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@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/} }
Gareth A. Jones. Noncongruence subgroups of the Hecke groups G (...).. Journal für die reine und angewandte Mathematik, Tome 314 (1980), pp. 174-176. http://geodesic.mathdoc.fr/item/JRAM_1980__314_152216/