@article{MAN_1983__263_163727,
     author = {J.L. Brenner and R.C. Lyndon},
     title = {Maximal {Nonparabolic} {Subgroups} of the {Modular} {Group.}},
     journal = {Mathematische Annalen},
     pages = {1--12},
     publisher = {mathdoc},
     volume = {263},
     year = {1983},
     zbl = {0491.20037},
     url = {http://geodesic.mathdoc.fr/item/MAN_1983__263_163727/}
}