1. N.I.Achieser and I.M.Glasmann,Theorie der linearen Operatoren im Hilbertraum,Akademie Verlag,Berlin,1981.
2. S.
Albeverio, F.
Gesztesy, R.
Hø;
Egh-Kroi-V and H.
Holden,
Solvable models in quantum mechanics,Springer-Verlag,Berlin,1988.
3. S. Albeverio and P. Kurasov, Singular perturbations of differential operators, London Math. Soc. Lecture Note Series 271 Cambridge University Press, Cambridge, 2000.
4. T. Ando and K. Nismo, Positive selfadjoint extensions of positive symmetric operators, Tollóku Math. J., 22 (1970), 65–75.
5. Yu. M. Arlinskiĭ, Positive spaces of of boundary values and sectorial extensions of nonnegative symmetric operators, Ukrainian Math. J., 40 (1988), 8–15.