1. Grza̧ślewicz, A.: Some remarks to additive functions. Math. Jpn. 23 (1978/79), 573–578
2. Grza̧ślewicz, A.: On the solution of the system of functional equations related to quadratic functionals. Glas. Mat. Ser. III 14(34), 77–82 (1979)
3. Halperin, I.: Problem 448. Colloq. Math. 11, 140 (1963)
4. Jurkat, W.B.: On Cauchy’s functional equation. Proc. Am. Math. Soc. 16, 683–686 (1965)
5. Kannappan, Pl, Kurepa, S.: Some relations between additive functions I. Aequationes Math. 4, 163–175 (1970)