Fushchich , V.I. and Serov , N.I. (1991) “Conditional invariance of the nonlinear wave equation”, Ukrains’kyi Matematychnyi Zhurnal, 43(3), pp. 394–399. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/9621 (Accessed: 12 January 2026).