Lahno, V. I., and S. V. Spichak. 2007. “Group Classification of Quasilinear Elliptic-Type Equations. I. Invariance With Respect to Lie Algebras With Nontrivial Levi Decomposition”. Ukrains’kyi Matematychnyi Zhurnal 59 (11), 1532–1545. https://umj.imath.kiev.ua/index.php/umj/article/view/3409.