@article{DE_1965_1_9_a1,
     author = {Yu. S. Bogdanov},
     title = {Generalized characteristic numbers of non-autonomous systems},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1140--1148},
     publisher = {mathdoc},
     volume = {1},
     number = {9},
     year = {1965},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1965_1_9_a1/}
}