1. Albeverio, S., Kuzhel, S., Nizhnik, L.: Singularly perturbed self-adjoint operators in scales of Hilbert spaces (in Russian). Ukraïn. Mat. Zh. 59(6), 723–743 (2007). (English transl. in Ukrainian Math. J. 59 (2007), 787–810)
2. Albeverio, S., Kuzhel, S., Nizhnik, L.: On the perturbation theory of self-adjoint operators. Tokyo J. Math. 31(2), 273–292 (2008)
3. Alonso, A., Simon, B.: The Birman–Kreĭn–Vishik theory of selfadjoint extensions of semibounded operators. J. Oper. Theory 4, 251–270 (1980)
4. Arlinskiĭ, Yu.: On m-accretive extensions and restrictions. Methods Funct. Anal. Topol. 4, 1–26 (1998)
5. Arlinskiĭ, Yu.: M-accretive extensions of sectorial operators and Krein spaces. Oper. Theory Adv. Appl. 118, 67–82 (2000)