GORBACHUK, M. L. On the spectral functions of differential equations of the second order with operator coefficients. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 18, n. 2, p. 3–21, 1966. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/8713. Acesso em: 6 dec. 2025.