%0 Journal Article %A A. G. Ramm %T The existence of periodic solutions for certain nonlinear problems %J Differencialʹnye uravneniâ %D 1977 %P 1701-1708 %V 13 %N 9 %I mathdoc %U http://geodesic.mathdoc.fr/item/DE_1977_13_9_a15/ %G ru %F DE_1977_13_9_a15