External finite-element approximations of eigenfunctions in the case of multiple eigenvalues (Q1334751)

The paper deals with the finite element analysis of second-order elliptic eigenvalue problems when the domains \(\Omega_ h\) are not contained in the original domain \(\Omega\subset \mathbb{R}^ 2\). The main aim of the paper is to study the convergence of approximate eigenfunctionals in the case of multiple exact eigenvalues, extending an approach formerly established by the authors in the case of simple eigenvalues. It differs from the approach of \textit{I. Babuška} and \textit{J. E. Osborn} [Math. Comput. 52, No. 186, 275-297 (1989; Zbl 0675.65108)] in that no use is made of operator theory.