Goi, T. P., and B. I. Ptashnik. “Problem With Nonlocal Conditions for Weakly Nonlinear Hyperbolic Equations”. Ukrains’kyi Matematychnyi Zhurnal, vol. 49, no. 2, Feb. 1997, pp. 186–195, https://umj.imath.kiev.ua/index.php/umj/article/view/4996.