S. A. Aldashev, “Correctness of multi-dimensional Darboux problems for the wave equation”, Ukr. Mat. Zhurn., vol. 45, no. 9, pp. 1299–1306, Sep. 1993, Accessed: Apr. 09, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/5933