@article{AL_1977_16_1_a6,
     author = {N. S. Romanovskii},
     title = {Free subgroups of finitely-presented groups},
     journal = {Algebra i logika},
     pages = {88--97},
     year = {1977},
     volume = {16},
     number = {1},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/AL_1977_16_1_a6/}
}