Voir la notice de l'article provenant de la source Math-Net.Ru
@article{DE_1965_1_2_a4, author = {V. A. Lunkevich and K. S. Sibirskii}, title = {On conditions for a center}, journal = {Differencialʹnye uravneni\^a}, pages = {176--181}, publisher = {mathdoc}, volume = {1}, number = {2}, year = {1965}, language = {ru}, url = {http://geodesic.mathdoc.fr/item/DE_1965_1_2_a4/} }
V. A. Lunkevich; K. S. Sibirskii. On conditions for a center. Differencialʹnye uravneniâ, Tome 1 (1965) no. 2, pp. 176-181. http://geodesic.mathdoc.fr/item/DE_1965_1_2_a4/