In the preface to the first edition we have described the origin of this book rooted
at the beginning in a course of lectures. With this fact in mind, we were naturally
pleased to learn that the volume was used as a course text in many points of the
world and we gladly accepted the offer of Springer Verlag which inherited the rights
from our original publisher, to consider preparation of a second edition.
It was our ambition to bring the reader close to the places where real life dwells,
and therefore this edition had to be more than a corrected printing. The field is
developing rapidly and since the first edition various new subjects have appeared;
as a couple of examples let us mention quantum computing or the major progress in
the investigation of random Schr¨odinger operators. There are, however, good sources
in the literature where the reader can learn about these and other new developments.