terminal vertices, allowing for the easy production 55U10 of examples. All the quasi-categorical notions introduced here are equiv- Keywords: alent to the established ones but our proofs are independent Quasi-categories and more “formal”. In particular, these results generalise 2-Category theory Formal category theory immediately to model categories enriched over quasi-cate- gories. © 2015 Elsevier Inc. -
Geometric Relativity
GRADUATE STUDIES IN MATHEMATICS 201 Geometric Relativity Dan A. Lee 10.1090/gsm/201 Geometric Relativity GRADUATE STUDIES IN MATHEMATICS 201 Geometric Relativity Dan A. Lee EDITORIAL COMMITTEE Daniel S. Freed (Chair) Bjorn Poonen Gigliola Staffilani Jeff A. Viaclovsky 2010 Mathematics Subject Classification. Primary 53-01, 53C20, 53C21, 53C24, 53C27, 53C44, 53C50, 53C80, 83C05, 83C57. For additional information and updates on this book, visit www.ams.org/bookpages/gsm-201 Library of Congress Cataloging-in-Publication Data Names: Lee, Dan A., 1978- author. Title: Geometric relativity / Dan A. Lee. Description: Providence, Rhode Island : American Mathematical Society, [2019] | Series: Gradu- ate studies in mathematics ; volume 201 | Includes bibliographical references and index. Identifiers: LCCN 2019019111 | ISBN 9781470450816 (alk. paper) Subjects: LCSH: General relativity (Physics)–Mathematics. | Geometry, Riemannian. | Differ- ential equations, Partial. | AMS: Differential geometry – Instructional exposition (textbooks, tutorial papers, etc.). msc | Differential geometry – Global differential geometry – Global Riemannian geometry, including pinching. msc | Differential geometry – Global differential geometry – Methods of Riemannian geometry, including PDE methods; curvature restrictions. msc | Differential geometry – Global differential geometry – Rigidity results. msc — Differential geometry – Global differential geometry – Spin and Spin. msc | Differential geometry – Global differential geometry – Geometric evolution equations (mean curvature flow, -
Monotonicity Formulas for Parabolic Flows on Manifolds
COMMUNICATIONS IN ANALYSIS AND GEOMETRY Volume 1, Number 1, 127-137, 1993 MONOTONICITY FORMULAS FOR PARABOLIC FLOWS ON MANIFOLDS RICHARD S. HAMILTON Recently Michael Struwe [S] and Gerhard Huisken [Hu2] have independently derived monotonicity formulas for the Harmonic Map heat flow on a Euclidean domain and for the Mean Curvature flow of a hypersurface in Euclidean space. In this paper we show how to generalize these results to the case of flows on a general compact manifold, and we also give the analogous monotonicity for- mula for the Yang-Mills heat flow. The key ingredient is a matrix Harnack estimate for positive solutions to the scalar heat equation given in [H]. In [GrH] the authors show how to use the monotonicity formula to prove that rapidly forming singularities in the Harmonic Map heat flow are asymptotic to homo- thetically shrinking solitons; similar results may be expected in other cases, as Huisken does in [Hu2] for the Mean Curvature flow in Euclidean space. We only obtain strict monotonicity for a special class of metrics, but in general there is an error term which is small enough to give the same effect. (Chen Yummei and Michael Struwe [CS] give a different approach to the error on manifolds.) The special class of metrics are those which are Ricci parallel (so that DiRjk = 0) and have weakly positive sectional curvature (so that RijkiViWjVkWt > 0 for all vectors V and W). This holds for example if M is flat or a sphere or a complex projective space, or a product of such, or a quotient of a product by a finite free group of isometries. -
Convex Ancient Solutions of the Mean Curvature Flow
j. differential geometry 101 (2015) 267-287 CONVEX ANCIENT SOLUTIONS OF THE MEAN CURVATURE FLOW Gerhard Huisken & Carlo Sinestrari Abstract We study solutions of the mean curvature flow which are defined for all negative times, usually called ancient solutions. We give various conditions ensuring that a closed convex ancient solution is a shrinking sphere. Examples of such conditions are: a uniform pinching condition on the curvatures, a suitable growth bound on the diameter, or a reverse isoperimetric inequality. We also study the behaviour of uniformly k-convex solutions, and consider generalizations to ancient solutions immersed in a sphere. 1. Introduction In this paper, we study convex ancient solutions to the mean cur- vature flow. We recall that a family of smooth immersions F : M× n+1 (t0,t1) → R ,whereM is an n-dimensional manifold, is called a mean curvature flow if it satisfies ∂F (1.1) (p, t)=−H(p, t)ν(p, t),p∈M,t∈ (t0,t1), ∂t where H(p, t)andν(p, t) are the mean curvature and the outer normal respectively at the point F (p, t)ofthesurfaceMt = F (·,t)(M). The signs are chosen such that −Hν = H is the mean curvature vector and the mean curvature of a convex surface is positive. We assume that n ≥ 2andthatM is closed. A solution is called ancient if it is defined for t ∈ (−∞, 0). While mean curvature flow is parabolic and hence in general ill-posed backward in time, ancient solutions are of interest for several reasons: They arise as tangent flows near singularities of the flow and therefore model the as- ymptotic profile of the surfaces as they approach a singularity, see e.g. -
Arxiv:1102.3001V2 [Math.DG]
ON LARGE VOLUME PRESERVING STABLE CMC SURFACES IN INITIAL DATA SETS MICHAEL EICHMAIR AND JAN METZGER Abstract. Let (M,g) be a complete 3-dimensional asymptotically flat manifold with everywhere positive scalar curvature. We prove that, given a compact subset K ⊂ M, all volume preserving stable constant mean curvature surfaces of sufficiently large area will avoid K. This com- plements the results of G. Huisken and S.-T. Yau [17] and of J. Qing and G. Tian [26] on the uniqueness of large volume preserving stable constant mean curvature spheres in initial data sets that are asymptot- ically close to Schwarzschild with mass m> 0. The analysis in [17] and [26] takes place in the asymptotic regime of M. Here we adapt ideas from the minimal surface proof of the positive mass theorem [32] by R. Schoen and S.-T. Yau and develop geometric properties of volume preserving stable constant mean curvature surfaces to handle surfaces that run through the part of M that is far from Euclidean. 1. Introduction A classical result in the calculus of variations is that the isoperimetric re- gions of Rn are precisely the round balls. A surface which is critical for the associated variational problem has constant mean curvature. In 1951, H. Hopf proved that immersed two-spheres of constant mean curvature in R3 are necessarily round, and then, in 1958, A. D. Alexandrov showed that so are compact embedded constant mean curvature hypersurfaces in Rn. Note arXiv:1102.3001v2 [math.DG] 17 May 2012 that translations preserve such surfaces and fully account for their non- uniqueness, once we fix their area.