@article{10_4153_CJM_1980_116_7,
     author = {Fine, Benjamin},
     title = {Congruence {Subgroups} of the {Picard} {Group}},
     journal = {Canadian journal of mathematics},
     pages = {1474--1481},
     year = {1980},
     volume = {32},
     number = {6},
     doi = {10.4153/CJM-1980-116-7},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CJM-1980-116-7/}
}