Bücher von Jurgen Appell

Das Buch gibt in sechs Kapiteln eine Einführung in die Theorie der reellen Funktionen einer und mehrerer Variabler. Hierbei stehen nicht so sehr abstrakte Ergebnisse im Vordergrund, sondern es werden besonders viele Beispiele und Gegenbeispiele präsentiert, anhand derer man die Bedeutung mathematischer Sätze besonders gut erkennen kann.In den ersten drei Kapiteln werden die wesentlichen Ergebnisse über stetige, differenzierbare und integrierbare Funktionen zusammengestellt. Das vierte Kapitel geht etwas über den üblichen Analysisstoff hinaus und ist "merkwürdigen" Teilmengen der reellen Achse und zugehörigen Funktionen gewidmet. Funktionen mehrerer Variabler werden im fünften und sechsten Kapitel behandelt.Über die starke Betonung von Beispielen hinaus ist ein weiteres Merkmal des Buches die große Anzahl von Übungsaufgaben am Ende jedes Kapitels. Es ist daher auch sehr gut als Aufgabensammlung zur Prüfungsvorbereitung geeignet.

In view of the eminent importance of spectral theory of linear operators in many fields of mathematics and physics, it is not surprising that various attempts have been made to define and study spectra also for nonlinear operators. This book provides a comprehensive and self-contained treatment of the theory, methods, and applications of nonlinear spectral theory. The first chapter briefly recalls the definition and properties of the spectrum and several subspectra for bounded linear operators. Then some numerical characteristics for nonlinear operators are introduced which are useful for describing those classes of operators for which there exists a spectral theory. Since spectral values are closely related to solvability results for operator equations, various conditions for the local or global invertibility of a nonlinear operator are collected in the third chapter. The following two chapters are concerned with spectra for certain classes of continuous, Lipschitz continuous, or differentiable operators. These spectra, however, simply adapt the corresponding definitions from the linear theory which somehow restricts their applicability. Other spectra which are defined in a completely different way, but seem to have useful applications, are defined and studied in the following four chapters. The remaining three chapters are more application-oriented and deal with nonlinear eigenvalue problems, numerical ranges, and selected applications to nonlinear problems. The only prerequisite for understanding this book is a modest background in functional analysis and operator theory. It is addressed to non-specialists who want to get an idea of the development of spectral theory for nonlinear operators in the last 30 years, as well as a glimpse of the diversity of the directions in which current research is moving.

This book is a self-contained account of knowledge of the theory of nonlinear superposition operators: a generalization of the notion of functions. The theory developed here is applicable to operators in a wide variety of function spaces, and it is here that the modern theory diverges from classical nonlinear analysis.