Fumio Hiai, Anna Jenčová (Apr 12 2024).
Abstract: We study the $\alpha$-$z$-Rényi divergences $D_{\alpha,z}(\psi|\varphi)$ where $\alpha,z>0$ ($\alpha\ne1$) for normal positive functionals $\psi,\varphi$ on general von Neumann algebras, introduced in [S.~Kato and Y.~Ueda, arXiv:2307.01790] and [S.~Kato, arXiv:2311.01748]. We prove the variational expressions and the data processing inequality (DPI) for the $\alpha$-$z$-Rényi divergences. We establish the sufficiency theorem for $D_{\alpha,z}(\psi|\varphi)$, saying that for $(\alpha,z)$ inside the DPI bounds, the equality $D_{\alpha,z}(\psi\circ\gamma|\varphi\circ\gamma)=D_{\alpha,z}(\psi|\varphi)<\infty$ in the DPI under a quantum channel (or a normal $2$-positive unital map) $\gamma$ implies the reversibility of $\gamma$ with respect to $\psi,\varphi$. Moreover, we show the monotonicity properties of $D_{\alpha,z}(\psi|\varphi)$ in the parameters $\alpha,z$ and their limits to the normalized relative entropy as $\alpha\nearrow1$ and $\alpha\searrow1$.