Lahno, V. I., & Spichak, S. V. (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. Retrieved from https://umj.imath.kiev.ua/index.php/umj/article/view/3409