@article{PDM_2009_3_a0,
     author = {N. G. Parvatov},
     title = {Lower and upper neighborhoods in set with closure property},
     journal = {Prikladna\^a diskretna\^a matematika},
     pages = {5--13},
     publisher = {mathdoc},
     number = {3},
     year = {2009},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/PDM_2009_3_a0/}
}