POKUTNYI, Oleksandr. Weakly nonlinear hyperbolic differential equation of the second order in Hilbert space. Topological Methods in Nonlinear Analysis, [S. l.], v. 64, n. 1, p. 279–294, 2024. DOI: 10.12775/TMNA.2023.056. Disponível em: https://apcz.umk.pl/TMNA/article/view/55211. Acesso em: 30 dec. 2025.