LUKASHENKO, G. V.; GUBREEV, G. M. About nilpotent $C_0$-semigroups of operators in the Hilbert spaces and criteria for similarity to the integration operator. Methods of Functional Analysis and Topology, [S. l.], v. 14, n. 1, p. 60–66, 2008. Disponível em: https://zen.imath.kiev.ua/index.php/mfat/article/view/370. Acesso em: 18 dec. 2025.