1. Aczél, J.: 1966, Lectures on functional equations and their applications. Academic Press, New York-London.
2. Aczél, J., H. Haruki, M.A. McKiernan and G.N. Saković: 1968, General and regular solutions of functional equations characterizing harmonic polynomials. Aeguationes Math. 1, 37–53.
3. Fenyö, I.: 1966, Uber eine Funktionalgleichung. Math. Nachr. 31, 103–109.
4. Flatto, L.: 1963, Functions with a mean value property II. Amer. J. Math. 85, 248–270.
5. Friedman, A. and W. Littman: 1962, Functions satisfying the mean value property. Trans. Amer. Math. Soc. 102, 167–180.