Burskii, V. P. “On the Third Boundary-Value Problem for an Improperly Elliptic Equation in a Disk”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 66, no. 2, Feb. 2014, pp. 279–283, https://umj.imath.kiev.ua/index.php/umj/article/view/2130.