ANDROSHCHUK, A. A. On transformation operators for a second order differential equation with operator coefficients . Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 23, n. 1, p. 52–55, 1970. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/8479. Acesso em: 6 dec. 2025.