Lower semicontinuity conjecture for convex integral functionals of matricial measures

Let H+,m\mathfrak{H}_{+,m} denote the cone of positive semidefinite Hermitian m×mm\times m matrices, let Td\mathbb T^d be the dd-dimensional torus, and let dM\mathrm{d}M be a nonnegative matricial measure on Td\mathbb T^d. Write its Lebesgue decomposition as dM=Φ(θ)dμ(θ)+dMs\mathrm{d}M=\Phi(\boldsymbol{\theta})\,\mathrm{d}\mu(\boldsymbol{\theta})+\mathrm{d}M_{\mathrm{s}}, where Φ\Phi is the absolutely continuous part and μ\mu is Lebesgue measure. Let f:H+,m×TdRf:\mathfrak{H}_{+,m}\times\mathbb T^d\to\mathbb R be such that f(,θ)f(\,\cdot\,,\boldsymbol{\theta}) is convex in its first argument for every fixed θTd\boldsymbol{\theta}\in\mathbb T^d. Lower semicontinuity conjecture. The functional

g:dMTdf(Φ(θ),θ)dμ(θ)g:\mathrm{d}M\longmapsto\int_{\mathbb T^d}f(\Phi(\boldsymbol{\theta}),\boldsymbol{\theta})\,\mathrm{d}\mu(\boldsymbol{\theta})

is lower semicontinuous with respect to the weak* topology. The conjecture is intended to provide a mathematical basis for the chosen divergence index and to ensure existence of a solution to the associated primal problem when the prior is arbitrary; the source discussion does not provide a proof, and its resolution remains open.

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Primary source

Bin Zhu, Augusto Ferrante, Johan Karlsson and Mattia Zorzi, “M^2-Spectral Estimation: A Flexible Approach Ensuring Rational Solutions”, arXiv:2004.14778 (2021).

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