Optimal trace norms for Helmholtz problems

The natural $H^1(\Omega)$ energy norm for Helmholtz problems is weighted with the wavenumber modulus $\sigma$ and induces natural weighted norms on the trace spaces $H^{\pm1/2}(\Gamma)$ by minimial extension to $\Omega\subset\mathbb R^n$. This paper presents a rigorous analysis for these trace norms with an explicit characterisation by weighted Sobolev-Slobodeckij norms and scaling estimates, highlighting their dependence on the geometry of the extension set $\Omega\subset\mathbb R^n$ and the weight $\sigma$. The analysis identifies conditions under which these trace norms are intrinsic to the isolated boundary component $\Gamma\subset\partial\Omega$ and provides $\sigma$-explicit estimates for trace inequalities in weighted spaces. In these natural wavenumber-weighted norms, the boundary integral operators allow improved continuity estimates that do \emph{not} deterioriate as $\sigma\to 0$.