AldashevS. A. “Darboux–Protter Spectral Problems for One Class of Multidimensional Hyperbolic Equations”. Ukrains’kyi Matematychnyi Zhurnal 55, no. 1 (January 25, 2003): 100-107. Accessed November 22, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3891.