@article{DE_1974_10_9_a1,
     author = {V. V. Amel'kin and I. V. Gaishun},
     title = {The periodic solutions of second order systems},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1560--1564},
     publisher = {mathdoc},
     volume = {10},
     number = {9},
     year = {1974},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1974_10_9_a1/}
}