@article{DE_1992_28_5_a22,
     author = {Z. I. Ismailov},
     title = {Formally normal extensions of an operator},
     journal = {Differencialʹnye uravneni\^a},
     pages = {905--907},
     publisher = {mathdoc},
     volume = {28},
     number = {5},
     year = {1992},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1992_28_5_a22/}
}