Arlinskii, Y.M. and Tsekanovskii, E. (2003) “Some remarks on singular perturbations of self-adjoint operators”, Methods of Functional Analysis and Topology, 9(4), pp. 287–308. Available at: https://zen.imath.kiev.ua/index.php/mfat/article/view/223 (Accessed: 17 December 2025).