@article{MAN_1992__293_4_164945,
     author = {A.W. Mason},
     title = {Free quotients of congruence subgroups of {Bianchi} groups.},
     journal = {Mathematische Annalen},
     pages = {65--76},
     publisher = {mathdoc},
     volume = {293},
     number = {4},
     year = {1992},
     zbl = {0734.20021},
     url = {http://geodesic.mathdoc.fr/item/MAN_1992__293_4_164945/}
}