@article{AL_1996_35_2_a2,
     author = {Yu. L. Ershov},
     title = {Free $\Delta^*$-groups},
     journal = {Algebra i logika},
     pages = {154--172},
     publisher = {mathdoc},
     volume = {35},
     number = {2},
     year = {1996},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/AL_1996_35_2_a2/}
}