Robin eigenvalues on domains with peaks.

Let $\Omega\subset\mathbb{R}^N$, $N\ge 2,$ be a bounded domain with an outward power-like peak which is assumed not too sharp in a suitable sense. We consider the Laplacian $u\mapsto -\Delta u$ in $\Omega$ with the Robin boundary condition $\partial_n u=\alpha u$ on $\partial\Omega$ with $\partial_n$ being the outward normal derivative and $\alpha>0$ being a parameter. We show that for large $\alpha$ the associated eigenvalues $E_j(\alpha)$ behave as $E_j(\alpha)\sim -\epsilon_j \alpha^\nu$, where $\nu>2$ and $\epsilon_j>0$ depend on the dimension and the peak geometry. This is in contrast with the well-known estimate $E_j(\alpha)=O(\alpha^2)$ for the Lipschitz domains.