@article{DAN_1974_216_4_a6,
     author = {A. V. Pokrovskii and M. A. Krasnosel'skii},
     title = {Periodic oscillations in systems with relay nonlinearities},
     journal = {Doklady Akademii Nauk},
     pages = {733--736},
     year = {1974},
     volume = {216},
     number = {4},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1974_216_4_a6/}
}