Berezansky, Y.M. and Brasche, J.F. (2002) “Generalized selfadjoint operators and their singular perturbations”, Methods of Functional Analysis and Topology, 8(4), pp. 1–14. Available at: https://zen.imath.kiev.ua/index.php/mfat/article/view/197 (Accessed: 17 December 2025).