DudkinM. Ye., and KoshmanenkoV. D. “On the Point Spectrum of Self-Adjoint Operators That Appears under Singular Perturbations of Finite Rank”. Ukrains’kyi Matematychnyi Zhurnal 55, no. 9 (September 25, 2003): 1269-1276. Accessed July 27, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/4000.