[CLLT93] Chan, S.-P., Lang, M.-L., Lim, C.-H. and Tan, S. P., Special polygons for subgroups of the modular group and applications, Internat. J. Math., 4 (1) (1993), 11--34.
[Kul91] Kulkarni, R. S., An arithmetic-geometric method in the study of the subgroups of the modular group, Amer. J. Math., 113 (6) (1991), 1053--1133.
[LLT95a] Lang, M.-L., Lim, C.-H. and Tan, S. P., An algorithm for determining if a subgroup of the modular group is congruence, J. London Math. Soc. (2), 51 (3) (1995), 491--502.
[LLT95b] Lang, M.-L., Lim, C.-H. and Tan, S. P., Independent generators for congruence subgroups of Hecke groups, Math. Z., 220 (4) (1995), 569--594.
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