@article{SM_2011_202_11_a4,
     author = {S. R. Safin},
     title = {Powers of sets in free groups},
     journal = {Sbornik. Mathematics},
     pages = {1661--1666},
     publisher = {mathdoc},
     volume = {202},
     number = {11},
     year = {2011},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_2011_202_11_a4/}
}