Koshmanenko, V. D., and O. V. Samoilenko. “Singular Perturbations of Finite Rank. Point Spectrum”. Ukrains’kyi Matematychnyi Zhurnal 49, no. 9 (September 25, 1997): 1186–1194. Accessed May 21, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/5117.