@article{AL_1967_6_3_a2,
     author = {Yu. M. Gor\v{c}akov},
     title = {On the central series of the free groups of varieties},
     journal = {Algebra i logika},
     pages = {13--24},
     publisher = {mathdoc},
     volume = {6},
     number = {3},
     year = {1967},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/AL_1967_6_3_a2/}
}