This book is a product of the experience of the authors in teaching
partial differential equations to students of mathematics, physics, and
engineering over a period of 20 years. Our goal in writing it has been
to introduce the subject with precise and rigorous analysis on the one
hand, and interesting and significant applications on the other. The
starting level of the book is at the first-year graduate level in a U.S.
university. Previous experience with partial differential equations is
not required, but the use of classical analysis to find solutions of
specific problems is not emphasized. From that perspective our treatment
is decidedly theoretical. We have avoided abstraction and full
generality in many situations, however. Our plan has been to introduce
fundamental ideas in relatively simple situations and to show their
impact on relevant applications. The student is then, we feel, well
prepared to fight through more specialized treatises. There are parts of
the exposition that require Lebesgue integration, distributions and
Fourier transforms, and Sobolev spaces. We have included a long
appendix, Chapter 8, giving precise statements of all results used. This
may be thought of as an introduction to these topics. The reader who is
not familiar with these subjects may refer to parts of Chapter 8 as
needed or become somewhat familiar with them as prerequisite and treat
Chapter 8 as Chapter O.