Gadjiev, T. S., and V. A. Mamedova. “On Removable Sets of Solutions of Second-Order Elliptic and Parabolic Equations in Nondivergent Form”.
Ukrains’kyi Matematychnyi Zhurnal, vol. 61, no. 11, Nov. 2009, pp. 1485-96,
https://umj.imath.kiev.ua/index.php/umj/article/view/3116.