@article{DE_1985_21_2_a5,
     author = {O. I. Nikonov},
     title = {Some extremal properties of observable differential systems},
     journal = {Differencialʹnye uravneni\^a},
     pages = {236--240},
     publisher = {mathdoc},
     volume = {21},
     number = {2},
     year = {1985},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1985_21_2_a5/}
}