Lyapunov center theorem for some nonlinear PDE's : a simple proof
Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Volume 29 (2000) no. 4, p. 823-837 
@article{ASNSP_2000_4_29_4_823_0,
     author = {Bambusi, Dario},
     title = {Lyapunov center theorem for some nonlinear PDE's : a simple proof},
     journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze},
     publisher = {Scuola normale superiore},
     volume = {Ser. 4, 29},
     number = {4},
     year = {2000},
     pages = {823-837},
     zbl = {1008.35003},
     mrnumber = {1822409},
     language = {en},
     url = {http://www.numdam.org/item/ASNSP_2000_4_29_4_823_0}
}

Bambusi, Dario. Lyapunov center theorem for some nonlinear PDE's : a simple proof. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Volume 29 (2000) no. 4, pp. 823-837. http://www.numdam.org/item/ASNSP_2000_4_29_4_823_0/

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