@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/}
}