@article{DE_1997_33_5_a9,
     author = {Yu. A. Il'in},
     title = {On the asymptotic perturbation of a nonlinear center},
     journal = {Differencialʹnye uravneni\^a},
     pages = {632--637},
     publisher = {mathdoc},
     volume = {33},
     number = {5},
     year = {1997},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1997_33_5_a9/}
}