@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/