BilusyakN.I., and PtashnikB. I. “Boundary-Value Problem for Weakly Nonlinear Hyperbolic Equations With Variable Coefficients”. Ukrains’kyi Matematychnyi Zhurnal 53, no. 9 (September 25, 2001): 1281-1286. Accessed October 6, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/4347.