Behrndt, JussiJonas, Peter2021-12-172021-12-172003-02-282197-8085https://depositonce.tu-berlin.de/handle/11303/15509http://dx.doi.org/10.14279/depositonce-14282The aim of this paper is to prove two perturbation results for a selfadjoint operator A in a Krein space H which can roughly be described as follows: (1) If Δ is an open subset of R, and all spectral subspaces for A corresponding to compact subsets of Δ have finite rank of negativity, the same is true for a selfadjoint operator B in H for which the difference of the resolvents of A and B is compact. (2) The property that there exists some neighbourhood Δ∞ of ∞ such that the restriction of A to a spectral subspace for A corresponding to Δ∞ is a nonnegative operator in H, is preserved under relative Sp perturbations in form sense if the resulting operator is again selfadjoint. The assertion (1) is proved for selfadjoint relations A and B. (1) and (2) generalize some known results.en510 Mathematikselfadjoint operators in Krein spacescompact perturbationsdefinitizable operatorsspectral points of positive and negative typeselfadjoint linear relationsResearch Paper