# Convex functional

A functional, defined on a convex subset of a linear vector space, the supergraph of which is a convex set. A functional which does not assume the value on a convex set is convex on if and only if the inequality

is satisfied. If the inequality sign is reversed, the functional is called concave. Operations which convert a convex functional into a convex functional include addition , multiplication by a positive number, taking the upper bound

and the infimal convolution

A convex functional bounded from above in a neighbourhood of some point is continuous at that point. If a convex functional is finite at some point , it has a (finite or infinite) derivative in any direction at that point. Closed convex functionals (i.e. functionals with convex and closed supergraphs) in locally convex linear topological spaces may be described in a dual way: Such a functional is the least upper bound of the affine functions which it dominates. This duality makes it possible to relate to each convex functional a dual object, the conjugate functional

Properties of convex functionals, operations on such functionals, and the relationship between a convex functional and its conjugate, are studied in convex analysis.

#### References

[1] | Z.W. Birnbaum, W. Orlicz, "Ueber die Verallgemeinerung des Begriffes der zueinander konjugierter Potenzen" Studia Math. , 3 (1931) pp. 1–67 |

[2] | G.H. Hardy, J.E. Littlewood, G. Pólya, "Inequalities" , Cambridge Univ. Press (1934) MR2591337 MR1601748 MR1472649 MR1478445 MR1234454 MR1146141 MR1136994 MR0944909 MR0197653 MR0046395 MR0083530 MR0008854 MR1563622 MR1545539 MR1545530 Zbl 0010.10703 Zbl 60.0169.01 |

[3] | M.A. Krasnosel'skii, Ya.B. Rutitskii, "Convex functions and Orlicz spaces" , Noordhoff (1961) (Translated from Russian) Zbl 0095.09103 |

[4] | W. Fenchel, "On conjugate convex functions" Canad. J. Math. , 1 (1949) pp. 73–77 MR0028365 Zbl 0038.20902 |

[5] | R.T. Rockafellar, "Convex analysis" , Princeton Univ. Press (1970) MR0274683 Zbl 0193.18401 |

**How to Cite This Entry:**

Convex functional.

*Encyclopedia of Mathematics.*URL: http://www.encyclopediaofmath.org/index.php?title=Convex_functional&oldid=28163