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@article{DE_1969_5_2_a10, author = {A. P. Sadovskii}, title = {On the problem of discrimination of center and focus}, journal = {Differencialʹnye uravneni\^a}, pages = {326--330}, publisher = {mathdoc}, volume = {5}, number = {2}, year = {1969}, language = {ru}, url = {http://geodesic.mathdoc.fr/item/DE_1969_5_2_a10/} }
A. P. Sadovskii. On the problem of discrimination of center and focus. Differencialʹnye uravneniâ, Tome 5 (1969) no. 2, pp. 326-330. http://geodesic.mathdoc.fr/item/DE_1969_5_2_a10/