ORUDZHEV, G. D. Polynomial approximations of solutions of higher-order operator-differential equations. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 46, n. 7, p. 952–955, 1994. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5689. Acesso em: 6 dec. 2025.