| Summary: | We consider the Helmholtz equation with real analytic coefficients on a bounded domain $\Omega\subset\mathbb{R}^{d}$. We take $d+1$ prescribed boundary conditions $f^{i}$ and frequencies $\omega$ in a fixed interval $[a,b]$. We consider a constraint on the solutions $u_{\omega}^{i}$ of the form $\zeta(u_{\omega}^{1},\ldots,u_{\omega}^{d+1},\nabla u_{\omega}^{1},\ldots,\nabla u_{\omega}^{d+1})\neq0$, where $\zeta$ is analytic, which is satisfied in $\Omega$ when $\omega=0$. We show that for any $\Omega^{\prime}\Subset\Omega$ and almost any $d+1$ frequencies $\omega_{k}$ in $[a,b]$, there exist $d+1$ subdomains $\Omega_{k}$ such that $\Omega^{\prime}\subset\cup_{k}\Omega_{k}$ and $\zeta(u_{\omega_{k}}^{1},\ldots,u_{\omega_{k}}^{d+1},\nabla u_{\omega_{k}}^{1},\ldots,\nabla u_{\omega_{k}}^{d+1})\neq0$ in $\Omega_{k}$. This question comes from hybrid imaging inverse problems. The method used is not specific to the Helmholtz model and can be applied to other frequency dependent problems.
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