1. Theory of Linear Operators in Hilbert Space, vol. II;Akhiezer,1981
2. Singularly perturbed self-adjoint operators in scales of Hilbert spaces;Albeverio;Ukr. Math. J.,2007
3. Boundary Value Problems, Weyl Functions, and Differential Operators;Behrndt,2020
4. Expansions in Eigenfunctions of Selfadjoint Operators;Berezanskii,1968
5. Functional Analysis, vol. II;Berezansky,1996