This work would not have been possible without the help and encouragement of
many people. Our ideas of how to present the material were clarified by critical
questions of the participants in our lecture course. The rough draft of these notes
was subject to detailed criticism by P. Armbruster, P. Braun-Munzinger, K. Dietrich,
D. Fick, W. Frahn, R. Fuller, K. Gelbke, H.L. Harney, D. Pelte and W. von Oertzen,
to all of whom we are very grateful. Needless to say, all remaining mistakes are
ours. We are indebted to J. Knoll and R. Schaeffer for the permission to use
unpublished material. Miss Browning, Mrs. Faulbaum and Mrs. Karl-Kratky typed various
versions of the manuscript, and we owe thanks to each of them for their infinite
patience and care.
These notes do not aim at a review of the field. We apologize to all authors
whose work was not included, or cited incompletely or incorrectly.