@article{GDZPPN002408732, title = {Subgroups of Finite Index of Fuchsian Groups.}, journal = {Mathematische Zeitschrift}, volume = {121}, year = {1971}, pages = {289-298}, zbl = {0223.20053}, url = {http://dml.mathdoc.fr/item/GDZPPN002408732} }
Karrass, A.; Solitar, D.; Hoare, A.H.M. Subgroups of Finite Index of Fuchsian Groups.. Mathematische Zeitschrift, Tome 121 (1971) pp. 289-298. http://gdmltest.u-ga.fr/item/GDZPPN002408732/