Geodesic
On the problem of distinguishing the center and focus in a certain case
Differencialʹnye uravneniâ, Tome 27 (1991) no. 11, pp. 1852-1862 
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/
@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},
     year = {1991},
     volume = {27},
     number = {11},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1991_27_11_a2/}
}
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