@article{VUU_2016_26_3_a4,
     author = {E. G. Pytkeev and A. G. Chentsov},
     title = {Some representations  of free ultrafilters},
     journal = {Vestnik Udmurtskogo universiteta. Matematika, mehanika, kompʹ\^uternye nauki},
     pages = {345--365},
     publisher = {mathdoc},
     volume = {26},
     number = {3},
     year = {2016},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/VUU_2016_26_3_a4/}
}