@article{GDZPPN002436876, title = {On Elliptic Lower Dimensional Tori in Hamiltonian Systems.}, journal = {Mathematische Zeitschrift}, volume = {202}, year = {1989}, pages = {559-608}, zbl = {0662.58037}, url = {http://dml.mathdoc.fr/item/GDZPPN002436876} }
Pöschel, Jürgen. On Elliptic Lower Dimensional Tori in Hamiltonian Systems.. Mathematische Zeitschrift, Tome 202 (1989) pp. 559-608. http://gdmltest.u-ga.fr/item/GDZPPN002436876/