KUZHEL', A. V. Evolution of the concept of the characteristic function of a linear operator. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 45, n. 6, p. 731–743, 1993. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5862. Acesso em: 5 dec. 2025.