@article{DE_1997_33_2_a12,
     author = {A. P. Sadovskii},
     title = {Solution of the center and focus problem for a cubic system of nonlinear oscillations},
     journal = {Differencialʹnye uravneni\^a},
     pages = {236--244},
     publisher = {mathdoc},
     volume = {33},
     number = {2},
     year = {1997},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1997_33_2_a12/}
}