@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/