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@article{DE_1989_25_5_a7, 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}, publisher = {mathdoc}, volume = {25}, number = {5}, year = {1989}, language = {ru}, url = {http://geodesic.mathdoc.fr/item/DE_1989_25_5_a7/} }
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/