The Board of Trustees of the American Mathematical Society, expressing its belief that a great deal of time would be saved for mathematicians if they could study a textbook of Russian precisely adapted to their needs, granted to the present author nine months leave of absence from his duties as Editor of Translations. To the Board, and to Gordon L. Walker, the Exec utive Director of the Society, who took the initiative in this matter with his customary energy and good will, the author is deeply gratefUl for the opportunity to write such a book. For indispensable help and advice in the...