DUDKIN, M. Ye.; KOSHMANENKO, V. D. On the Point Spectrum of Self-Adjoint Operators That Appears under Singular Perturbations of Finite Rank. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 55, n. 9, p. 1269–1276, 2003. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4000. Acesso em: 27 apr. 2025.