T. P. Goi and B. I. Ptashnik, “Problem with nonlocal conditions for weakly nonlinear hyperbolic equations”, Ukr. Mat. Zhurn., vol. 49, no. 2, pp. 186–195, Feb. 1997, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/4996