@article{SM_1986_54_2_a4,
     author = {Yu. V. Tishin},
     title = {Normal subgroups of free constructions},
     journal = {Sbornik. Mathematics},
     pages = {367--385},
     publisher = {mathdoc},
     volume = {54},
     number = {2},
     year = {1986},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1986_54_2_a4/}
}