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@article{DE_1968_4_11_a8, author = {A. P. Sadovskii}, title = {The problem of center and focus}, journal = {Differencialʹnye uravneni\^a}, pages = {2002--2009}, publisher = {mathdoc}, volume = {4}, number = {11}, year = {1968}, language = {ru}, url = {http://geodesic.mathdoc.fr/item/DE_1968_4_11_a8/} }
A. P. Sadovskii. The problem of center and focus. Differencialʹnye uravneniâ, Tome 4 (1968) no. 11, pp. 2002-2009. http://geodesic.mathdoc.fr/item/DE_1968_4_11_a8/