@article{IM_1973__20_142203,
     author = {Alan Weinstein},
     title = {Normal {Modes} for {Nonlinear} {Hamiltonian} {Systems.}},
     journal = {Inventiones mathematicae},
     pages = {47--58},
     publisher = {mathdoc},
     volume = {20},
     year = {1973},
     zbl = {0264.70020},
     url = {http://geodesic.mathdoc.fr/item/IM_1973__20_142203/}
}