@article{MZ_1972__125_171689,
     author = {A. Howard M. Hoare and Abraham Karrass},
     title = {Subgroups of {Infinite} {Index} in {Fuchsian} {Groups.}},
     journal = {Mathematische Zeitschrift},
     pages = {59--69},
     publisher = {mathdoc},
     volume = {125},
     year = {1972},
     zbl = {0223.20054},
     url = {http://geodesic.mathdoc.fr/item/MZ_1972__125_171689/}
}