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KAM for the nonlinear beam equation 1: small-amplitude solutions

In this paper we prove a KAM result for the non linear beam equation on the d-dimensional torus $$u_{tt}+Δ^2 u+m u + g(x,u)=0\ ,\quad t\in { \mathbb{R}} , \; x\in {\mathbb T}^d, \qquad \qquad (*) $$ where $g(x,u)=4u^3+ O(u^4)$. Namely, we show that, for generic $m$, most of the small amplitude invariant finite dimensional tori of the linear equation $(*)_{g=0}$, written as the system $$ u_t=-v,\quad v_t=Δ^2 u+mu, $$, persist as invariant tori of the nonlinear equation $(*)$, re-written similarly. If $d\ge2$, then not all the persisted tori are linearly stable, and we construct explicit examples of partially hyperbolic invariant tori. The unstable invariant tori, situated in the vicinity of the origin, create around them some local instabilities, in agreement with the popular belief in nonlinear physics that small-amplitude solutions of space-multidimensonal hamiltonian PDEs behave in a chaotic way. The proof uses an abstract KAM theorem from another our publication.