Bilusyak, N.I., and B. I. Ptashnik. “A Boundary-Value Problem for Weakly Nonlinear Hyperbolic Equations With Data on the Entire Boundary of a Domain”. Ukrains’kyi Matematychnyi Zhurnal 53, no. 2 (February 25, 2001): 244-249. Accessed May 16, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/4240.