  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 $\alpha=z$.
