RADZIEVSKII, G. V. On an upper bound for the number of characteristic values of an operator function. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 50, n. 2, p. 211–224, 1998. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4956. Acesso em: 5 dec. 2025.