Spectral stability estimates for elliptic operators subject to domain transformations with non-uniformly bounded gradients

We consider uniformly elliptic operators with Dirichlet or Neumann homogeneous boundary conditions on a domain $\Omega$ in ${\mathbb{R}}^N$. We consider deformations $\phi (\Omega)$ of $\Omega$ obtained by means of a locally Lipschitz homeomorphism $\phi$ and we estimate the variation of the eigenfunctions and eigenvalues upon variation of $\phi$. We prove general stability estimates without using uniform upper bounds for the gradients of the maps $\phi$. As an application, we obtain estimates on the rate of convergence for eigenvalues and eigenfunctions when a domain with an outward cusp is approximated by a sequence of Lipschitz domains.