@article{PORMA_2006__63_1_52975,
     author = {Dinca, George and Goeleven, Daniel and Pa\c{s}ca, Daniel},
     title = {Duality mapping and the existence of periodic solutions for non-autonomous second order systems.},
     journal = {Portugaliae mathematica},
     pages = {47--68},
     publisher = {mathdoc},
     volume = {63},
     number = {1},
     year = {2006},
     zbl = {1105.34023},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/PORMA_2006__63_1_52975/}
}