Eigenvalues and resonances of dissipative acoustic operator for strictly convex obstacles

Abstract

We examine the wave equation in the exterior of a strictly convex bounded domain $K$ with dissipative boundary condition $\partial_{\nu} u - \gamma(x) \partial_t u = 0$ on the boundary $\Gamma$ and $0 < \gamma(x) <1, \:\forall x \in \Gamma.$ The solutions are described by a contraction semigroup $V(t) = e^{tG}, \: t \geq 0.$ The poles $\lambda$ of the meromorphic incoming resolvent $(G - \lambda)^{-1}: \:{ \mathcal H}_{comp} \rightarrow {\mathcal D}_{loc}$ are eigenvalues of G if ${\rm Re}\: \lambda < 0$ and incoming resonances if ${\rm Re}\: \lambda > 0$. We obtain sharper results for the location of the eigenvalues of $G$ and incoming resonances in $\Lambda = \{\lambda \in \mathbb C:\: |{\rm Re}\: \lambda| \leq C_2(1 + |{\rm Im}\: \lambda|)^{-2},\: |{\rm Im}\: \lambda| \geq A_2 > 1\}$ and we prove a Weyl formula for their asymptotic. For $K = \{x \in {\mathbb R}^3:\:|x| \leq 1\}$ and $\gamma$ constant we show that $G$ has no eigenvalues so the Weyl formula concerns only the incoming resonances.