Georgiev, Svetlin, and Mohamed Majdoub. “Two Nonnegative Solutions for Two-Dimensional Nonlinear Wave Equations”. CUBO, A Mathematical Journal 24, no. 3 (December 21, 2022): 393–412. Accessed May 4, 2024. https://cubo.ufro.cl/ojs/index.php/cubo/article/view/3206.