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

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 ${\mathcal{E}}_{\mathrm{\Lambda },\mathrm{\Omega }}$. A sharp submap remains sharp if and only if ${\mathcal{E}}_{\mathrm{\Lambda },\mathrm{\Omega }}$ 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.