The Dictionary of Classical and Theoretical Mathematics, one volume of the Comprehensive Dictionary of Mathematics, includes entries from the fields of geometry, logic, number theory, set theory, and topology. The authors who contributed their work to this volume are professional mathematicians, active in both teaching and research. The goal in writing this dictionary has been to define each term rigorously, not to author a large and comprehensive survey text in mathematics. Though it has remained our purpose to make each definition self-contained, some definitions unavoidably depend on others, and a modicum of “definition chasing” is necessitated. We hope this is minimal | F CLASSICAL AND THEORETICAL MATHEMATICS 2001 by CRC Press LLC A VOLUME IN THE COMPREHENSIVE DICTIONARY OF MATHEMATICS DICTIONARY OF CLASSICAL AND THEORETICAL MATHEMATICS Edited by Catherine Cavagnaro William T. Haight II CRC Press Boca Raton London New York Washington . 2001 by CRC Press .