BilusyakN., Komarnyts’kaL. I., and PtashnikB. I. “Dirichlet-Type Problems for Systems of Partial Differential Equations Unresolved With Respect to the Highest Time Derivative”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 54, no. 12, Dec. 2002, pp. 1592-0, https://umj.imath.kiev.ua/index.php/umj/article/view/4198.