The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Typesetting: Camera-ready copy by translator Cover-design: de'blik, Berlin Printed on acid-free paper 62 / 3020 hu-5 4 3 2 I 0 'polishing' the English of the book was very appreciated, and Dr. D.Merkle and the staff of Springer-Verlag for assistance in preparation and publication of the book. The author is also grateful to Prof. Ju.Kaplunov for useful discussion of a number of aspects of this work and to colleagues who have rendered help in realization of some numerical computations. And finally, I am externally grateful to my wife and colleague Dr. G.Mel'nikova, without whose daily help and support this book would never have been ready for publication.
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