@article{DE_1985_21_6_a1,
     author = {A. A. Zevin},
     title = {Estimates for periodic solutions of systems of second-order nonlinear differential equations},
     journal = {Differencialʹnye uravneni\^a},
     pages = {932--937},
     publisher = {mathdoc},
     volume = {21},
     number = {6},
     year = {1985},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1985_21_6_a1/}
}