@article{DE_1970_6_4_a1,
     author = {A. M. Letov},
     title = {Some unsolved problems of automatic control theory},
     journal = {Differencialʹnye uravneni\^a},
     pages = {592--615},
     publisher = {mathdoc},
     volume = {6},
     number = {4},
     year = {1970},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1970_6_4_a1/}
}