@article{MOMA_1979__87_177918,
     author = {W. Imrich and R. Razen},
     title = {On the number of subgroups of given index in the modular group.},
     journal = {Monatshefte f\"ur Mathematik},
     pages = {273--280},
     publisher = {mathdoc},
     volume = {87},
     year = {1979},
     zbl = {0407.20037},
     url = {http://geodesic.mathdoc.fr/item/MOMA_1979__87_177918/}
}