A First Course in Functional Analysis

A concise introduction to the major concepts of functional analysis Requiring only a preliminary knowledge of elementary linear algebra and real analysis, A First Course in Functional Analysis provides an introduction to the basic principles and practical applications of functional analysis. Key concepts are illustrated in a straightforward manner, which facilitates a complete and fundamental understanding of the topic. This book is based on the author's own class-tested material and uses clear language to explain the major concepts of functional analysis, including Banach spaces, Hilbert spaces, topological vector spaces, as well as bounded linear functionals and operators. As opposed to simply presenting the proofs, the author outlines the logic behind the steps, demonstrates the development of arguments, and discusses how the concepts are connected to one another. Each chapter concludes with exercises ranging in difficulty, giving readers the opportunity to reinforce their comprehension of the discussed methods. An appendix provides a thorough introduction to measure and integration theory, and additional appendices address the background material on topics such as Zorn's lemma, the Stone-Weierstrass theorem, Tychonoff's theorem on product spaces, and the upper and lower limit points of sequences. References to various applications of functional analysis are also included throughout the book. A First Course in Functional Analysis is an ideal text for upper-undergraduate and graduate-level courses in pure and applied mathematics, statistics, and engineering. It also serves as a valuable reference for practitioners across various disciplines, including the physical sciences, economics, and finance, who would like to expand their knowledge of functional analysis.

Table of Contents:
Preface xi 1. Linear Spaces and Operators 1 1.1 Introduction 1 1.2 Linear Spaces 2 1.3 Linear Operators 5 1.4 Passage from Finite- to Infinite-Dimensional Spaces 7 Exercises 8 2. Normed Linear Spaces: The Basics 11 2.1 Metric Spaces 11 2.2 Norms 12 2.3 Space of Bounded Functions 18 2.4 Bounded Linear Operators 19 2.5 Completeness 21 2.6 Comparison of Norms 30 2.7 Quotient Spaces 31 2.8 Finite-Dimensional Normed Linear Spaces 34 2.9 Lᵖ Spaces 38 2.10 Direct Products and Sums 51 2.11 Schauder Bases 53 2.12 Fixed Points and Contraction Mappings 53 Exercises 54 3. Major Banach Space Theorems 59 3.1 Introduction 59 3.2 Baire Category Theorem 59 3.3 Open Mappings 61 3.4 Bounded Inverses 63 3.5 Closed Linear Operators 64 3.6 Uniform Boundedness Principle 66 Exercises 68 4. Hilbert Spaces 71 4.1 Introduction 71 4.2 Semi-Inner Products 72 4.3 Nearest Points and Convexity 77 4.4 Orthogonality 80 4.5 Linear Functionals on Hilbert Spaces 86 4.6 Linear Operators on Hilbert Spaces 88 4.7 Order Relation on Self-Adjoint Operators 97 Exercises 98 5. Hahn–Banach Theorem 103 5.1 Introduction 103 5.2 Basic Version of Hahn–Banach Theorem 104 5.3 Complex Version of Hahn–Banach Theorem 105 5.4 Application to Normed Linear Spaces 107 5.5 Geometric Versions of Hahn–Banach Theorem 108 Exercises 118 6. Duality 121 6.1 Examples of Dual Spaces 121 6.2 Adjoints 130 6.3 Double Duals and Reflexivity 133 6.4 Weak and Weak* Convergence 136 Exercises 140 7. Topological Linear Spaces 143 7.1 Review of General Topology 143 7.2 Topologies on Linear Spaces 148 7.3 Linear Functionals on Topological Linear Spaces 151 7.4 Weak Topology 153 7.5 Weak* Topology 156 7.6 Extreme Points and Krein–Milman Theorem 160 7.7 Operator Topologies 164 Exercises 164 8. The Spectrum 167 8.1 Introduction 167 8.2 Banach Algebras 169 8.3 General Properties of the Spectrum 170 8.4 Numerical Range 176 8.5 Spectrum of a Normal Operator 177 8.6 Functions of Operators 180 8.7 Brief Introduction to C_-Algebras 183 Exercises 184 9. Compact Operators 187 9.1 Introduction and Basic Definitions 187 9.2 Compactness Criteria in Metric Spaces 188 9.3 New Compact Operators from Old 192 9.4 Spectrum of a Compact Operator 194 9.5 Compact Self-Adjoint Operators on Hilbert Spaces 197 9.6 Invariant Subspaces 201 Exercises 203 10. Application to Integral and Differential Equations 205 10.1 Introduction 205 10.2 Integral Operators 206 10.3 Integral Equations 211 10.4 Second-Order Linear Differential Equations 214 10.5 Sturm–Liouville Problems 217 10.6 First-Order Differential Equations 223 Exercises 226 11. Spectral Theorem for Bounded, Self-Adjoint Operators 229 11.1 Introduction and Motivation 229 11.2 Spectral Decomposition 231 11.3 Extension of Functional Calculus 235 11.4 Multiplication Operators 240 Exercises 243 Appendix A Zorn’s Lemma 245 Appendix B Stone–Weierstrass Theorem 247 B.1 Basic Theorem 247 B.2 Nonunital Algebras 250 B.3 Complex Algebras 252 Appendix C Extended Real Numbers and Limit Points of Sequences 253 C.1 Extended Reals 253 C.2 Limit Points of Sequences 254 Appendix D Measure and Integration 257 D.1 Introduction and Notation 257 D.2 Basic Properties of Measures 258 D.3 Properties of Measurable Functions 259 D.4 Integral of a Nonnegative Function 261 D.5 Integral of an Extended Real-Valued Function 265 D.6 Integral of a Complex-Valued Function 267 D.7 Construction of Lebesgue Measure on R 267 D.8 Completeness of Measures 273 D.9 Signed and Complex Measures 274 D.10 Radon–Nikodym Derivatives 276 D.11 Product Measures 278 D.12 Riesz Representation Theorem 280 Appendix E Tychonoff’s Theorem 289 Symbols 293 References 297 Index 299