Skrypnik, I. I. 2004. “A Necessary Condition for the Regularity of a Boundary Point for Degenerating Parabolic Equations With Measurable Coefficients”. Ukrains’kyi Matematychnyi Zhurnal 56 (6): 818–836. https://umj.imath.kiev.ua/index.php/umj/article/view/3801.