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@article{DE_1968_4_5_a19, author = {A. P. Sadovskii}, title = {A solution of the center and focus problem in a special case}, journal = {Differencialʹnye uravneni\^a}, pages = {943--945}, publisher = {mathdoc}, volume = {4}, number = {5}, year = {1968}, language = {ru}, url = {http://geodesic.mathdoc.fr/item/DE_1968_4_5_a19/} }
A. P. Sadovskii. A solution of the center and focus problem in a special case. Differencialʹnye uravneniâ, Tome 4 (1968) no. 5, pp. 943-945. http://geodesic.mathdoc.fr/item/DE_1968_4_5_a19/