@article{DE_1984_20_4_a21,
     author = {D. Kh. Safarov},
     title = {Systems of second-order partial differential equations},
     journal = {Differencialʹnye uravneni\^a},
     pages = {716--719},
     year = {1984},
     volume = {20},
     number = {4},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1984_20_4_a21/}
}