This series is devoted to thorough, yet reasonably concise treatments of topics in any branch of mathematics. Typically, a Tract takes up a single thread in a wide subject, and follows its ramifications, thus throwing light on various of its aspects. Tracts are expected to be rigorous, definitive and of lasting value to mathematicians working in the relevant disciplines. Exercises can be included to illustrate techniques, summarize past work and enhance the book's value as a seminar text.