1. S. Albeverio, R. Hoegh-Krohn, Point interaction as limits of short range interactions on quantum mechanics. J. Oper. Theory 6, 313–339 (1981)
2. S. Albeverio, J.E. Fenstad, R. Hoegh-Krohn, Singular perturbation and non standard analysis. Trans. Am. Math. Soc. 252, 275–295 (1979)
3. K. Alonso, B. Simon, The theory of self-extensions. J. Oper. Theory 4, 251–270 (1980)
4. M. Birman, On the theory of self-adjoint extension s of positive definite operators. Math. Sb. N.S. 38, 431–480 (1956)
5. V. Benci, Ultrafunction and generalized solutions. Adv. Nonlinear Stud. 13, 461–486b (2013)