MitropolskiyY. A., and KhomaL. G. “The Existence of a Classical Solution to the Mixed Problem for a Linear Second-Order Hyperbolic Equation”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 45, no. 9, Sept. 1993, pp. 1232–1238, https://umj.imath.kiev.ua/index.php/umj/article/view/5925.