KirilichV. M., and FirmanT. I. 2016. “Problem Without Initial Conditions for a Countable Semilinear Hyperbolic System of First-Order Equations”. Ukrains’kyi Matematychnyi Zhurnal 68 (8), 1043-55. https://umj.imath.kiev.ua/index.php/umj/article/view/1900.