@article{MZM_1994_55_5_a8,
     author = {L. M. Lerman and Ya. L. Umanskii},
     title = {Isoenergetic classification of integrable {Hamiltonian} systems in a neighborhood of a simple elliptic point},
     journal = {Matemati\v{c}eskie zametki},
     pages = {88--97},
     year = {1994},
     volume = {55},
     number = {5},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1994_55_5_a8/}
}