The Fisher metric as a metric on the cotangent bundle

Hiroshi Nagaoka

doi:10.1007/s41884-023-00126-9

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arXiv · 2310.13237

The Fisher metric as a metric on the cotangent bundle

Hiroshi Nagaoka

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Abstract

The Fisher metric on a manifold of probability distributions is usually treated as a metric on the tangent bundle. In this paper, we focus on the metric on the cotangent bundle induced from the Fisher metric with calling it the Fisher co-metric. We show that the Fisher co-metric can be defined directly without going through the Fisher metric by establishing a natural correspondence between cotangent vectors and random variables. This definition clarifies a close relation between the Fisher co-metric and the variance/covariance of random variables, whereby the Cram\'{e}r-Rao inequality is trivialized. We also discuss the monotonicity and the invariance of the Fisher co-metric with respect to Markov maps, and present a theorem characterizing the co-metric by the invariance, which can be regarded as a cotangent version of \v{C}encov's characterization theorem for the Fisher metric. The obtained theorem can also viewed as giving a characterization of the variance/covariance.

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Hiroshi Nagaoka. 2023-10-20. The Fisher metric as a metric on the cotangent bundle. https://doi.org/10.1007/s41884-023-00126-9

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