Valitskii, Yu. N., B. I. Golets, and T. I. Zelenyak. “Multipoint Boundary Conditions for Differential Operators”. Ukrains’kyi Matematychnyi Zhurnal 55, no. 1 (January 25, 2003): 126–131. Accessed April 11, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/3894.