The purpose of this book is to describe as simply as possible a number of the ideas and methods which seem to be particularly
helpful in the study of nonlinear boundary value problems for
differential equations of the second order. Apart from the
restriction to second order, the kinds of equations treated and
the level of treatment are those common to introductory courses
in differential equations which treat initial value problems. The
reason for this restriction is that whereas nth order initial value
problems are not essentially more difficult than first or second
order, higher order boundary value problems are. Those of
second order already show the difficulties but are becoming
fairly well understood. Much of the material has only recently
appeared in the mathematical literature, however, and cannot
yet be found in the textbooks.