@article{IIMI_2015_2_a18,
     author = {E. G. Pytkeev and A. G. Chentsov},
     title = {Some properties of open ultrafilters},
     journal = {Izvestiya Instituta Matematiki i Informatiki Udmurtskogo Gosudarstvennogo Universiteta},
     pages = {140--148},
     year = {2015},
     number = {2},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/IIMI_2015_2_a18/}
}