# Jensen-Shannon divergence is compositional

Let \(\mathsf{FinStoch}\) be the category of finite sets and stochastic matrices.
Given two stochastic matrices, \(f_1,f_2: X \to Y\), we can define their
**Jensen-Shannon distance** as \(d(f_1,f_2) := \sup_x \sqrt{\operatorname{JSD}(f_1(x),f_2(x))}\),
where JSD is the Jensen-Shannon divergence.
It’s a standard result that the root of JSD defines a metric on the space of
probability measures - hence the above defines a metric on the set
\(\mathsf{FinStoch}(X,Y)\). My aim here is to show that
*this gives an enrichment of \(\mathsf{FinStoch}\) in the category \(\mathsf{Met}\)
of metric spaces and short, i.e distance nonincreasing, maps*

The content of this statement is that the composition map

\[\mathsf{FinStoch}(X,Y) \otimes \mathsf{FinStoch}(Y,Z) \to \mathsf{FinStoch}(X,Z)\] is a short map. The monoidal structure on \(\mathsf{Met}\) that we’re considering is given by the “1-metric”, i.e \[d_{X \otimes Y}((x,y),(x’,y’)) = d_X(x,x’) + d_Y(y,y’)\] This has the convenient property that a map is short if and only if it’s “short in each variable separately”. In other words, we must show that the map \[f \circ - :\mathsf{FinStoch}(X,Y) \to \mathsf{FinStoch}(X,Z)\] is short for each \(f\), and that the map \[- \circ f : \mathsf{FinStoch}(Y,Z) \to \mathsf{FinStoch}(X,Z)\] is short for each \(f\).

## Fact about \(\operatorname{JSD}\).

We will use the following characterization of \(\operatorname{JSD}(p,q)\): Let \(B\) be a “fair coinflip”, i.e a random variable which is \(0\) with probability \(1/2\) and \(1\) otherwise. Let \(X\) be a random variable which is distributed according to \(p\) if \(B=0\) and \(q\) if \(B=1\). Then the mutual information of \(X\) and \(B\) is exactly the Jensen-Shannon divergence of \(p\) and \(q\)

## Postcomposition

We want to show that \(d(fg_0,fg_1) \leq d(g_0,g_1)\). Clearly we may as well square both sides, so that we’re trying to show that \[\sup_x \operatorname{JSD}(fg_0(x),fg_1(x)) \leq \sup_x \operatorname{JSD}(g_0(x),g_1(x))\]

It suffices to show that this inequality holds for each \(x\), so let an \(x\) be given. Then \(\operatorname{JSD}(g_0(x),g_1(x))\) is the mutual information between a fair coin \(B\) and a variable \(Y\) distributed according to \(g_B(x)\). and \(\operatorname{JSD}(fg_1(x),fg_2(x))\) is the mutual information between \(B\) and a variable distributed as \(f(Y)\). But “postprocessing” by \(f\) can’t possible put more information about \(B\) into the random variable, since it depends on \(Z\) only through \(B\). Hence we have the desired inequality.

## Precomposition

We want to show \(d(g_0f,g_1f) \leq d(g_0,g_1)\) Again it suffices to show \[\sup_x \operatorname{JSD}(g_0f(x),g_1f(x)) \leq \sup_y \operatorname{JSD}(g_0(y),g_1(y))\] It’s enough to show this for each specific \(x\), so let’s assume wlog that \(X=*\) and \(f\) is simply a distribution, and we’re showing \[\operatorname{JSD}(g_0f,g_1f) \leq \sup_y \operatorname{JSD}(g_0(y),g_1(y))\]

To see this, let \(Y\) be distributed according to \(f\), let \(B\) be an unbiased coin, and let \(Z\) be distributed according to \(g_B(Y)\). Then \(\operatorname{JSD}(g_0f,g_1f)\) is \(I(Z;B)\), and \(\operatorname{JSD}(g_0(y),g_1(y))\) is \(I(Z;B | Y = y)\).

Our claim is that \(I(Z;B) \leq I(Z;B | Y=y)\) for at least one \(y\). We insert the entropy formula for mutual information: \[H(B) - H(B|Z) \leq H(B|Y=y) - H(B|Z, Y=y)\] Since \(B,Y\) independent, \(H(B|Y=y) = H(B)\) (equals one bit), so we may rearrange this as \[H(B|Z,Y=y) \leq H(B|Z)\] Now that the expected value \(E_y[H(B|Z,Y=y)]\) is precisely \(H(B|Z,Y)\) by definition. By a standard inequality \(H(B|Z,Y) \leq H(B|Z)\). Hence we also have the inequality \(H(B|Z,Y=y)\) for at least one \(y\), as desired.

## Followup questions

- Is there a way to build something like this “canonically” - the way entropy is characterized by a set of axioms in A characterization of Entropy in Terms of Information Loss?
- How does this relate to other metrics on probability, i.e the description from A probability monad as the colimit of spaces of finite samples