KONSTANTINOV, A. Yu. Spectral theory of some matrix differential operators of mixed order. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 50, n. 8, p. 1064–1072, 1998. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4855. Acesso em: 14 apr. 2025.