Ptashnik, B. I., and S. M. Repetylo. “Boundary-Value Problem With Mixed Conditions for Linear Typeless Partial Differential Equations”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 68, no. 5, May 2016, pp. 665-82, https://umj.imath.kiev.ua/index.php/umj/article/view/1870.