@article{IM_1966__2_141853,
     author = {R.A. RANKIN},
     title = {Lattice {Subgroups} of {Free} {Congruence} {Groups.}},
     journal = {Inventiones mathematicae},
     pages = {215--221},
     publisher = {mathdoc},
     volume = {2},
     year = {1966},
     zbl = {0146.04105},
     url = {http://geodesic.mathdoc.fr/item/IM_1966__2_141853/}
}