Amanov, R. A., and F. I. Mamedov. “On Some Properties of Solutions of Quasilinear Degenerate Equations”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 60, no. 7, July 2008, pp. 918–936, https://umj.imath.kiev.ua/index.php/umj/article/view/3209.