@article{GDZPPN002103931,
title = {Birkhoff periodic orbits for small perturbations of completely integrable Hamiltonian Systems with convex Hamiltonians.},
journal = {Inventiones mathematicae},
volume = {88},
year = {1987},
pages = {225-242},
zbl = {0642.58040},
url = {http://dml.mathdoc.fr/item/GDZPPN002103931}
}
Katok, A.; Bernstein, D. Birkhoff periodic orbits for small perturbations of completely integrable Hamiltonian Systems with convex Hamiltonians.. Inventiones mathematicae, Tome 88 (1987) pp. 225-242. http://gdmltest.u-ga.fr/item/GDZPPN002103931/