AldashevS. A. “Criterion for the Uniqueness of a Solution of the Darboux-Protter Problem for Multidimensional Hyperbolic Equations With Chaplygin Operator”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 56, no. 8, Aug. 2004, pp. 1119–1127, https://umj.imath.kiev.ua/index.php/umj/article/view/3826.