@article{DE_1967_3_5_a7,
     author = {L. A. Cherkas},
     title = {Perturbations of a differential equation},
     journal = {Differencialʹnye uravneni\^a},
     pages = {781--786},
     publisher = {mathdoc},
     volume = {3},
     number = {5},
     year = {1967},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1967_3_5_a7/}
}