1. Aczél, J., Haruki, H., McKiernan, M. A., andSakovič, G. N.,General and Regular Solutions of Functional Equations Characterizing Harmonic Polynomials, Aequationes Math.1, 37–53 (1968).
2. Djoković, D. Ž.,Triangle Functional Equation and its Generalization, Univ. Beograd. Publ. Elektrotehn. Fak. Ser. Mat. Fiz.189, 47–52 (1967).
3. Flatto, L.,Functions with a Mean Value Property, J. Math. Mech.10, 11–18 (1961).
4. Flatto, L.,Functions with a Mean Value Property II, Amer. J. Math.85, 248–270 (1963),
5. Flatto, L.,Partial Differential Equations and Difference Equations Proc. Amer. Math. Soc.16., 858–863 (1965).