@article{02096772, title = {Congruence subgroups of }, journal = {Experimental Mathematics}, volume = {12}, year = {2003}, pages = {243-255}, doi = {10.1080/10586458.2003.10504495}, zbl = {1060.11021}, language = {en}, url = {http://dml.mathdoc.fr/item/02096772} }
Cummins, C.J.; Pauli, S. Congruence subgroups of . Experimental Mathematics, Tome 12 (2003) pp. 243-255. doi : 10.1080/10586458.2003.10504495. http://gdmltest.u-ga.fr/item/02096772/