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, vol. 47, no. 9, Sept. 1995, pp. 1197–1208, https://umj.imath.kiev.ua/index.php/umj/article/view/5519.