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@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/} }
V. V. Amel'kin; I. V. Gaishun. The periodic solutions of second order systems. Differencialʹnye uravneniâ, Tome 10 (1974) no. 9, pp. 1560-1564. http://geodesic.mathdoc.fr/item/DE_1974_10_9_a1/