1. Aczél, J.,Lectures on functional equations and their applications. Academic Press, New York, London, 1966.
2. Aczél, J.,The general solution of two functional equations by reducing to functions additive in two variables and with the aid of Hamel basis. Glasnik Mat. fiz. astr.20 (1965), 65–72.
3. Aczél, J. andDhombres, J.,Functional equations in several variables. Cambridge University Press, Cambridge, 1988.
4. Baker, J. A.,On quadratic functionals continuous along rays. Glasnik Mat.3 (23) (1968), 215–229.
5. Gleason, A. M.,The definition of a quadratic form. Amer. Math. Monthly73 (1966), 1049–1056.