The $L^p$-Fisher-Rao metric and Amari-Cencov $\alpha$-connections

TL;DR

This paper introduces a family of $L^p$-Fisher-Rao metrics that generalize the classical Fisher-Rao metric, explores their relation to Amari-Cencov $ abla^{(eta)}$-connections, and provides new insights into the geometry of probability densities and their geodesics.

Contribution

It defines $L^p$-Fisher-Rao metrics, links them to Amari-Cencov connections, and solves geodesic equations, revealing new geometric structures in information geometry.

Findings

$L^p$-Fisher-Rao metrics generalize classical Fisher-Rao.

Geodesic equations of $F_p$ and $ abla^{(eta)}$ coincide for specific $p$ and $eta$.

Solutions to geodesic equations cease to exist after finite time when leaving the positive sphere.

Abstract

We introduce a family of Finsler metrics, called the $L^p$-Fisher-Rao metrics $F_p$, for $p\in (1,\infty)$, which generalizes the classical Fisher-Rao metric $F_2$, both on the space of densities Dens$_+(M)$ and probability densities Prob$(M)$. We then study their relations to the Amari-\u{C}encov $\alpha$-connections $\nabla^{(\alpha)}$ from information geometry: on Dens$_+(M)$, the geodesic equations of $F_p$ and $\nabla^{(\alpha)}$ coincide, for $p = 2/(1-\alpha)$. Both are pullbacks of canonical constructions on $L^p(M)$, in which geodesics are simply straight lines. In particular, this gives a new variational interpretation of $\alpha$-geodesics as being energy minimizing curves. On Prob$(M)$, the $F_p$ and $\nabla^{(\alpha)}$ geodesics can still be thought as pullbacks of natural operations on the unit sphere in $L^p(M)$, but in this case they no longer coincide unless $p=2$. Using this transformation, we solve the geodesic equation of the $\alpha$-connection by showing that the geodesic are pullbacks of projections of straight lines onto the unit sphere, and they always cease to exists after finite time when they leave the positive part of the sphere. This unveils the geometric structure of solutions to the generalized Proudman-Johnson equations, and generalizes them to higher dimensions. In addition, we calculate the associate tensors of $F_p$, and study their relation to $\nabla^{(\alpha)}$.