@article{CRMATH_2007__344_12_779_0,
     author = {Birtea, Petre and Puta, Mircea and Tudoran, R\u{a}zvan Micu},
     title = {Periodic orbits in the case of a zero eigenvalue},
     journal = {Comptes Rendus. Math\'ematique},
     pages = {779--784},
     publisher = {Elsevier},
     volume = {344},
     number = {12},
     year = {2007},
     doi = {10.1016/j.crma.2007.05.003},
     language = {en},
     url = {http://www.numdam.org/articles/10.1016/j.crma.2007.05.003/}
}