There are many inequalities which in the class of continuous functions are equivalent to convexity (for example the Jensen inequality and the Hermite-Hadamard inequalities). We show that this is not a coincidence: every nontrivial linear inequality which is valid for all convex functions is valid only for convex functions.
@article{bwmeta1.element.bwnjournal-article-doi-10_4064-sm192-1-3, author = {Jacek Tabor and J\'ozef Tabor}, title = {Characterization of convex functions}, journal = {Studia Mathematica}, volume = {192}, year = {2009}, pages = {29-37}, zbl = {1167.26003}, language = {en}, url = {http://dml.mathdoc.fr/item/bwmeta1.element.bwnjournal-article-doi-10_4064-sm192-1-3} }
Jacek Tabor; Józef Tabor. Characterization of convex functions. Studia Mathematica, Tome 192 (2009) pp. 29-37. http://gdmltest.u-ga.fr/item/bwmeta1.element.bwnjournal-article-doi-10_4064-sm192-1-3/