The present volume is concerned with the theory of partial dif- ferential equations, in particular with parts of this wide field that are
related to concepts of physics and mechanics. Even with this restric-
tion, completeness seems unattainable; to a certain extent the ma-
terial selected corresponds to my personal experience and taste. The
intention is to make an important branch of mathematical analysis
more accessible by emphasizing concepts and methods rather than
presenting a collection of theorems and farts, and by leading from
an elementary level to key points on the frontiers of our knowledge