1. Aczél J.: Some unsolved problems in the theory of functional equations. Arch. Math., 15, 435–444 (1964)
2. J. Aczél, The general solution of two functional equations by reduction to functions additive in two variables and with the aid of Hamel bases, Glasnik Mat.-Fiz. Astronom. Ser. II Društvo Mat. Fiz. Hrvatske, 20 (1965), 65–73.
3. J. Aczél and J. Dhombres, Functional Equations in Several Variables, Encyclopaedia of Mathematics and its Applications, vol. 31, Cambridge Univ. Press (Cambridge–New York–New Rochelle–Melbourne–Sydney, 1989).
4. Benz W.: 5 Problem (in Report of Meeting: The twenty-seventh international symposium on functional equations). Aequationes Math., 39, 302 (1990)
5. Boros Z., Erdei P.: A conditional equation for additive functions. Aequationes Math., 70, 309–313 (2005)