Vlasii, O. D., and B. I. Ptashnik. “A Problem With Nonlocal Conditions for Partial Differential Equations Unsolved With Respect to the Leading Derivative”. Ukrains’kyi Matematychnyi Zhurnal 55, no. 8 (August 25, 2003): 1022-1034. Accessed May 9, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3978.