Vasylyshyn, P. B., I. S. Klyus, and B. I. Ptashnik. “Multipoint Problem for Hyperbolic Equations With Variable Coefficients”. Ukrains’kyi Matematychnyi Zhurnal 48, no. 11 (November 25, 1996): 1468–1476. Accessed December 5, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/5200.