This book is the first of a series of monographs on mathematical subjects which are to be published under the auspices of the Mathematical Association of America and whose publication has been made possible by a very generous gift to the Association by Mrs. Mary Hegeler Carus as trustee for the Edward C. Hegeler Trust Fund. The purpose of the monographs is to make the essential features of various mathematical theories accessible and attractive to as many persons as possible who have an interest in mathematics but who may not be specialists in the particular theory presented, a purpose which Mrs. Carus has very appropriately described to be “the diffusion of mathematical and formal thought as contributory to exact knowledge and clear thinking, not only for mathematicians and teachers of mathematics but also for other scientists and the public at large.”

The attainment of this end will not always be easy for authors who have long specialized in unraveling the intricacies of the domains in which their principal activities lie, and the clientele of readers which they may reasonably hope to interest will vary greatly with the subjects presented. It would obviously be unwise to regard this first attempt as in any final sense a model for the many monographs which it is hoped will follow.