@article{JRAM_1994__451_153626,
     author = {Thomas Bartsch and Michel Willem},
     title = {Periodic solutions of non-autonomous {Hamiltonian} systems with symmetries.},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {149--160},
     publisher = {mathdoc},
     volume = {451},
     year = {1994},
     zbl = {0794.58037},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1994__451_153626/}
}