Trunk, C., F. Philipp, and J. Behrndt. 2006. “Properties of the Spectrum of Type $\pi_{+}$ and Type $\pi_{-}$ of Self-Adjoint Operators in Krein Spaces”. Methods of Functional Analysis and Topology 12 (4): 326-40. https://zen.imath.kiev.ua/index.php/mfat/article/view/326.