Hypograph (mathematics)

In mathematics, the hypograph or subgraph of a function 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.

Hypograph of a function

The domain (rather than the codomain) of the function is not particularly important for this definition; it can be an arbitrary set[1] instead of .

Definition

The definition of the hypograph was inspired by that of the graph of a function, where the graph of is defined to be the set

The hypograph or subgraph of a function valued in the extended real numbers is the set[2]

Similarly, the set of points on or above the function is its epigraph. The strict hypograph is the hypograph with the graph removed:

Despite the fact that might take one (or both) of as a value (in which case its graph would not be a subset of ), the hypograph of is nevertheless defined to be a subset of rather than of

Properties

The hypograph of a function is empty if and only if 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 is a halfspace in

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

See also

Citations

  1. Charalambos D. Aliprantis; Kim C. Border (2007). Infinite Dimensional Analysis: A Hitchhiker's Guide (3rd ed.). Springer Science & Business Media. pp. 8–9. ISBN 978-3-540-32696-0.
  2. Rockafellar & Wets 2009, pp. 1–37.

References

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