@article{DE_1977_13_9_a15,
     author = {A. G. Ramm},
     title = {The existence of periodic solutions for certain nonlinear problems},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1701--1708},
     publisher = {mathdoc},
     volume = {13},
     number = {9},
     year = {1977},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1977_13_9_a15/}
}