KOSHMANENKO, V. D.; SAMOILENKO, O. V. Singular perturbations of finite rank. Point spectrum. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 49, n. 9, p. 1186–1194, 1997. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5117. Acesso em: 5 dec. 2025.