An Introduction to Metric Spaces

This book serves as a textbook for an introductory course in metric spaces for undergraduate or graduate students. The goal is to present the basics of metric spaces in a natural and intuitive way and encourage students to think geometrically while actively participating in the learning of this subject. In this book, the authors illustrated the strategy of the proofs of various theorems that motivate readers to complete them on their own. Bits of pertinent history are infused in the text, including brief biographies of some of the central players in the development of metric spaces. The textbook is divided into seven chapters that contain the main materials on metric spaces; namely, introductory concepts, completeness, compactness, connectedness, continuous functions and metric fixed point theorems with applications. Some of the noteworthy features of this book include · Diagrammatic illustrations that encourage readers to think geometrically · Focus on systematic strategy to generate ideas for the proofs of theorems · A wealth of remarks, observations along with a variety of exercises · Historical notes and brief biographies appearing throughout the text

Table of Contents:
Contents Preface ix A Note to the Reader xiii Authors xv 1 Set Theory 1 1.1 Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1.1 The empty set . . . . . . . . . . . . . . . . . . . . . . 4 1.1.2 Operations on sets . . . . . . . . . . . . . . . . . . . . 5 1.1.3 Uniqueness of the empty set . . . . . . . . . . . . . . . 9 1.1.4 Power sets . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.1.5 Cartesian products . . . . . . . . . . . . . . . . . . . . 9 1.2 Relations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1.2.1 Types of relations . . . . . . . . . . . . . . . . . . . . 12 1.2.2 Equivalence relations . . . . . . . . . . . . . . . . . . . 13 1.2.3 Partition of sets . . . . . . . . . . . . . . . . . . . . . 15 1.2.4 Partial order relations . . . . . . . . . . . . . . . . . . 16 1.3 Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 1.3.1 Composition of functions . . . . . . . . . . . . . . . . 24 1.3.2 Inverse of a function . . . . . . . . . . . . . . . . . . . 26 1.3.3 Images of sets under functions . . . . . . . . . . . . . 32 1.3.4 Inverse images of sets under functions . . . . . . . . . 36 1.4 Countability of Sets . . . . . . . . . . . . . . . . . . . . . . . 39 1.4.1 Finite sets . . . . . . . . . . . . . . . . . . . . . . . . . 41 1.4.2 Countable sets . . . . . . . . . . . . . . . . . . . . . . 44 Problem Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 Biographical Notes . . . . . . . . . . . . . . . . . . . . . . . . . . 51 2 Metric Spaces 55 2.1 Review of Real Number System and Absolute Value . . . . . 55 2.2 Young, H¨older, andMinkowski Inequalities . . . . . . . . . . 57 2.3 Notion ofMetric Space . . . . . . . . . . . . . . . . . . . . . 64 2.4 Open Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 2.4.1 Subspace topology . . . . . . . . . . . . . . . . . . . . 96 2.4.2 Product topology . . . . . . . . . . . . . . . . . . . . . 97 2.5 Closed Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 2.6 Interior, Exterior, and Boundary Points . . . . . . . . . . . . 101 2.7 Limit and Cluster Points . . . . . . . . . . . . . . . . . . . . 104 2.8 Bounded Sets . . . . . . . . . . . . . . . . . . . . . . . . . . 110 2.9 Distance Between Sets . . . . . . . . . . . . . . . . . . . . . 112 2.10 EquivalentMetrics . . . . . . . . . . . . . . . . . . . . . . . . 115 Problem Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 Biographical Notes . . . . . . . . . . . . . . . . . . . . . . . . . . 