Voir la notice de l'article provenant de la source Math-Net.Ru
@article{DE_1978_14_9_a5, author = {\`E. F. Kirnitskaya and K. S. Sibirskii}, title = {Conditions for two centers for a quadratic differential system}, journal = {Differencialʹnye uravneni\^a}, pages = {1589--1593}, publisher = {mathdoc}, volume = {14}, number = {9}, year = {1978}, language = {ru}, url = {http://geodesic.mathdoc.fr/item/DE_1978_14_9_a5/} }
È. F. Kirnitskaya; K. S. Sibirskii. Conditions for two centers for a quadratic differential system. Differencialʹnye uravneniâ, Tome 14 (1978) no. 9, pp. 1589-1593. http://geodesic.mathdoc.fr/item/DE_1978_14_9_a5/