1. Aczél, J.,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. Društvo Mat. Fiz. Hrvatske [2]20, 65–73 (1965).
2. Yosida, K., Functional Analysis (Springer-Verlag, Berlin—Heidelberg—New York 1966), esp. pp. 39–40.