@article{DE_1971_7_12_a23,
     author = {A. P. Sadovskii},
     title = {The conditions for the center of a certain system of differential equations},
     journal = {Differencialʹnye uravneni\^a},
     pages = {2285--2286},
     publisher = {mathdoc},
     volume = {7},
     number = {12},
     year = {1971},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1971_7_12_a23/}
}