Three-Point Trace Positivity:Schatten Norm Compression and Interpolating Metrics

Let $(\mathcal N,ν)$ be a finite tracial von Neumann algebra and set $\mathcal{K}_r(P,Q)=ν((P^{1/2}QP^{1/2})^r)$. We prove that $$ [\mathcal{K}_r(P_i,P_j)]_{i,j=1}^3\ge0, \qquad 1/2\le r\le1. $$ The range $1/2<r<1$ is new even for matrices, and order three is sharp for $1/2\le r<1$. Th