1. B. Simon,Quantum Mechanics for Hamiltonians Defined as Quadratic Forms (Princeton University Press, Princeton, New Jersey, 1971), p. 204.
2. R. D. Richtmeyer,Principles of Advanced Mathematical Physics (Springer, New York, 1978), Vol. I, Chapter 7.
3. M. Reed and B. Simon,Methods of Modern Mathematical Physics (Academic Press, New York, 1972), Vol. I, p. 255.
4. N. I. Akhiezer and I. M. Glazman,Theory of Linear Operators in Hilbert Space (Pitman, London, 1980), Vol. I, pp. 128 and 156.
5. F. Riesz and B. Sz-Nagy,Leçons d'Analyse Fonctionelle (Gauthier-Villars, Paris, 1972), 6th edn., p. 305;Functional Analysis (Ungar, New York, 1955).