@article{10_1017_S0017089500000641,
     author = {Mason, A. W.},
     title = {Lattice subgroups of free congruence groups},
     journal = {Glasgow mathematical journal},
     pages = {106--115},
     year = {1969},
     volume = {10},
     number = {2},
     doi = {10.1017/S0017089500000641},
     url = {http://geodesic.mathdoc.fr/articles/10.1017/S0017089500000641/}
}