Egorchenko, I.A. and Popovich, R.O. (2001) “Group Classification of Generalized Eikonal Equations”, Ukrains’kyi Matematychnyi Zhurnal, 53(11), pp. 1513–1520. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/4371 (Accessed: 6 December 2025).