@article{DE_1968_4_3_a19,
     author = {V. B. Ryvkin},
     title = {Letter to the editor},
     journal = {Differencialʹnye uravneni\^a},
     pages = {556},
     publisher = {mathdoc},
     volume = {4},
     number = {3},
     year = {1968},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1968_4_3_a19/}
}