Burskii, V. P. “Conditions of Regularity of a General Differential Boundary-Value Problem for Improperly Elliptic Equations”. Ukrains’kyi Matematychnyi Zhurnal 62, no. 6 (June 25, 2010): 754–761. Accessed May 6, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/2906.