@article{DE_1981_17_8_a21,
     author = {A. G. Sokolskii},
     title = {On the stability of {Hamiltonian} systems in case of zero frequency},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1509--1510},
     publisher = {mathdoc},
     volume = {17},
     number = {8},
     year = {1981},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1981_17_8_a21/}
}