Geodesic
The problem of distinguishing the center and a focus for systems with nonzero linear part
Differencialʹnye uravneniâ, Tome 12 (1976) no. 7, pp. 1238-1246 
A. P. Sadovskii. The problem of distinguishing the center and a focus for systems with nonzero linear part. Differencialʹnye uravneniâ, Tome 12 (1976) no. 7, pp. 1238-1246. http://geodesic.mathdoc.fr/item/DE_1976_12_7_a10/
@article{DE_1976_12_7_a10,
     author = {A. P. Sadovskii},
     title = {The problem of distinguishing the center and a focus for systems with nonzero linear part},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1238--1246},
     year = {1976},
     volume = {12},
     number = {7},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1976_12_7_a10/}
}
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