Saturating the Data Processing Inequality for $α-z$ Renyi Relative Entropy

Abstract: It has been shown that the $\alpha-z$ R{\'e}nyi relative entropy satisfies the Data Processing Inequality (DPI) for a certain range of $\alpha$'s and $z$'s. Moreover, the range is completely characterized by Zhang in `20. We prove necessary and algebraically sufficient conditions to saturate the DPI for the $\alpha-z$ R{\'e}nyi relative entropy whenever $1<\alpha\leq 2$ and $\frac{\alpha}{2}\leq z\leq\alpha$. Moreover, these conditions coincide whenever $\alpha=z$.