@article{DAN_1991_319_4_a9,
     author = {M. A. Krasnosel'skii and A. V. Pokrovskii},
     title = {Operators of a problem on forced oscillations in systems with},
     journal = {Doklady Akademii Nauk},
     pages = {819--822},
     publisher = {mathdoc},
     volume = {319},
     number = {4},
     year = {1991},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1991_319_4_a9/}
}