@article{GDZPPN002086271,
title = {Lattice Subgroups of Free Congruence Groups.},
journal = {Inventiones mathematicae},
volume = {1},
year = {1966},
pages = {215-221},
zbl = {0146.04105},
url = {http://dml.mathdoc.fr/item/GDZPPN002086271}
}
RANKIN, R.A. Lattice Subgroups of Free Congruence Groups.. Inventiones mathematicae, Tome 1 (1966) pp. 215-221. http://gdmltest.u-ga.fr/item/GDZPPN002086271/