1. Aczél J., Lectures on Functional Equations and Their Applications, volume 19 of Mathematics in Science and Engineering, Academic Press, New York-London 1966.
2. Aczél J., Dhombres J., Functional Equations in Several Variables, Encyclopedia of Mathematics and its Applications 31, Cambridge University Press, Cambridge 1989.
3. Choquet G., Les cônes convexes faiblement complets dans l'Analyse, Proc. Intern. Congr. Mathematicians, Stockholm (1962), 317-330.
4. Daróczy Z., Problem 3, in: Report of Meeting. Second DKWS, Hajdúszoboszló 2002, Ann. Math. Sil. 16 (2003), pp. 95-96.
5. Di-Lian Yang, The quadratic functional equation on groups, Publ. Math. Debrecen 66/3-4 (2005), 327-348.
6. Dragomir S.S., On Hadamard's inequality for the convex mappings defined on a ball in the space and applications, Math. Inequal. Appl. 3 (2000), no. 2, 177-187.
7. Dragomir S.S., On Hadamard's inequality on a disk, JIPAM. J. Inequal. Pure Appl. Math. 1 (2000), no. 1, Article 2, 11 pp. (electronic).
8. Dragomir S.S., On the Hadamard's inequality for convex functions on the co-ordinates in a rectangle from the plane, Taiwanese J. Math. 5 (2001), no. 4, 775-788.
9. Dragomir S.S., Pearce C.E.M., Selected Topics on Hermite-Hadamard Inequalities, RGMIA Monographs (http://rgmia.vu.edu.au/monographs/hermite_hadamard.html), Victoria University, 2000.
10. Gavrea B., On Hadamard's inequality for the convex mappings defined on a convex domain in the space, JIPAM. J. Inequal. Pure Appl. Math. 1 (2000), no. 1, Article 9, 6 pp. (electronic).
11. Ger R., Remark 4, in: Report of Meeting. Second DKWS, Hajdúszoboszló 2002, Ann. Math. Sil. 16 (2003), pp. 96-97.
12. Hadamard J., Étude sur les propriétés des fonctions entieres et en particulier d'une fonction considérée par Riemann, J. Math. Pures Appl. 58 (1893), 171-215.
13. Jarczyk W., Almost iterable functions., Aequationes Math. (University of Waterloo) 42 (1991) 202-219.
14. Kannappan PI., On quadratic functional equation, Int. J. Math. Stat. Sci. 9 (2000), no. 1, 35-60.
15. Minty G.J., On the monotonicity of the gradient of a convex function, Pacific J. Math. 14 (1964), 243-247.
16. Niculescu C.P., The Hermite-Hadamard inequality for convex functions of a vector variable, Math. Inequal. Appl. 5 (2002), no. 4, 619-623.
17. Niculescu C.P., Persson L.-E., Old and new on the Hermite-Hadamard inequality, Real Anal. Exchange 29 (2003/2004), vol. 2, 619-623.
18. Rockafellar R.T., Characterization of the subdifferentials of convex functions, Pacific J. Math. IT (1966), 497-510.
19. Rockafellar R.T., On the maximal monotonicity of subdifferential mappings, Pacific J. Math. 33 (1970), 209-216.
20. Targoński Gy., New directions and open problems in iteration theory [Ber. Math.-Statist. Sekt. Forschungsgesellsch. Joanneum, No. 229]. Forschungszentrum, Graz, 1984. Google Scholar