# A Cheat Sheet for Information Theory Fundamentals in ML

Posted on Dec 31, 2022

All of the definitions stem from Kevin Murphy’s Probabilistic Machine Learning: An Introduction.

Many of the following primitives are defined for discrete as well as continuous random variables and distributions. Since they are analogous, I chose to only represent discrete versions for the sake of simplicity.

Entropy$$\mathbb{H}(X) = \mathbb{H}(p) := -\sum_\mathcal{X} p(X=x)\log_2 p(X=x)$$
Cross-entropy$$\mathbb{H}(p, q) := -\sum_\mathcal{X} p(X=x) \log_2 q(X=x)$$
Joint entropy$$\mathbb{H}(X, Y) := -\sum_{\mathcal{X}, \mathcal{Y}} p(X=x, Y=y)\log_2 p(X=x, Y=y)$$
Conditional entropy\begin{aligned} \mathbb{H}(X|Y) &:= \mathbb{E}_{p(X)}[\mathbb{H}(p(Y|X))] \\ &= \mathbb{H}(X, Y) - \mathbb{H}(X) \end{aligned}
Chain rule for entropy$$\mathbb{H}(X_1, X_2, \dots, X_n) = \sum_{i=1}^n \mathbb{H}(X_i|X_1, \dots X_{i-1})$$
KL-divergence (a.k.a relative entropy)\begin{aligned} D_{KL}(p||q) &:= \sum_{\mathcal{X}} p(X=x) \log_2 \frac{p(X=x)}{q(X=X)} \\ &= \mathbb{H}(p, q) - \mathbb{H}(p) \end{aligned}
Forwards KL-divergenceApproximating $p$ with $q$ by minimizing $D_{KL}(p||q)$ w.r.t. $q$
Reverse KL-divergenceApproximating $p$ with $q$ by minimizing $D_{KL}(q||p)$ w.r.t. $q$
(Expected) Mutual Information (a.k.a. Information Gain)\begin{aligned} \mathbb{I}(X;Y) &:= D_{KL}(p(X, Y) || p(X)p(Y)) \\ &= \mathbb{H}(X) - \mathbb{H}(X|Y) \\ &= \mathbb{H}(X) + \mathbb{H}(Y) - \mathbb{H}(X, Y) \end{aligned}
Conditional Mutual Information\begin{aligned} \mathbb{I}(X;Y|Z) &:= \mathbb{E}_{p(Z)} [ \mathbb{I}(X;Y) | Z] \\ &= \mathbb{I}(Y; X, Z) - \mathbb{I}(Y;Z) \end{aligned}
Chain rule for MI$$\mathbb{I}(Z_1, \dots, Z_n; X) = \sum_{i=1}^n \mathbb{I}(Z_i; X | Z_1, \dots, Z_{i-1})$$