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arXiv · 2609.09915

Sharpness Loss for Radon--Nikodym Derivatives of Completely Positive Maps

Mohsen Kian
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Abstract

The Radon--Nikodym theorem for completely positive maps identifies the order interval below a map with positive contractions in the commutant of its minimal Stinespring representation. In this paper, we study the behavior of projection-valued Radon--Nikodym derivatives (sharp submaps) under composition. We prove that sharpness loss is determined by the multiplicative defect of the induced pullback map. In the finite-dimensional case, we show that this defect is determined by the orthogonal complement of the composite Kraus relation space, denoted $\mathcal{E}_{Λ,Ω}$. A sharp submap remains sharp if and only if $\mathcal{E}_{Λ,Ω}$ reduces the lifted Radon--Nikodym projection. Furthermore, we establish a tensor-factor criterion for the global preservation of sharpness and explicitly quantify the sharpness-loss defect.

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Mohsen Kian. 2026-09-09. Sharpness Loss for Radon--Nikodym Derivatives of Completely Positive Maps. https://arxiv.org/abs/2609.09915

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