PARASYUK, E. M.; ZORY, L. M. On the multiplication problem of ordinary differential equations . Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 23, n. 6, p. 804–807, 1971. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/8631. Acesso em: 6 dec. 2025.