Konstantinov, A. Yu. “Spectral Theory of Some Matrix Differential Operators of Mixed Order”. Ukrains’kyi Matematychnyi Zhurnal 50, no. 8 (August 25, 1998): 1064–1072. Accessed February 17, 2026. https://umj.imath.kiev.ua/index.php/umj/article/view/4855.