Discontinuous Groups of Isometries in the Hyperbolic Plane - Werner Fenchel, Jakob Nielsen - Google Books

http://books.google.de/books?id=IJC3ojxMamEC

This is an introductory textbook on isometry groups of the hyperbolic plane. Interest in such groups dates back more than 120 years. Examples appear in number theory (modular groups and triangle groups), the theory of elliptic functions, and the theory of…