@article{DE_1976_12_12_a19,
     author = {A. P. Sadovskii},
     title = {On the problem of distinguishing the center and focus for systems of nonlinear oscillations},
     journal = {Differencialʹnye uravneni\^a},
     pages = {2273--2274},
     publisher = {mathdoc},
     volume = {12},
     number = {12},
     year = {1976},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1976_12_12_a19/}
}