Derezi\'{n}ski J., Fr\"uboes R.
Level Shift Operator and 2nd order perturbation theory
(50K, LATeX 2e)

ABSTRACT.  e study spectral properties of linear operators. The main tool is the
so-called {\em Level Shift Operator}, which expresses the effects of
2nd order perturbation theory on the point spectrum. We give
estimates for the location of the spectrum in a selected region and
approximate formulas for the corresponding spectral projections.
Presented results are quite robust---they are
applicable even if the spectrum in this region
has infinite multiplicity.