@article{CMUC_2014_55_2_a10,
     author = {Protasov, I. and Slobodianiuk, S.},
     title = {Ultracompanions of subsets of a group},
     journal = {Commentationes Mathematicae Universitatis Carolinae},
     pages = {257--265},
     year = {2014},
     volume = {55},
     number = {2},
     mrnumber = {3193930},
     zbl = {06391542},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/CMUC_2014_55_2_a10/}
}