@article{10_1007_s10492_010_0013_9,
     author = {Zhang, Xingyong and Zhou, Yinggao},
     title = {On periodic solutions of non-autonomous second order {Hamiltonian} systems},
     journal = {Applications of Mathematics},
     pages = {373--384},
     year = {2010},
     volume = {55},
     number = {5},
     doi = {10.1007/s10492-010-0013-9},
     mrnumber = {2737718},
     zbl = {1224.34126},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.1007/s10492-010-0013-9/}
}