GUBREEV, G. M.; VOLKOVA, M. G.; TARASENKO, A. A. On one class of nonselfadjoint operators with a discrete spectrum. Methods of Functional Analysis and Topology, [S. l.], v. 17, n. 3, p. 211–218, 2011. Disponível em: https://zen.imath.kiev.ua/index.php/mfat/article/view/486. Acesso em: 16 dec. 2025.