@article{10_4064_ap85_2_5,
     author = {M. Sabatini},
     title = {Period function's convexity for {Hamiltonian} centers
 with separable variables},
     journal = {Annales Polonici Mathematici},
     pages = {153--163},
     publisher = {mathdoc},
     volume = {85},
     number = {2},
     year = {2005},
     doi = {10.4064/ap85-2-5},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/ap85-2-5/}
}