@article{DAN_1976_231_4_a14,
     author = {V. V. Beletskii and A. N. Shlyakhtin},
     title = {Extremum properties of resonance motions},
     journal = {Doklady Akademii Nauk},
     pages = {829--832},
     publisher = {mathdoc},
     volume = {231},
     number = {4},
     year = {1976},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1976_231_4_a14/}
}