@article{CMJ_2007__57_1_a24,
     author = {Wang, Genqiang and Cheng, Sui Sun},
     title = {Even periodic solutions of higher order duffing differential equations},
     journal = {Czechoslovak Mathematical Journal},
     pages = {331--343},
     publisher = {mathdoc},
     volume = {57},
     number = {1},
     year = {2007},
     mrnumber = {2309968},
     zbl = {1174.34037},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/CMJ_2007__57_1_a24/}
}