@article{DAN_1957_113_1_a0,
     author = {I. Berstein},
     title = {On the problem of periodical solutions of non-linear systems with a small parameter},
     journal = {Doklady Akademii Nauk},
     pages = {9--11},
     publisher = {mathdoc},
     volume = {113},
     number = {1},
     year = {1957},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1957_113_1_a0/}
}