@article{SM_1979_35_1_a3,
     author = {Yu. A. Ignatov},
     title = {Free and nonfree subgroups of $PSL_2(\mathbf C)$ generated by two parabolic elements},
     journal = {Sbornik. Mathematics},
     pages = {49--55},
     year = {1979},
     volume = {35},
     number = {1},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1979_35_1_a3/}
}