@article{DE_1971_7_2_a4,
     author = {I. A. Konovalov},
     title = {Certain sufficient tests for the existence of limit cycles of a certain dynamic system},
     journal = {Differencialʹnye uravneni\^a},
     pages = {226--231},
     publisher = {mathdoc},
     volume = {7},
     number = {2},
     year = {1971},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1971_7_2_a4/}
}