NIZHNIK, L. P.; DUDKIN, M. E. Singularly perturbed normal operators. Methods of Functional Analysis and Topology, [S. l.], v. 16, n. 4, p. 298–303, 2010. Disponível em: https://zen.imath.kiev.ua/index.php/mfat/article/view/459. Acesso em: 16 dec. 2025.