Lahno VI, Spichak SV. Group classification of quasilinear elliptic-type equations. I. Invariance with respect to Lie algebras with nontrivial Levi decomposition. Ukr. Mat. Zhurn. [Internet]. 2007 Nov. 25 [cited 2025 Dec. 7];59(11):1532–1545. Available from: https://umj.imath.kiev.ua/index.php/umj/article/view/3409