An eigenvalue multiplicity formula for the Schur complement of a 3 × 3 block operator matrix

Abstract

We consider the Schur complement S(λ) with real spectral parameter λ corresponding to a certain 3 × 3 block operator matrix. In our case the essential spectrum of S(λ) can have gaps. We obtain formulas for the number and multiplicities of eigenvalues belonging to an arbitrary interval outside the essential spectrum of S(λ).