“Reduction of Multi-Dimensional D’Alembert Equations to Two-Dimensional Equations: Ansatzes, Compatibility of the Reduction Conditions, Reduced Equations”. Transactions of Institute of Mathematics, the NAS of Ukraine, vol. 3, no. 2, Nov. 2006, pp. 98-112, https://trim.imath.kiev.ua/index.php/trim/article/view/385.