PRZYBYŁA, Maciej. Mixed stability of the d’Alembert functional equation. Annales Mathematicae Silesianae, [S. l.], v. 19, p. 53–57, 2005. Disponível em: https://journals.us.edu.pl/index.php/AMSIL/article/view/14084. Acesso em: 5 dec. 2025.