Vasylyshyn, P. B., Klyus, I. S., & Ptashnik, B. I. (1996). Multipoint problem for hyperbolic equations with variable coefficients. Ukrains’kyi Matematychnyi Zhurnal, 48(11), 1468-1476. https://umj.imath.kiev.ua/index.php/umj/article/view/5200