@article{DE_1989_25_12_a9,
     author = {V. I. Mironenko},
     title = {Simple systems and periodic solutions of differential equations},
     journal = {Differencialʹnye uravneni\^a},
     pages = {2109--2114},
     year = {1989},
     volume = {25},
     number = {12},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1989_25_12_a9/}
}