@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/