@article{MAN_1992__292_2_164922,
     author = {Hidekazu Ito},
     title = {Integrability of {Hamiltonian} systems and {Birkhoff} normal forms in the simple resonance case.},
     journal = {Mathematische Annalen},
     pages = {411--444},
     publisher = {mathdoc},
     volume = {292},
     number = {2},
     year = {1992},
     zbl = {0735.58022},
     url = {http://geodesic.mathdoc.fr/item/MAN_1992__292_2_164922/}
}