An Entropy-Energy Identity for Predictive Kullback-Leibler Regret in Infinitely Divisible Location Models

We consider predictive density estimation under logarithmic score for $d$-dimensional infinitely divisible location models. Taking the formal Bayes predictive density under the Lebesgue prior as a benchmark, we study the Kullback-Leibler regret of competing Bayes predictive densities. Our main contribution is an exact entropy-energy identity: the integrated regret of a Bayes predictive density ${\widehat{p}}^{\pi }$ under prior $\pi$ relative to the benchmark admits an exact representation as the Dirichlet-form energy of the square-rooted marginal distribution $\sqrt{M^{\pi }}$ for the symmetric Markov semigroup induced by the benchmark kernel. This converts regret comparisons into a potential-theoretic problem and yields a sharp recurrence/transience characterization of when the benchmark predictive density can or cannot be uniformly improved. We introduce an $\mathcal{A}$-harmonic class of improper priors – defined through the generator $\mathcal{A}$ of the induced process – and give explicit tail conditions – an integral test on the induced marginal, equivalent to power-law prior decay in heavy-tailed models – that guarantee admissibility of the resulting Bayes predictive density. We illustrate the theory with new results for several distributions.