@article{DAN_1980_251_1_a15,
     author = {V. V. Beletskii and G. V. Kasatkin},
     title = {Extremal properties of resonant motions},
     journal = {Doklady Akademii Nauk},
     pages = {58--62},
     publisher = {mathdoc},
     volume = {251},
     number = {1},
     year = {1980},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1980_251_1_a15/}
}