1. Aczél, J.: Lectures on functional equations and their applications. In: Oser, H. (ed.) Mathematics in Science and Engineering, vol. 19. Academic Press, New York (1966) (Translated by Scripta Technica, Inc. Supplemented by the author)
2. Aczél, J., Haruki, H., McKiernan, M.A., Sakovič, G.N.: General and regular solutions of functional equations characterizing harmonic polynomials. Aequ. Math. 1, 37–53 (1968)
3. Bahyrycz, A., Olko, J.: Hyperstability of some functional equations on restricted domain. J. Funct. Spaces. Art. ID 1946394, 6 (2017)
4. Bahyrycz, A., Olko, J.: On stability and hyperstability of an equation characterizing multi-Cauchy–Jensen mappings. Results Math. 73(2), 55 (2018)
5. Chung, J.K., Ebanks, B.R., Ng, C.T., Sahoo, P.K., Zeng, W.B.: On generalized rectangular and rhombic functional equations. Publ. Math. Debr. 47(3–4), 249–270 (1995)