LagnoV. I., and SpichakS. V. “Group Classification of Quasilinear Elliptic-Type Equations. II. Invariance under Solvable Lie Algebras”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 63, no. 2, Feb. 2011, pp. 200-15, https://umj.imath.kiev.ua/index.php/umj/article/view/2711.