GORBACHUK, V. I.; GORBACHUK, M. L. On completeness of the set of root vectors for unbounded operators. Methods of Functional Analysis and Topology, [S. l.], v. 12, n. 4, p. 353–362, 2006. Disponível em: https://zen.imath.kiev.ua/index.php/mfat/article/view/328. Acesso em: 16 dec. 2025.