@article {Berti20081671, title = {Cantor families of periodic solutions for wave equations via a variational principle}, journal = {Advances in Mathematics}, volume = {217}, number = {4}, year = {2008}, note = {cited By (since 1996)6}, pages = {1671-1727}, abstract = {We prove existence of small amplitude periodic solutions of completely resonant wave equations with frequencies in a Cantor set of asymptotically full measure, via a variational principle. A Lyapunov-Schmidt decomposition reduces the problem to a finite dimensional bifurcation equation-variational in nature-defined on a Cantor set of non-resonant parameters. The Cantor gaps are due to "small divisors" phenomena. To solve the bifurcation equation we develop a suitable variational method. In particular, we do not require the typical "Arnold non-degeneracy condition" of the known theory on the nonlinear terms. As a consequence our existence results hold for new generic sets of nonlinearities. {\textcopyright} 2007 Elsevier Inc. All rights reserved.}, issn = {00018708}, doi = {10.1016/j.aim.2007.11.004}, author = {Massimiliano Berti and Philippe Bolle} }