@article{DAN_1997_352_6_a4,
     author = {L. Z. Fishman},
     title = {Preservation of the properties of second-order differential},
     journal = {Doklady Akademii Nauk},
     pages = {739--741},
     publisher = {mathdoc},
     volume = {352},
     number = {6},
     year = {1997},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1997_352_6_a4/}
}