Generalised Weyl theorems and spectral pollution in the Galerkin method

The new version deals with Galerkin sequences which are dense in the form domain of A, when A is bounded from belowInternational audienceWe consider a general framework for investigating spectral pollution in the Galerkin method. We show how this phenomenon is characterised via the existence of particular Weyl sequences which are singular in a suitable sense. For a semi-bounded selfadjoint operator A we identify relative compactness conditions on a selfadjoint perturbation B ensuring that the limiting set of spectral pollution of A and B coincide. Our results show that, under perturbation, this limiting set behaves in a similar fashion as the essential spectrum