Fushchich , W. I., and R. Z. Zhdanov. “Conditional Symmetry and Reduction of Differential Equations With Partial Derivatives ”. Ukrains’kyi Matematychnyi Zhurnal, vol. 44, no. 7, Aug. 1992, pp. 970-82, https://umj.imath.kiev.ua/index.php/umj/article/view/8129.