Preface to the Third Edition In the new edition,supplements,additional explanations and cross references have been added at numerous places,including new formulations of the prob- lems.Fig res have been redra wn and the lay out has been improved.in all additions I have intended to cha ct c lenweber.It was a pleasure to work with Dr.R.Hilton,in order to convey the spirit and the subtleties of the German text into the English translation. Also,I wish to thank Prof.U.Tauber for occasional advice.Special thanks go to them and to Mrs.Jorg-Muiller for general supervision.I would like to thank all colleagues and students who have made suggestions to improve the book,as well as the publisher,Dr.Thorsten Schneider and Mrs.J.Lenz for the excellent cooperation. Munich,May 2005 F.SchwablPreface to the Third Edition In the new edition, supplements, additional explanations and cross references have been added at numerous places, including new formulations of the prob￾lems. Figures have been redrawn and the layout has been improved. In all these additions I have intended not to change the compact character of the book. The proofs were read by E. Bauer, E. Marquard–Schmitt and T. Wol￾lenweber. It was a pleasure to work with Dr. R. Hilton, in order to convey the spirit and the subtleties of the German text into the English translation. Also, I wish to thank Prof. U. T¨auber for occasional advice. Special thanks go to them and to Mrs. J¨org-M¨uller for general supervision. I would like to thank all colleagues and students who have made suggestions to improve the book, as well as the publisher, Dr. Thorsten Schneider and Mrs. J. Lenz for the excellent cooperation. Munich, May 2005 F. Schwabl