1. Adamjan, V., Langer, H., Möller, M.: Compact perturbation of definite type spectra of self-adjoint quadratic operator pencils. Integr. Equ. Oper. Theory 39, 127–152 (2001)
2. Arens, R.: Operational calculus of linear relations. Pac. J. Math. 11, 9–23 (1961)
3. Azizov, T.Ya., Iokhvidov, I.S.: Linear Operators in Spaces with an Indefinite Metric. Wiley, Chichester (1989)
4. Azizov, T.Ya., Jonas, P.: On compact perturbations of normal operators in a Krein space. Ukrainskii Matem. Zurnal 42, 1299–1306 (1990)
5. Azizov, T.Ya., Strauss, V.A.: Spectral decompositions for special classes of self-adjoint and normal operators on Krein spaces. In: Spectral Theory and Applications. Theta Ser. Adv. Math., vol. 2, pp. 45–67. Theta, Bucharest (2003)