@article{GDZPPN00232332X, title = {Maximal Nonparabolic Subgroups of the Modular Group.}, journal = {Mathematische Annalen}, volume = {264}, year = {1983}, pages = {1-12}, zbl = {0491.20037}, url = {http://dml.mathdoc.fr/item/GDZPPN00232332X} }
Brenner, J.L.; Lyndon, R.C. Maximal Nonparabolic Subgroups of the Modular Group.. Mathematische Annalen, Tome 264 (1983) pp. 1-12. http://gdmltest.u-ga.fr/item/GDZPPN00232332X/