@article{10_1017_S0017089500004419,
     author = {Cohen, Jeffrey},
     title = {On {non-Hurwitz} groups and non-congruence subgroups of the modular group},
     journal = {Glasgow mathematical journal},
     pages = {1--7},
     year = {1981},
     volume = {22},
     number = {1},
     doi = {10.1017/S0017089500004419},
     url = {http://geodesic.mathdoc.fr/articles/10.1017/S0017089500004419/}
}