@article{MZ_1989__201_4_174049,
     author = {Antonio Ambrosetti and V. Coti Zelati},
     title = {Perturbation of {Hamiltonian} {Systems} with {Keplerian} {Potentials.}},
     journal = {Mathematische Zeitschrift},
     pages = {227--242},
     publisher = {mathdoc},
     volume = {201},
     number = {4},
     year = {1989},
     zbl = {0653.34032},
     url = {http://geodesic.mathdoc.fr/item/MZ_1989__201_4_174049/}
}