PtashnikВ. I. (1971) “An analogue of the $n$-point problem for a linear hyperbolic equation ”, Ukrains’kyi Matematychnyi Zhurnal, 23(4), pp. 472-480. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/8581 (Accessed: 1September2024).