# Inequalities in information theory

Inequalities in information theory

Inequalities are very important in the study of information theory. There are a number of different contexts in which these inequalities appear.

hannon-type inequalities

Consider a finite collection of finitely (or at most countably) supported random variables on the same probability space. For a collection of "n" random variables, there are $2^n-1$ such non-empty subsets for which entropies can be defined. For example, when $n=2,$ we may consider the entropies $H\left(X_1\right),$ $H\left(X_2\right),$ and $H\left(X_1, X_2\right),$ and express the following inequalities (which together characterize the range of the marginal and joint entropies of two random variables):
* $H\left(X_1\right) ge 0$
* $H\left(X_2\right) ge 0$
* $H\left(X_1\right) le H\left(X_1, X_2\right)$
* $H\left(X_2\right) le H\left(X_1, X_2\right)$
* $H\left(X_1, X_2\right) le H\left(X_1\right) + H\left(X_2\right)$In fact, these can all be expressed as special cases of a single inequality involving the conditional mutual information, namely:$I\left(A;B|C\right) ge 0,$where $A$, $B$, and $C$ each denote the joint distribution of some arbitrary subset of our collection of random variables. Inequalities that can be derived from this are known as Shannon-type inequalities. More formally, (following the notation of Yeung), define $Gamma^*_n$ to be the set of all "constructable" points in $mathbb R^\left\{2^n-1\right\},$ where a point is said to be constructable if and only if there is a joint, discrete distribution of "n" random variables such that each coordinate of that point, indexed by a non-empty subset of $\left\{1,2,...n\right\},$ is equal to the joint entropy of the corresponding subset of the "n" random variables. The closure of $Gamma^*_n$ is denoted $overline\left\{Gamma^*_n\right\}.$

The cone in $mathbb R^\left\{2^n-1\right\}$ characterized by all Shannon-type inequalities among "n" random variables is denoted $Gamma_n.$ Software has been developed to automate the task of proving such inequalities. [ [http://user-www.ie.cuhk.edu.hk/~ITIP/ ITIP - Information Theoretic Inequality Prover] ] [ [http://xitip.epfl.ch/ Xitip - Information Theoretic Inequalities Prover] ] Given an inequality, such software is able to determine whether the given inequality contains the cone $Gamma_n,$ in which case the inequality can be verified, since $Gamma^*_n subseteq Gamma_n.$

Non-Shannon-type inequalities

Other, less trivial inequalities have been discovered among the entropies and joint entropies of four or more random variables, which cannot be derived from Shannon's basic inequalities. [Z. Zhang, R.W. Yeung, "A non-Shannon-type conditional inequality of information quantities." IEEE Transactions on Information Theory. New York: Nov 1997. Vol. 43, Iss. 6; p. 1982] [K. Makarychev et al. "A new class of non-Shannon-type inequalities for entropies." Communications in Information and Systems, Vol. 2, No. 2, pp. 147-166, December 2002 [http://www.cs.princeton.edu/~ymakaryc/papers/nonshann.pdf PDF] ] These are known as non-Shannon-type inequalities.

It was shown that:$Gamma^*_n subset overline\left\{Gamma^*_n\right\} subset Gamma_n,$where the inclusions are proper for $n ge 4.$ All three of these sets are, in fact, convex cones.

Other inequalities

Gibbs' inequality

"Main article: Gibbs' inequality"

This fundamental inequality states that the Kullback–Leibler divergence is non-negative.

Kullback's inequality

Another inequality concerning the Kullback–Leibler divergence is known as Kullback's inequality [Aimé Fuchs and Giorgio Letta, "L'inégalité de Kullback. Application à la théorie de l'estimation." Séminaire de probabilités (Strasbourg), vol. 4, pp. 108-131, 1970. http://www.numdam.org/item?id=SPS_1970__4__108_0] . If $P$ and $Q$ are equivalent (i.e. absolutely continuous with respect to one another) probability distributions on the same measurable space, whose first moments exist, then:$D_\left\{KL\right\}\left(P|Q\right) ge Psi_Q^*\left(mu\text{'}_1\left(P\right)\right),$where $Psi_Q^*$ is the rate function, i.e. the convex conjugate of the cumulant-generating function, of $Q$, and $mu\text{'}_1\left(P\right)$ is the first moment of $P.$

The Cramér–Rao bound is an immediate consequence of this inequality.

Hirschman uncertainty

In 1957, [I.I. Hirschman, "A Note on Entropy", American Journal of Mathematics, 1957] Hirschman showed that for a (reasonably well-behaved) function $f:mathbb R ightarrow mathbb C$ such that $int_\left\{-infty\right\}^infty |f\left(x\right)|^2,dx = 1,$ and its Fourier transform $g\left(y\right)=int_\left\{-infty\right\}^infty f\left(x\right) e^\left\{-2 pi i x y\right\},dx,$ the sum of the differential entropies of $|f|^2$ and $|g|^2$ is non-negative, i.e.:$-int_\left\{-infty\right\}^infty |f\left(x\right)|^2 log |f\left(x\right)|^2 ,dx -int_\left\{-infty\right\}^infty |g\left(y\right)|^2 log |g\left(y\right)|^2 ,dy ge 0.$Hirschman conjectured, and it was later proved, [W. Beckner. "Inequalities in Fourier Analysis." Annals of Mathematics, vol. 102, no. 6, pp. 159-182. 1975] that a sharper bound of $1-log 2,$ which is attained in the case of a Gaussian distribution, could replace the right-hand side of this inequality. This is especially significant since it implies, and is stronger than, Weyl's formulation of Heisenberg's uncertainty principle.

This inequality can be generalized, and somewhat more easily proven, [S. Zozor, C. Vignat. "On classes of non-Gaussian asymptotic minimizers in entropic uncertainty principles." [http://arxiv.org/abs/math/0605510v1 arXiv:math/0605510v1] ] via the concept of Rényi entropy. [Iwo Bialynicki-Birula. "Formulation of the uncertainty relations in terms of the Renyi entropies." [http://arxiv.org/abs/quant-ph/0608116v2 arXiv:quant-ph/0608116v2] ] Namely, if and it can be shown that:from which the lower bound for the sum of the Shannon entropies can be obtained by taking the limit as

Tao's inequality

Given discrete random variables $X,$ $Y,$ and $Y\text{'},$ such that $X$ takes values only in the interval $\left[-1,1\right]$ and $Y\text{'}$ is determined by $Y$ (so that $H\left(Y\text{'}|Y\right)=0$), we have [ T. Tao, "Szemeredi's regularity lemma revisited", Contrib. Discrete Math., 1 (2006), 8–28. Preprint: [http://arxiv.org/abs/math/0504472v2 arXiv:math/0504472v2] ] [Rudolf Ahlswede, "The final form of Tao's inequality relating conditional expectation and conditional mutual information." Advances in Mathematics of Communications. Volume 1, No. 2, 2007, pp. 239–242 [http://www.aimsciences.org/journals/pdfs.jsp?paperID=2565&mode=full PDF] ] :relating the conditional expectation to the conditional mutual information. (Note: the correction factor $log 2$ inside the radical arises because we are measuring the conditional mutual information in bits rather than nats.)

ee Also

*Cramér–Rao bound
*Entropy power inequality
*Fano's inequality
*Jensen's inequality
*Kraft inequality

References

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