125 3 Complete Metric Spaces 129 3.1 Sequences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 3.1.1 Subsequences . . . . . . . . . . . . . . . . . . . . . . . 130 3.2 Convergence of Sequence . . . . . . . . . . . . . . . . . . . . 131 3.3 CompleteMetric Spaces . . . . . . . . . . . . . . . . . . . . . 139 3.4 Completion ofMetric Spaces . . . . . . . . . . . . . . . . . . 143 3.4.1 Construction of the set Z . . . . . . . . . . . . . . . . 145 3.4.2 Embedding X in Z . . . . . . . . . . . . . . . . . . . . 147 3.4.3 Proving Z is complete . . . . . . . . . . . . . . . . . . 147 3.4.4 Uniqueness of extension up to isometry . . . . . . . . 148 3.5 Baire Category Theorem . . . . . . . . . . . . . . . . . . . . 149 3.5.1 Category of sets . . . . . . . . . . . . . . . . . . . . . 149 3.5.2 Baire category theorem . . . . . . . . . . . . . . . . . 151 3.5.3 Applications of Baire category theorem . . . . . . . . 153 Problem Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 Biographical Notes . . . . . . . . . . . . . . . . . . . . . . . . . . 158 4 Compact Metric Spaces 161 4.1 Open Cover and Compact Sets . . . . . . . . . . . . . . . . . 161 4.2 General Properties of Compact Sets . . . . . . . . . . . . . . 165 4.3 Sufficient Conditions for Compactness . . . . . . . . . . . . . 169 4.4 Sequential Compactness . . . . . . . . . . . . . . . . . . . . . 172 4.5 Compactness: Characterizations . . . . . . . . . . . . . . . . 174 Problem Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 Biographical Notes . . . . . . . . . . . . . . . . . . . . . . . . . . 180 5 Connected Spaces 183 5.1 Connectedness . . . . . . . . . . . . . . . . . . . . . . . . . . 183 5.1.1 Connected subsets . . . . . . . . . . . . . . . . . . . . 185 5.2 Components . . . . . . . . . . . . . . . . . . . . . . . . . . . 190 5.3 Totally Disconnected Spaces . . . . . . . . . . . . . . . . . . 192 Problem Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193 6 Continuity 195 6.1 Continuity of Real Valued Functions . . . . . . . . . . . . . . 195 6.2 Continuous Functions in ArbitraryMetric Spaces . . . . . . 197 6.2.1 Equivalent definitions of continuity and other characterizations . . . . . . . . . . . . . . . . . . . . . 202 6.2.2 Results on continuity . . . . . . . . . . . . . . . . . . . 210 6.3 Uniform Continuity . . . . . . . . . . . . . . . . . . . . . . . 217 6.4 Continuous Functions on Compact Spaces . . . . . . . . . . . 224 6.5 Continuous Functions on Connected Spaces . . . . . . . . . . 229 6.5.1 Path connectedness . . . . . . . . . . . . . . . . . . . . 237 6.6 Equicontinuity and Arzela-Ascoli’s Theorem . . . . . . . . . 242 6.7 Open and ClosedMaps . . . . . . . . . . . . . . . . . . . . . 245 6.8 Homeomorphism . . . . . . . . . . . . . . . . . . . . . . . . . 246 Problem Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251 Biographical Notes . . . . . . . . . . . . . . . . . . . . . . . . . . 252 7 Banach Fixed Point Theorem and Its Applications 255 7.1 Banach Contraction Theorem . . . . . . . . . . . . . . . . . 255 7.2 Applications of Banach Contraction Principle . . . . . . . . . 260 7.2.1 Root finding problem . . . . . . . . . . . . . . . . . . 260 7.2.2 Solution of systemof linear algebraic equations . . . . 261 7.2.3 Picard existence theorem for differential equations . . 264 7.2.4 Solutions of integral equations . . . . . . . . . . . . . 267 7.2.5 Solutions of initial value and boundary value problems . . . . . . . . . . . . . . . . . . . . . . . . . 271 7.2.6 Implicit function theorem . . . . . . . . . . . . . . . . 273 Problem Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274 Biographical Notes . . . . . . . . . . . . . . . . . . . . . . . . . . 276 Appendix A 277 Bibliography 281 Index 283