@article{10_1017_S0017089500003347,
     author = {Stothers, W. W.},
     title = {Subgroups of infinite index in the modular group},
     journal = {Glasgow mathematical journal},
     pages = {33--43},
     year = {1978},
     volume = {19},
     number = {1},
     doi = {10.1017/S0017089500003347},
     url = {http://geodesic.mathdoc.fr/articles/10.1017/S0017089500003347/}
}