AndreevaN. V., SerovN. I., and TulupovaL. O. “$Q$-Conditional Symmetry of a Nonlinear Two-Dimensional Heat-Conduction Equation”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 52, no. 6, June 2000, pp. 846-9, https://umj.imath.kiev.ua/index.php/umj/article/view/4481.