@article{DE_1991_27_4_a20,
     author = {I. G. Kozubovskaya},
     title = {Periodic solutions of some systems of nonlinear differential equations},
     journal = {Differencialʹnye uravneni\^a},
     pages = {705--707},
     publisher = {mathdoc},
     volume = {27},
     number = {4},
     year = {1991},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1991_27_4_a20/}
}