@article{DAN_1969_186_1_a4,
     author = {L. V. Ovsyannikov},
     title = {Partial invariance},
     journal = {Doklady Akademii Nauk},
     pages = {22--25},
     year = {1969},
     volume = {186},
     number = {1},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1969_186_1_a4/}
}