@article{MZ_1989__202_1_174109,
     author = {J\"urgen P\"oschel},
     title = {On {Elliptic} {Lower} {Dimensional} {Tori} in {Hamiltonian} {Systems.}},
     journal = {Mathematische Zeitschrift},
     pages = {559--608},
     publisher = {mathdoc},
     volume = {202},
     number = {1},
     year = {1989},
     zbl = {0662.58037},
     url = {http://geodesic.mathdoc.fr/item/MZ_1989__202_1_174109/}
}