xvi Preface
Craig Jackson read the English version of the book and made many
useful corrections and remarks.
The final part of the work on the book was done during my visits to the
Institut des Hautes
´
Etudes Scientifiques (Bures-sur-Yvette, France) and the
Max Planck Institute of Mathematics (Bonn, Germany). I am very grateful
to both institutions for their hospitality.
In conclusion I want to thank my teachers, friends, colleagues, and es-
pecially my students, from whom I learned so much.
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