1. Akhiezer N.I., Glazman I.M.: Theory of Linear Operators in Hilbert Space, Two Volumes Bound as One. Dover Publications, New York (1993)
2. Akhiezer N.I., Glazman I.M.: Theory of Linear Operators in Hilbert Space. (Russian) vol. II, Third edition. Vishcha Shkola, Kharkov (1978)
3. Alpay D., Gohberg I.: Pairs of selfadjoint operators and their invariants. St. Petersburg Math. J. 16, 59–104 (2005)
4. Azizov T.Y., Behrndt J., Trunk C.: On finite rank perturbations of definitizable operators. J. Math. Anal. Appl. 339, 1161–1168 (2008)
5. Azizov T.Y., Iokhvidov I.S.: Linear Operators in Spaces with an Indefinite Metric. John Wiley & Sons, London (1990)