@article{IM_1982__69_142957,
     author = {A.L. Edmonds and J.H. Ewing and R. Kulkarni},
     title = {Torsion {Free} {Subgroups} of {Fuchsian} {Groups} and {Tessalations} of {Surfaces.}},
     journal = {Inventiones mathematicae},
     pages = {331--346},
     publisher = {mathdoc},
     volume = {69},
     year = {1982},
     zbl = {0498.20033},
     url = {http://geodesic.mathdoc.fr/item/IM_1982__69_142957/}
}