Kirilich, V. M., and T. I. Firman. “Problem Without Initial Conditions for a Countable Semilinear Hyperbolic System of First-Order Equations”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 68, no. 8, Aug. 2016, pp. 1043-55, https://umj.imath.kiev.ua/index.php/umj/article/view/1900.