  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.
