1. T.Ya. Azizov, I.S. Iokhvidov, Foundations of the Theory of Linear Operators in Spaces with an Indefinite Metric, Nauka, Moscow, 1986 (English Trans.: Linear Operators in Spaces with an Indefinite Metric, Wiley, New York, 1990)
2. Indefinite Inner Product Spaces;Bognar,1974
3. B. Bodenstorfer, A. Dijksma, H. Langer, Dissipative eigenvalue problems for a Sturm–Liouville operator with a singular potential, Proc. Roy. Soc. Edinburgh 130A (2000) 1237–1257
4. Self-adjoint subspaces and eigenfunction expansions for ordinary differential subspaces;Coddington;J. Differential Equations,1976
5. Extensions of symmetric relations;Bruk;Mat. Zametki,1977