Beresnevich, V. V., V. I. Bernik, P. B. Vasylyshyn, and B. I. Ptashnik. 1999. “A Multipoint Problem With Multiple Nodes for Linear Hyperbolic Equations”. Ukrains’kyi Matematychnyi Zhurnal 51 (10), 1311–1316. https://umj.imath.kiev.ua/index.php/umj/article/view/4729.