Discontinuous groups of isometries in the hyperbolic plane

the long version

"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 linear differential equations in the complex domain (giving rise to the alternative name Fuchsian groups.)" "It is intended for students and researchers in the many areas of mathematics that involve the use of discontinuous groups."--Jacket.