@article{MZ_1995__220_1_174827,
     author = {M.-L. Lang and C.-H. Lim and S.-P. Tan},
     title = {Independent generators for congruence subgroups of {Hecke} groups.},
     journal = {Mathematische Zeitschrift},
     pages = {569--594},
     publisher = {mathdoc},
     volume = {220},
     number = {1},
     year = {1995},
     zbl = {0845.11017},
     url = {http://geodesic.mathdoc.fr/item/MZ_1995__220_1_174827/}
}