@article{RM_1974_29_1_a20,
     author = {A. Yu. Ol'shanskii},
     title = {On characteristic subgroups of free groups},
     journal = {Trudy Matematicheskogo Instituta imeni V.A. Steklova},
     year = {1974},
     volume = {29},
     number = {1},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/RM_1974_29_1_a20/}
}