@article{DE_1978_14_2_a25,
     author = {A. P. Sadovskii},
     title = {A solution for the problem of center and focus for certain systems of non-linear oscillations},
     journal = {Differencialʹnye uravneni\^a},
     pages = {380--382},
     publisher = {mathdoc},
     volume = {14},
     number = {2},
     year = {1978},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1978_14_2_a25/}
}