S. A. Aldashev, “Darboux–Protter Spectral Problems for One Class of Multidimensional Hyperbolic Equations”, Ukr. Mat. Zhurn., vol. 55, no. 1, pp. 100–107, Jan. 2003, Accessed: Feb. 16, 2026. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/3891