Changes in a Finite Part of the Spectrum of the Laplace Operator under Delta-Like Perturbations

Abstract

We study the spectrum of the Laplace operator in a bounded simply connected domain with the zero Dirichlet condition on the boundary under delta-like perturbations of the operator at an interior point of the domain. We determine the maximal operator for the perturbations and single out a class of invertible restrictions of this operator whose spectra differ from the spectrum of the original operator by a finite (possibly, empty) set. These results can be viewed as transferring some of H. Hochstadt’s results for Sturm-Liouville operators to Laplace operators.