Komarnyts’ka, L. I., and B. I. Ptashnik. “Boundary-Value Problems for Differential Equations Unsolvable With Respect to the Higher Time Derivative”. Ukrains’kyi Matematychnyi Zhurnal 47, no. 9 (September 25, 1995): 1197–1208. Accessed June 2, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/5519.