Voir la notice de l'article provenant de la source Math-Net.Ru
@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/} }
A. P. Sadovskii. The conditions for the center of a certain system of differential equations. Differencialʹnye uravneniâ, Tome 7 (1971) no. 12, pp. 2285-2286. http://geodesic.mathdoc.fr/item/DE_1971_7_12_a23/