@article{10_4064_aa_24_3_313_323,
     author = {R. Rankin},
     title = {Subgroups of the modular group defined by a single linear congruence},
     journal = {Acta Arithmetica},
     pages = {313--323},
     publisher = {mathdoc},
     volume = {24},
     number = {3},
     year = {1973},
     doi = {10.4064/aa-24-3-313-323},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/aa-24-3-313-323/}
}