AldashevS. A. (2004). Criterion for the uniqueness of a solution of the Darboux-Protter problem for multidimensional Hyperbolic equations with Chaplygin operator. Ukrains’kyi Matematychnyi Zhurnal, 56(8), 1119–1127. Retrieved from https://umj.imath.kiev.ua/index.php/umj/article/view/3826