Periodic solutions for completely resonant nonlinear wave equations with Dirichlet boundary conditions

We consider the nonlinear string equation with Dirichlet boundary conditions $u_{xx}-u_{tt}=\f(u)$, with $\f(u)=\F u^{3} + O(u^{5})$ odd and analytic, $\F\neq0$, and we construct small amplitude periodic solutions with frequency $\o$ for a large Lebesgue measure set of $\o$ close to $1$. This extends previous results where only a zero-measure set of frequencies could be treated (the ones for which no small divisors appear). The proof is based on combining the Lyapunov-Schmidt decomposition, which leads to two separate sets of equations dealing with the resonant and non-resonant Fourier components, respectively the Q and the P equations, with resummation techniques of divergent powers series, allowing us to control the small divisors problem. The main difficulty with respect the nonlinear wave equations $u_{xx}-u_{tt}+ M u = \f(u)$, $M\neq0$, is that not only the P equation but also the Q equation is infinite-dimensional.