@article{DE_1968_4_4_a6,
     author = {N. I. Avdonin},
     title = {Certain tests for the existence and absence of closed trajectories of a certain system of differential equations},
     journal = {Differencialʹnye uravneni\^a},
     pages = {639--645},
     publisher = {mathdoc},
     volume = {4},
     number = {4},
     year = {1968},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1968_4_4_a6/}
}