This book is one of a series written by professional mathematicians in order to make some important mathematical ideas interesting and understandable to a large audience of high school students and laymen. Most of the volumes in the New Mathematical Library cover topics not usually included in the high school curriculum; they vary in difficulty, and, even within a single book, some parts require a greater degree of concentration than others. Thus, while the readers need little technical knowledge to understand most of these books, they will have to make an intellectual effort.

If readers have encountered mathematics so far only in the classroom, they should keep in mind that a book on mathematics cannot be read quickly. Nor must they expect to understand all parts of the book on first reading. They should feel free to skip complicated parts and return to them later; often an argument will be clarified by a subsequent remark. On the other hand, sections containing thoroughly familiar material may be read very quickly.

The best way to learn mathematics is to do mathematics. Readers are urged to acquire the habit of reading with paper and pencil in hand; in this way mathematics will become increasingly meaningful to them.

The authors and editorial committee are interested in reactions to the books in this series, and hope that readers will write to: Anneli Lax, Editor, New Mathematical Library, New York University, The Courant Instituteof Mathematical Sciences, 251 Mercer Street, New York, N. Y. 10012.