@article{DAN_1986_287_1_a15,
     author = {V. A. Yakubovich},
     title = {A frequency theorem for periodic systems},
     journal = {Doklady Akademii Nauk},
     pages = {70--73},
     publisher = {mathdoc},
     volume = {287},
     number = {1},
     year = {1986},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1986_287_1_a15/}
}