Lahno, V. I., and S. V. Spichak. “Group Classification of Quasilinear Elliptic-Type Equations. I. Invariance With Respect to Lie Algebras With Nontrivial Levi Decomposition”. Ukrains’kyi Matematychnyi Zhurnal, vol. 59, no. 11, Nov. 2007, pp. 1532–1545, https://umj.imath.kiev.ua/index.php/umj/article/view/3409.