Pokutnyi, Oleksandr. “Weakly Nonlinear Hyperbolic Differential Equation of the Second Order in Hilbert Space”. Topological Methods in Nonlinear Analysis 64, no. 1 (September 15, 2024): 279–294. Accessed December 31, 2025. https://apcz.umk.pl/TMNA/article/view/55211.