@article{IM_1987__88_143452,
     author = {A. Katok and D. Bernstein},
     title = {Birkhoff periodic orbits for small perturbations of completely integrable {Hamiltonian} {Systems} with convex {Hamiltonians.}},
     journal = {Inventiones mathematicae},
     pages = {225--242},
     publisher = {mathdoc},
     volume = {88},
     year = {1987},
     zbl = {0642.58040},
     url = {http://geodesic.mathdoc.fr/item/IM_1987__88_143452/}
}