# Differential entropy

The formal analogue of the concept of entropy for random variables having distribution densities. The differential entropy $h ( \xi )$ of a random variable $\xi$ defined on some probability space $( \Omega , \mathfrak A , P)$, assuming values in an $n$- dimensional Euclidean space $\mathbf R ^ {n}$ and having distribution density $p( x)$, $x \in \mathbf R ^ {n}$, is given by the formula

$$h ( \xi ) = \int\limits _ {\mathbf R ^ {n} } p ( x) \mathop{\rm log} p ( x) dx ,$$

where $0 \mathop{\rm log} 0$ is assumed to be equal to zero. Thus, the differential entropy coincides with the entropy of the measure $P ( \cdot )$ with respect to the Lebesgue measure $\lambda ( \cdot )$, where $P ( \cdot )$ is the distribution of $\xi$.

The concept of the differential entropy proves useful in computing various information-theoretic characteristics, in the first place the mutual amount of information (cf. Information, amount of) $J ( \xi , \eta )$ of two random vectors $\xi$ and $\eta$. If $h ( \xi )$, $h ( \eta )$ and $h ( \xi , \eta )$( i.e. the differential entropy of the pair $( \xi , \eta )$) are finite, the following formula is valid:

$$J ( \xi , \eta ) = - h ( \xi , \eta ) + h ( \xi )+ h ( \eta ).$$

The following two properties of the differential entropy are worthy of mention: 1) as distinct from the ordinary entropy, the differential entropy is not covariant with respect to a change in the coordinate system and may assume negative values; and 2) let $\phi ( \xi )$ be the discretization of an $n$- dimensional random variable $\xi$ having a density, with steps of $\Delta x$; then for the entropy $H ( \phi ( x))$ the formula

$$H ( \phi ( \xi )) = - n \mathop{\rm log} \Delta x + h ( \xi ) + o ( 1)$$

is valid as $\Delta \rightarrow 0$. Thus, $H ( \phi ( x )) \rightarrow + \infty$ as $\Delta x \rightarrow 0$. The principal term of the asymptotics of $H ( \phi ( \xi ))$ depends on the dimension of the space of values of $\xi$. The differential entropy defines the term next in order of the asymptotic expansion independent of $\Delta x$ and it is the first term involving a dependence on the actual nature of the distribution of $\xi$.

How to Cite This Entry:
Differential entropy. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Differential_entropy&oldid=46665
This article was adapted from an original article by R.L. DobrushinV.V. Prelov (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article