@article{IM2_2013_77_3_a9,
     author = {Vik. S. Kulikov and V. M. Kharlamov},
     title = {Covering semigroups},
     journal = {Izvestiya. Mathematics },
     pages = {594--626},
     publisher = {mathdoc},
     volume = {77},
     number = {3},
     year = {2013},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/IM2_2013_77_3_a9/}
}