Index: The Book of Statistical ProofsGeneral TheoremsInformation theory ▷ Differential entropy ▷ Addition upon matrix multiplication

Theorem: Let $X$ be a continuous random vector. Then, the differential entropy of $X$ increases additively when multiplied with an invertible matrix $A$:

$\label{eq:dent-addvec} \mathrm{h}(AX) = \mathrm{h}(X) + \log |A| \; .$

Proof: By definition, the differential entropy of $X$ is

$\label{eq:X-dent} \mathrm{h}(X) = - \int_{\mathcal{X}} f_X(x) \log f_X(x) \, \mathrm{d}x$

where $f_X(x)$ is the probability density function of $X$ and $\mathcal{X}$ is the set of possible values of $X$.

The probability density function of a linear function of a continuous random vector $Y = g(X) = \Sigma X + \mu$ is

$\label{eq:pdf-linfct} f_Y(y) = \left\{ \begin{array}{rl} \frac{1}{\left| \Sigma \right|} f_X(\Sigma^{-1}(y-\mu)) \; , & \text{if} \; y \in \mathcal{Y} \\ 0 \; , & \text{if} \; y \notin \mathcal{Y} \end{array} \right.$

where $\mathcal{Y} = \left\lbrace y = \Sigma x + \mu: x \in \mathcal{X} \right\rbrace$ is the set of possible outcomes of $Y$.

Therefore, with $Y = g(X) = AX$, i.e. $\Sigma = A$ and $\mu = 0_n$, the probability density function of $Y$ is given by

$\label{eq:Y-pdf} f_Y(y) = \left\{ \begin{array}{rl} \frac{1}{\left| A \right|} f_X(A^{-1}y) \; , & \text{if} \; y \in \mathcal{Y} \\ 0 \; , & \text{if} \; y \notin \mathcal{Y} \end{array} \right.$

where $\mathcal{Y} = \left\lbrace y = A x: x \in \mathcal{X} \right\rbrace$.

Thus, the differential entropy of $Y$ is

$\label{eq:Y-dent-s1} \begin{split} \mathrm{h}(Y) &\overset{\eqref{eq:X-dent}}{=} - \int_{\mathcal{Y}} f_Y(y) \log f_Y(y) \, \mathrm{d}y \\ &\overset{\eqref{eq:Y-pdf}}{=} - \int_{\mathcal{Y}} \left[ \frac{1}{\left| A \right|} f_X(A^{-1}y) \right] \log \left[ \frac{1}{\left| A \right|} f_X(A^{-1}y) \right] \, \mathrm{d}y \; . \end{split}$

Substituting $y = Ax$ into the integral, we obtain

$\label{eq:Y-dent-s2} \begin{split} \mathrm{h}(Y) &= - \int_{\mathcal{X}} \left[ \frac{1}{\left| A \right|} f_X(A^{-1}Ax) \right] \log \left[ \frac{1}{\left| A \right|} f_X(A^{-1}Ax) \right] \, \mathrm{d}(Ax) \\ &= - \frac{1}{\left| A \right|} \int_{\mathcal{X}} f_X(x) \log \left[ \frac{1}{\left| A \right|} f_X(x) \right] \, \mathrm{d}(Ax) \; . \end{split}$

Using the differential $\mathrm{d}(Ax) = \lvert A \rvert \mathrm{d}x$, this becomes

$\label{eq:Y-dent-s3} \begin{split} \mathrm{h}(Y) &= - \frac{\left| A \right|}{\left| A \right|} \int_{\mathcal{X}} f_X(x) \log \left[ \frac{1}{\left| A \right|} f_X(x) \right] \, \mathrm{d}x \\ &= - \int_{\mathcal{X}} f_X(x) \log f_X(x) \, \mathrm{d}x - \int_{\mathcal{X}} f_X(x) \log \frac{1}{\left| A \right|} \, \mathrm{d}x \; . \end{split}$

Finally, employing the fact that $\int_{\mathcal{X}} f_X(x) \, \mathrm{d}x = 1$, we can derive the differential entropy of $Y$ as

$\label{eq:Y-dent-s4} \begin{split} \mathrm{h}(Y) &= - \int_{\mathcal{X}} f_X(x) \log f_X(x) \, \mathrm{d}x + \log \left| A \right| \int_{\mathcal{X}} f_X(x) \, \mathrm{d}x \\ &\overset{\eqref{eq:X-dent}}{=} \mathrm{h}(X) + \log |A| \; . \end{split}$
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Metadata: ID: P261 | shortcut: dent-addvec | author: JoramSoch | date: 2021-10-07, 09:10.