The investigations of Siegel on discrete groups of motions of the hyperbolic plane with a fundamental region of finite volume ... make possible a simple characterization of groups conjugate to the modular group by a minimal condition.
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Lectures on modular functions of one complex variable. Vol. 29. Berlin-Heidelberg-New York: Springer, 1983 revised edition. (notes by Sunder Lal taken on lectures given by Maass at the Tata Institute of Fundamental Research in 1962–1963)