1. T. Kato, Perturbation theory for linear operators, Springer Verlag 1966.
2. M. Reed, B. Simon, Methods of modern mathematical physics, Academic Press, Vol. I, 1972, Vol. II 1975, Voll. III, 1977 /78.
3. A.G. Sigalov, The main mathematical problems in the theory of atomic spectra, Uspehi mat. Nauk 22, No. 2 (134), 3, 1967 (= Russ. Math. Surveys 22_, No. 2. (1967)
4. T. Kato, Some mathematical problems in quantum mechanics, Suppl. Progr. Theor. Phys. 40, 3, 1967.
5. B. Simon, Quantum mechanics for Hamiltonians defined as quadratic forms, Princeton University Press 1971.