Ptashnik В. I. An analogue of the $n$-point problem for a linear hyperbolic equation . Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 23, n. 4, p. 472–480, 1971. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/8581. Acesso em: 6 dec. 2025.