@article{DE_1987_23_2_a1,
     author = {A. A. Bel'kovich},
     title = {On the existence of invariant tori for a periodic {Hamiltonian} system having some of characteristic indices with nonzero real part},
     journal = {Differencialʹnye uravneni\^a},
     pages = {192--198},
     publisher = {mathdoc},
     volume = {23},
     number = {2},
     year = {1987},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1987_23_2_a1/}
}