1. Dirac, P.A.M., Principles of Quantum Mechanics, Oxford at the Clarendon Press, 1936.
2. RellichF., Math. Z. 49, 702?723 (1943?44).
3. StummelF., Math. Ann. 132, 150?176 (1956).
4. Dunford, N. and Schwartz, J.T., Linear Operators, Part I, Wiley, 1963. (See Lemma VI. 2.10.)
5. Kato, T., Perturbation Theory for Linear Operators, Springer-Verlag, 1966. (a. Theorem V. 4.6; b. Subsection V. 5.4; c. Problem V. 3.17.)