Egorchenko, I. A., and R. O. Popovich. “Group Classification of Generalized Eikonal Equations”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 53, no. 11, Nov. 2001, pp. 1513-20, https://umj.imath.kiev.ua/index.php/umj/article/view/4371.