@article{DE_1970_6_11_a5,
     author = {Yu. K. Metlyaev},
     title = {The use of operators that have certain properties in qualitative investigations of differential equations},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1976--1981},
     year = {1970},
     volume = {6},
     number = {11},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1970_6_11_a5/}
}