Klyus, I. S., and B. I. Ptashnik. “A Multipoint Problem for Partial Differential Equations Unresolved With Respect to the Higher Time Derivative”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 51, no. 12, Dec. 1999, pp. 1604–1613, https://umj.imath.kiev.ua/index.php/umj/article/view/4765.