We are extremely grateful to Springer-Verlag and to Prof. Dr. W. BeiglbOck for bring ing out the English edition of our book. We are also thankful to Dr. R. S. Wadhwa for a qualified translation. While preparing the manuscript for translation, we took the opportunity to go through the whole text, make necessary amendments, supplement the original material with new results, and considerably enlarge the lists of references. We hope that this book will serv to strengthen the bonds of international coopera tion in this field. July 1986 The authors Translator's Note The final form of the...