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@article{DE_1991_27_11_a2, author = {S. A. Zolotarev}, title = {On the problem of distinguishing the center and focus in a certain case}, journal = {Differencialʹnye uravneni\^a}, pages = {1852--1862}, publisher = {mathdoc}, volume = {27}, number = {11}, year = {1991}, language = {ru}, url = {http://geodesic.mathdoc.fr/item/DE_1991_27_11_a2/} }
S. A. Zolotarev. On the problem of distinguishing the center and focus in a certain case. Differencialʹnye uravneniâ, Tome 27 (1991) no. 11, pp. 1852-1862. http://geodesic.mathdoc.fr/item/DE_1991_27_11_a2/