@article{MZ_1971__120_171529,
     author = {A. Karrass and D. Solitar and A.H.M. Hoare},
     title = {Subgroups of {Finite} {Index} of {Fuchsian} {Groups.}},
     journal = {Mathematische Zeitschrift},
     pages = {289--298},
     publisher = {mathdoc},
     volume = {120},
     year = {1971},
     zbl = {0223.20053},
     url = {http://geodesic.mathdoc.fr/item/MZ_1971__120_171529/}
}