1. J. Aczél, Some general methods in the theory of functional equations in one variable. New applications of functional equations (Russian). Uspehi Mat. Nauk (N.S.) 11, No. 3 (69) (1956), 3–58.
2. H. Alzer, Ungleichungen für geometrische und arithmetische Mittelwerte. Proc. Kon. Nederl. Akad. Wetensch. 91 (1988), 365–374.
3. H. Alzer, T. Ando and Y. Nakamura, The inequalities of W. Sierpinski and Ky Fan. J. Math. Anal. Appl. 149 (1990), 497–512.
4. E.F. Beckenbach and R. Bellman, Inequalities. Springer Verlag, Berlin, 1983.
5. R. Bellman, On an inequality concerning an indefinite form. Amer. Math. Monthly 63 (1956), 108–109.