Fushchich W. I., and Zhdanov R. Z. “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.