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Periodic solutions for completely resonant nonlinear wave equations

Guido Gentile·Vieri Mastropietro·Michela Procesi
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Abstract

We consider the nonlinear string equation with Dirichlet boundary conditions $u_{xx}-u_{tt}=ϕ(u)$, with $ϕ(u)=Φu^{3} + O(u^{5})$ odd and analytic, $Φ\neq0$, and we construct small amplitude periodic solutions with frequency $ø$ for a large Lebesgue measure set of $ø$ 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 nonresonant 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 $u_{xx}-u_{tt}+ M u = ϕ(u)$, $M\neq0$, is that not only the P equation but also the Q equation is infinite-dimensional

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Guido Gentile, Vieri Mastropietro, Michela Procesi. 2004-02-16. Periodic solutions for completely resonant nonlinear wave equations. https://doi.org/10.1007/s00220-004-1255-8

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