@article{AL_1989_28_2_a6,
     author = {G. Rosenberger},
     title = {On free subgroups of generalized triangle groups},
     journal = {Algebra i logika},
     pages = {227--240},
     year = {1989},
     volume = {28},
     number = {2},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/AL_1989_28_2_a6/}
}