Borsuk, M. V. 1996. “On the Solvability of the Dirichlet Problem for Elliptic Nondivergent Equations of the Second Order in a Domain With Conical Point”.
Ukrains’kyi Matematychnyi Zhurnal 48 (1): 13-24.
https://umj.imath.kiev.ua/index.php/umj/article/view/5358.