# Definition:Dirac Measure

## Definition

Let $\struct {X, \Sigma}$ be a measurable space.

Let $x \in X$ be any point in $X$.

Then the Dirac measure at $x$, denoted $\delta_x$ is the measure defined by:

$\delta_x: \Sigma \to \overline \R, \ \map {\delta_x} E := \begin{cases}0 & \text{if } x \notin E \\ 1 & \text{if } x \in E \end{cases}$

where $\overline \R$ denotes the extended set of real numbers.

That $\delta_x$ actually is a measure is shown on Dirac Measure is Measure.

## Also known as

Alternatively, the Dirac measure at $x$ may be called Dirac's delta measure at $x$ or unit mass at $x$.

In physics, this measure is often (very informally) treated as a special function.

This obfuscates the rigid mathematical foundations the Dirac measure lies in, and thence should always be avoided.

## Source of Name

This entry was named for Paul Adrien Maurice Dirac.