KANNAPPAN, P.; KIM, G. H. On the stability of the generalized cosine functional equations. Annales Universitatis Paedagogicae Cracoviensis Studia Mathematica, [S. l.], v. 1, p. 49–58, 2001. Disponível em: https://studmath.uken.krakow.pl/article/view/13516. Acesso em: 30 oct. 2026.