@article{DE_1976_12_5_a21,
     author = {L. A. Cherkas},
     title = {The number of limit cycles of a certain second order autonumous system},
     journal = {Differencialʹnye uravneni\^a},
     pages = {944--946},
     year = {1976},
     volume = {12},
     number = {5},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1976_12_5_a21/}
}