most recent 30 from http://mathoverflow.net 2013-06-19T03:01:30Z http://mathoverflow.net/feeds/user/3738 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/2757/generators-for-congruence-subgroups-of-sl-2/13915#13915 Ignat Soroko 2010-02-03T02:40:40Z 2010-02-03T02:40:40Z

<p>Hi Andy,</p> <p>I don't know if you are still interested in this, but I just found the reference:</p> <p>MR0049937 (14,250d) Grosswald, Emil On the parabolic generators of the principal congruence subgroups of the modular group. Amer. J. Math. 74, (1952). 435--443.</p> <p>It is based on the previous work of H.Frasch (1933) who gave an explicit set of free generators for principal congruence subgroups Gamma(p) in PSL(2,Z), for prime p's.</p> <p>-Ignat</p>