@article{JRAM_1980__314_152216,
     author = {Gareth A. Jones},
     title = {Noncongruence subgroups of the {Hecke} groups {G} (...).},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {174--176},
     publisher = {mathdoc},
     volume = {314},
     year = {1980},
     zbl = {0417.10022},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1980__314_152216/}
}