1. Aczél, J.,Lectures on functional equations and their applications. Academic Press, New York-London, 1966.
2. van der Corput, J.,Goniometrische Functies gekarakteriseerd door een Functionalbetreklering II. Euclides (Gronigen)17 (1940), 65–75.
3. Ger, R.,On some properties of polynomial functions. Ann. Polon. Math.25 (1971), 195–203.
4. Kannappan, Pl.,The functional equation f(xy) +f(xy −1) = 2f(x)f(y)for groups. Proc. Amer. Math. Soc.19 (1968), 69–74.
5. Kuczma, M.,Functional equations in a single variable. PWN — Polish Scientific Publishers, Warszawa, 1968.