@article{DE_1975_11_6_a20,
     author = {G. S. Osipenko},
     title = {On a homotopy invariant},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1139--1140},
     year = {1975},
     volume = {11},
     number = {6},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1975_11_6_a20/}
}