Hypograph (mathematics)
# Hypograph (mathematics)

In mathematics, the hypograph or subgraph of a function $f:\mathbb {R} ^{n}\rightarrow \mathbb {R}$ is the set of points lying on or below its graph. A related definition is that of such a function's epigraph, which is the set of points on or above the function's graph.

The domain (rather than the codomain) of the function is not particularly important for this definition; it can be an arbitrary set instead of $\mathbb {R} ^{n}$.

## Definition

The definition of the hypograph was inspired by that of the graph of a function, where the graph of $f:X\to Y$ is defined to be the set

    $\operatorname {graph} f:=\left\{(x,y)\in X\times Y~:~y=f(x)\right\}.$

The hypograph or subgraph of a function $f:X\to [-\infty ,\infty ]$ valued in the extended real numbers $[-\infty ,\infty ]=\mathbb {R} \cup \{\pm \infty \}$ is the set

    ${\begin{alignedat}{4}\operatorname {hyp} f&=\left\{(x,r)\in X\times \mathbb {R} ~:~r\leq f(x)\right\}\\&=\left[f^{-1}(\infty )\times \mathbb {R} \right]\cup \bigcup _{x\in f^{-1}(\mathbb {R} )}\{x\}\times (-\infty ,f(x)].\end{alignedat}}$

Similarly, the set of points on or above the function is its epigraph.

The strict hypograph is the hypograph with the graph removed:

    ${\begin{alignedat}{4}\operatorname {hyp} _{S}f&=\left\{(x,r)\in X\times \mathbb {R} ~:~r<f(x)\right\}\\&=\operatorname {hyp} f\setminus \operatorname {graph} f\\&=\bigcup _{x\in X}\{x\}\times (-\infty ,f(x)).\end{alignedat}}$

Despite the fact that $f$ might take one (or both) of $\pm \infty$ as a value (in which case its graph would not be a subset of $X\times \mathbb {R}$), the hypograph of $f$ is nevertheless defined to be a subset of $X\times \mathbb {R}$ rather than of $X\times [-\infty ,\infty ].$

## Properties

The hypograph of a function $f$ is empty if and only if $f$ is identically equal to negative infinity.

A function is concave if and only if its hypograph is a convex set. The hypograph of a real affine function $g:\mathbb {R} ^{n}\to \mathbb {R}$ is a halfspace in $\mathbb {R} ^{n+1}.$

A function is upper semicontinuous if and only if its hypograph is closed.

