@article{VMUMM_1998_4_a11,
     author = {A. V. Karapetyan and Yu. G. Sitanskaya},
     title = {Invariant sets of nonnatural mechanical systems},
     journal = {Vestnik Moskovskogo universiteta. Matematika, mehanika},
     pages = {58--61},
     publisher = {mathdoc},
     number = {4},
     year = {1998},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/VMUMM_1998_4_a11/}
}