Lagno, V. I., & Spichak, S. V. (2011). Group classification of quasilinear elliptic-type equations. II. Invariance under solvable Lie algebras.
Ukrains’kyi Matematychnyi Zhurnal,
63(2), 200-215.
https://umj.imath.kiev.ua/index.php/umj/article/view/2711