“…We refer the reader to the seminal contributions in Vainikko [32,33], Bramble and Osborn [4], Strang and Fix [31], Osborn [28], Descloux et al [19,20], Babuška and Osborn [2], and to the more recent reviews in [3,21]. The approximation of elliptic spectral problems has also been studied by means of mixed finite element methods [12,27,26], discontinuous Galerkin methods [1,23], hybridizable discontinuous Galerkin methods [14,24], hybrid high-order methods [10,13], and virtual element methods [22]. All of these methods deliver optimally-convergent approximations, but since the eigenfunctions become more and more oscillatory in the upper part of the spectrum, the approximation is accurate only in the lower part of the spectrum.…”