@article{DE_1978_14_9_a2,
     author = {I. M. Vul'pe},
     title = {The determination of periodic solutions of nonlinear systems by the averaging method},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1558--1565},
     publisher = {mathdoc},
     volume = {14},
     number = {9},
     year = {1978},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1978_14_9_a2/}
}