DUDKIN, M. Ye. Singularly perturbed normal operators. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 51, n. 8, p. 1045–1053, 1999. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4695. Acesso em: 6 dec. 2025.