GORBACHUK, M. L. On analytic solutions of operator differential equations. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 52, n. 5, p. 596–607, 2000. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4454. Acesso em: 5 dec. 2025.