BarannykT. A., BarannykA. F., and YurykI. I. “Generalized Procedure of Separation of Variables and Reduction of Nonlinear Wave Equations”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 61, no. 7, July 2009, pp. 892-05, https://umj.imath.kiev.ua/index.php/umj/article/view/3065.