The author is very pleased that his book, first published in Russian in 2000 by MCCME Publishers, is now appearing in English under the auspices of such a truly classical publishing house as Springer-Verlag.
In this edition several pertinent remarks by the referees (to whom the author expresses his gratitude) were taken into account, and new exercises were added (mostly) to the first half of the book, thus achieving a better balance with the second half. Besides, some typos and minor errors, noticed in the Russian edition, were corrected. We are extremely grateful to all our readers who assisted us in this tiresome bug hunt. We are especially grateful to A. De Paris, I. S. Krasil’schik, and A. M. Verbovetski, who demonstrated their acute eyesight, truly of degli Lincei standards.
The English translation was carried out by A.B. Sossinsky (Chapters 1–8), I. S. Krasil’schik (Chapter 9), and S. V. Duzhin (Chapters 10–11) and reduced to a common denominator by the first of them; A. M. Astashov prepared new versions of the figures; all the TEX-nical work was done by M.M. Vinogradov.