Mathematics is playing an ever more important role in the physical and biological sciences, provo king a blurring of boundaries between scientific dis­ ciplines and a resurgence of interest in the modern as weil as the classical techniques of applied mathematics. This renewal of interest, both in research and teaching, has led to the establishment of the series: Texts in Applied Mathe­ matics (TAM). The development of new courses is a natural consequence of a . high level of excitement on the research frontier as newer techniques, such as numerical and symbolic computer systems, dynamical systems, and chaos, mix with and reinforce the traditional methods of applied mathematics. Thus, the purpose of this textbook series is to meet the current and future needs of these advances and encourage the teaching of new courses. TAM will publish textbooks suitable für use in advanced undergraduate and beginning graduate courses, and will complement the Applied Mathematical Sciences (AMS) series,which will focus on advanced textbooks and research level monographs. Preface A proper understanding of the theory of boundary value problems, as op­ posed to a knowledge of techniques for solving specific problems or classes of problems, requires some background in functional analysis.