AldashevS. A. 2003. “Criterion for the Uniqueness of a Solution of the Darboux–Protter Problem for Degenerate Multidimensional Hyperbolic Equations”. Ukrains’kyi Matematychnyi Zhurnal 55 (11), 1569-75. https://umj.imath.kiev.ua/index.php/umj/article/view/4024.