@article{TM_1983_154_a30,
     author = {H. Zieschang},
     title = {Subgroups of a~free product of cyclic groups},
     journal = {Trudy Matematicheskogo Instituta imeni V.A. Steklova},
     pages = {284--295},
     year = {1983},
     volume = {154},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/TM_1983_154_a30/}
}