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 reader needs little
technical knowledge to understand most of these books, he will have
to make an intellectual effort.