@article{GDZPPN002098687,
title = {Torsion Free Subgroups of Fuchsian Groups and Tessalations of Surfaces.},
journal = {Inventiones mathematicae},
volume = {69},
year = {1982},
pages = {331-346},
zbl = {0498.20033},
url = {http://dml.mathdoc.fr/item/GDZPPN002098687}
}
Edmonds, A.L.; Ewing, J.H.; Kulkarni, R. Torsion Free Subgroups of Fuchsian Groups and Tessalations of Surfaces.. Inventiones mathematicae, Tome 69 (1982) pp. 331-346. http://gdmltest.u-ga.fr/item/GDZPPN002098687/