@article{DE_1967_3_11_a6,
     author = {A. M. Samoilenko},
     title = {Periodic solutions of second-order nonlinear equations},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1903--1912},
     year = {1967},
     volume = {3},
     number = {11},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1967_3_11_a6/}
}