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@article{DE_1978_14_12_a10, author = {A. P. Sadovskii}, title = {Formal transformations and the problem of distinguishing the center and focus}, journal = {Differencialʹnye uravneni\^a}, pages = {2185--2189}, publisher = {mathdoc}, volume = {14}, number = {12}, year = {1978}, language = {ru}, url = {http://geodesic.mathdoc.fr/item/DE_1978_14_12_a10/} }
A. P. Sadovskii. Formal transformations and the problem of distinguishing the center and focus. Differencialʹnye uravneniâ, Tome 14 (1978) no. 12, pp. 2185-2189. http://geodesic.mathdoc.fr/item/DE_1978_14_12_a10/