@article {2004,
	title = {Multiplicity of periodic solutions of nonlinear wave equations},
	journal = {Nonlinear Anal. 56 (2004) 1011-1046},
	year = {2004},
	publisher = {Elsevier},
	doi = {10.1016/j.na.2003.11.001},
	url = {http://hdl.handle.net/1963/2974},
	author = {Massimiliano Berti and Philippe Bolle}
}
