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@article{DE_1973_9_4_a8, author = {A. P. Sadovskii}, title = {The problem of distinguishing the center and the focus}, journal = {Differencialʹnye uravneni\^a}, pages = {650--659}, publisher = {mathdoc}, volume = {9}, number = {4}, year = {1973}, language = {ru}, url = {http://geodesic.mathdoc.fr/item/DE_1973_9_4_a8/} }
A. P. Sadovskii. The problem of distinguishing the center and the focus. Differencialʹnye uravneniâ, Tome 9 (1973) no. 4, pp. 650-659. http://geodesic.mathdoc.fr/item/DE_1973_9_4_a8/