Galewski, Marek, and Andrzej Nowakowski. “Existence of Solutions for a Nonlinear Wave Equation”. Topological Methods in Nonlinear Analysis 28, no. 2 (December 1, 2006): 385–399. Accessed January 3, 2026. https://apcz.umk.pl/TMNA/article/view/TMNA.2006.034.