Hans Maass

"The investigations of Siegel on discrete groups of motions of the with a fundamental region of finite volume ... make possible a simple characterization of groups conjugate to the modular group by a minimal condition."

"... in 1949, a paper appeared by a German mathematician Hans Maass, which raised some rather interesting problems. You see, earlier the automorphic functions and forms — one had essentially thought of functions that were often called holomorphic, analytical. And Maass started studying functions that were not of that nature, but were instead solutions of a certain eigenvalue problem, which had a certain type of behavior with respect to the discrete group which corresponds to the modular forms. Maass also worked essentially just on the modular group and its subgroups, not on general groups."