KUZHEL, S. A. On some properties of unperturbed operators. Methods of Functional Analysis and Topology, [S. l.], v. 2, n. 3, p. 78–84, 1996. Disponível em: https://zen.imath.kiev.ua/index.php/mfat/article/view/22. Acesso em: 19 dec. 2025.