@article{DE_1985_21_11_a6,
     author = {V. N. Laptinskii},
     title = {Fourier approximations of periodic solutions of nonlinear differential equations},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1899--1904},
     publisher = {mathdoc},
     volume = {21},
     number = {11},
     year = {1985},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1985_21_11_a6/}
}