@article{DE_1984_20_9_a24,
     author = {M. C. Konstantinov},
     title = {Some sufficient conditions for the existence of periodic solutions of nonlinear systems of two differential equations},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1632--1634},
     publisher = {mathdoc},
     volume = {20},
     number = {9},
     year = {1984},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1984_20_9_a24/}
}