TRUNK, C.; PHILIPP, F.; BEHRNDT, J. Properties of the spectrum of type $\pi_{+}$ and type $\pi_{-}$ of self-adjoint operators in Krein spaces. Methods of Functional Analysis and Topology, [S. l.], v. 12, n. 4, p. 326–340, 2006. Disponível em: https://zen.imath.kiev.ua/index.php/mfat/article/view/326. Acesso em: 16 dec. 2025.