A reducibility result for a class of linear wave equations on $\mathbb{T}^d$

Abstract: We prove a reducibility result for a class of quasi-periodically forced linear wave equations on the $d$-dimensional torus $\mathbb{T}^d$ of the form $$ \partial_{tt} v - \Delta v + \varepsilon {\cal P}(\omega t)[v] = 0 $$ where the perturbation ${\cal P}(\omega t)$ is a second order operator of the form ${\cal P}(\omega t) = - a(\omega t) \Delta - {\cal R}(\omega t)$, the frequency $\omega \in {\cal R}^\nu$ is in some Borel set of large Lebesgue measure, the function $a : \mathbb{T}^\nu \to {\cal R}$ (independent of the space variable) is sufficiently smooth and ${\cal R}(\omega t)$ is a time-dependent finite rank operator. This is the first reducibility result for linear wave equations with unbounded perturbations on the higher dimensional torus $\mathbb{T}^d$. As a corollary, we get that the linearized Kirchhoff equation at a smooth and sufficiently small quasi-periodic function is reducible.