Addendum: Normal Congruence Subgroups of the Modular Group

1964 ◽  
Vol 86 (2) ◽  
pp. 465
Author(s):  
Morris Newman
1985 ◽  
Vol 27 ◽  
pp. 239-247 ◽  
Author(s):  
K. Wohlfahrt

The theory of algebraic curves associated with subgroups of finite index in the modular group Γ is highly developed for such subgroups of Γ as may be defined by means of congruences in the ring ℤ of rational integers. The situation in he case of non-congruence subgroups of Γ, on the other hand, is drastically different. It reduces to a few isolated examples, two of which may be found in [12]. Related research by A. O. L. Atkin and H. P. F. Swinnerton-Dyer began in [1].


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