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@article{DE_1989_25_6_a5, author = {A. P. Sadovskii}, title = {The problem of the center and focus for analytic systems with a zero linear part. {II}}, journal = {Differencialʹnye uravneni\^a}, pages = {950--956}, publisher = {mathdoc}, volume = {25}, number = {6}, year = {1989}, language = {ru}, url = {http://geodesic.mathdoc.fr/item/DE_1989_25_6_a5/} }
A. P. Sadovskii. The problem of the center and focus for analytic systems with a zero linear part. II. Differencialʹnye uravneniâ, Tome 25 (1989) no. 6, pp. 950-956. http://geodesic.mathdoc.fr/item/DE_1989_25_6_a5/