@article{DE_1979_15_9_a21,
     author = {A. P. Sadovskii},
     title = {On conditions for a center and focus for nonlinear oscillation equations},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1716--1719},
     publisher = {mathdoc},
     volume = {15},
     number = {9},
     year = {1979},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1979_15_9_a21/}
}