1. Aczél, J., 7. Problem. In Report of the twenty-first International Symposium on Functional Equations. Aequationes Math. 26 (1984), 258.
2. Albert, M. and Baker, J. A., Functions with bounded n-th differences. A.n. Polon. Math. 43 (1983), 93–103.
3. Alsina, C., On the stability of a functional equation related to associativity. Ann. Polon. Math. 53 (1991), 1–5.
4. Badora, R.,On some generalized invariant means and their application to the stability of the Hyers-Ulam type I. Manuscript.
5. Baker, J. A., The stability of the cosine equation. Proc. A.er. Math. Soc. 80 (1980), 411–416.