@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} }