@article{AL_1981_20_6_a2,
     author = {D. I. Zaitsev},
     title = {Nilpotent approximations of metabelian groups},
     journal = {Algebra i logika},
     pages = {638--653},
     year = {1981},
     volume = {20},
     number = {6},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/AL_1981_20_6_a2/}
}