Korzyuk, V., and J. V. Rudzko. “Curvilinear Parallelogram Identity and Mean-Value Property for a Semilinear Hyperbolic Equation of the Second Order”. Eurasian Mathematical Journal, vol. 15, no. 2, July 2024, pp. 61-74, https://emj.enu.kz/index.php/main/article/view/343.