@article{DE_1969_5_9_a9,
     author = {A. G. Terent'ev},
     title = {Certain properties of linear differential operators},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1635--1641},
     publisher = {mathdoc},
     volume = {5},
     number = {9},
     year = {1969},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1969_5_9_a9/}
}