@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}
}
