Geodesic
The problem of the center and focus for analytic systems with a zero linear part. I
Differencialʹnye uravneniâ, Tome 25 (1989) no. 5, pp. 790-799 
A. P. Sadovskii. The problem of the center and focus for analytic systems with a zero linear part. I. Differencialʹnye uravneniâ, Tome 25 (1989) no. 5, pp. 790-799. http://geodesic.mathdoc.fr/item/DE_1989_25_5_a7/
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     author = {A. P. Sadovskii},
     title = {The problem of the center and focus for analytic systems with a zero linear part. {I}},
     journal = {Differencialʹnye uravneni\^a},
     pages = {790--799},
     year = {1989},
     volume = {25},
     number = {5},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1989_25_5_a7/}
}
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