Closed convex function

In mathematics, a function $f:{\mathbb {R}}^{n}\rightarrow {\mathbb {R}}$ is said to be closed if for each $\alpha \in {\mathbb {R}}$, the sublevel set $\{x\in {\mbox{dom}}f\vert f(x)\leq \alpha \}$ is a closed set.

Equivalently, if the epigraph defined by ${\mbox{epi}}f=\{(x,t)\in {\mathbb {R}}^{{n+1}}\vert x\in {\mbox{dom}}f,\;f(x)\leq t\}$ is closed, then the function $f(x)$ is closed.

This definition is valid for any function, but most used for convex functions. A proper convex function is closed if and only if it is lower semi-continuous.^[1] For a convex function which is not proper there is disagreement as to the definition of the closure of the function.

Properties

• If $f:{\mathbb {R}}^{n}\rightarrow {\mathbb {R}}$ is a continuous function and ${\mbox{dom}}f$ is closed, then $f$ is closed.
• A closed proper convex function f is the pointwise supremum of the collection of all affine functions h such that h ≤ f (called the affine minorants of f).

References

1. Convex Optimization Theory. Athena Scientific. 2009. pp. 10, 11. ISBN 978-1886529311.

• Boyd, Stephen; Vandenberghe, Lieven (2004). Convex optimization (PDF). New York: Cambridge. pp. 639–640. ISBN 978-0521833783.
• Rockafellar, R. Tyrrell (1997) [1970]. Convex Analysis. Princeton, NJ: Princeton University Press. ISBN 978-0-691-01586-6.