# A computable lower bound for the KL from Hammersley-Chapman-Robbins inequality

I first read of this bound from:

```
Nishiyama, T., 2019. A New Lower Bound for Kullback-Leibler Divergence Based on Hammersley-Chapman-Robbins Bound. arXiv:1907.00288 [cs, math, stat].
```

The following notes are not very polished, but present the general idea. I hope they are useful.

The strategy of the paper is quite nice; first, Nishiyama shows that if we have two distributions \(P,Q\) and define the mixture \(R_t(x):=P(x)+t(Q(x)-P(x)), t\in [0,1]\), then:

\[\frac{d}{dt}D_a(P|R_t)=\frac{1-a}{t}D_a(P|R_t)+\frac{1+a}{t}D_{a+1}(P|R_t),\] for any \(a\) and \(D_a\) being the alpha-divergence. Setting \(a=1\) leaves us with the KL divergence and \(\chi^2\), which recovers the nice identity:

\[\frac{d}{dt}KL(P|R_t)=\frac{1}{t}\chi^2(P|R_t).\] Now, fix a function \(f\) for which \(E_Q, E_P, V_P, V_{R_t}\) (expectations and variances) are finite and \(V_{R_t}>0\) for all \(t\in [0,1]\). Then, applying the HCR inequality gives:

\[\frac{d}{dt}KL(P|R_t)\geq \frac{(E_{R_t}-E_P)^2}{tV_{R_t}}.\] Integrating the above in \([0,1]\) gives the result of the paper as \(\int_0^1 KL(P|R_t)dt=KL(P|Q).\)

We can also show that

\[KL(P|Q)=\int_0^1 \frac{\chi^2(P|R_t)}{t}dt=\int_0^1 t\int_{\Omega}\frac{(Q-P)^2}{P+t(Q-P)}dxdt.\] Assuming everything exists, we can exchange the integrals (ala. Fubini) and then expand the \(t\) function around \(t=1\) to introduce the chi-square divergence:

\[KL(P|Q)=\chi^2(P|Q)+...\] ## Actual lower bound

The lower bound is:

\[KL(P|Q)\geq \int_0^1\frac{(E_{R_t}-E_P)^2}{tV_{R_t}}dt,\] where the integral depends on \(E_P, E_Q, V_P, V_Q\) and can be computed analytically and written as a sum of logarithms (by using partial fractions).

# Tighter lower-bounds

Starting from the equality:

\[KL(P|Q)=\int_0^1 \frac{\chi^2(P|R_t)}{t}dt\] we can derive tighter lower bounds for the KL. First, we write the variational representation of \(\chi^2\)

\[\chi^2(P|R_t)=\sup_{h}\left \{ 2E_P[h]-E_Q[h^2]-1\right \}.\] Suppressing the family of functions leads to lower bounds of chi-square and thus to lower bounds of the KL. For example, the HCR bound can be derived by considering first degree polynomials.