GRUSHKA, Ya. I. Behavior of semigroups of normal operators in a neighborhood of zero. Methods of Functional Analysis and Topology, [S. l.], v. 5, n. 3, p. 4–12, 1999. Disponível em: https://zen.imath.kiev.ua/index.php/mfat/article/view/93. Acesso em: 17 dec. 2025.