Dudkin, M. Y., & Koshmanenko, V. D. (2003). On the Point Spectrum of Self-Adjoint Operators That Appears under Singular Perturbations of Finite Rank. Ukrains’kyi Matematychnyi Zhurnal, 55(9), 1269-1276. https://umj.imath.kiev.ua/index.php/umj/article/view/4000