BilusyakN.I., and PtashnikB. I. 2001. “Boundary-Value Problem for Weakly Nonlinear Hyperbolic Equations With Variable Coefficients”. Ukrains’kyi Matematychnyi Zhurnal 53 (9), 1281-86. https://umj.imath.kiev.ua/index.php/umj/article/view/4347.