AldashevS. A. “On the Well-Posedness of Derichlet Problems for the Many-Dimensional Wave Equation and lavrent’ev-Bitsadze Equation”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 48, no. 5, May 1996, pp. 702-6, https://umj.imath.kiev.ua/index.php/umj/article/view/5303.