H. Langer, M. Langer, A. Markus and C. Tretter,
Spectrum of definite type of self-adjoint operators in Krein spaces,
Linear and Multilinear Algebra 53 (2005), 115–136
Abstract:
For a self-adjoint operator in a Krein space we construct an interval [ν,μ]
outside of which the operator has only a spectrum of definite type and
possesses a local spectral function. As a consequence, a spectral subspace
corresponding to an interval outside [ν,μ] admits an angular operator representation.
We describe a defect subspace of the domain of the angular operator in
terms of the Schur complement, and we derive variational principles for the
discrete eigenvalues in such intervals of definite type.