Heavy-ball rate for a two-eigenvalue diagonal quadratic
= Heavy-ball rate for a two-eigenvalue diagonal quadratic
For $A=\operatorname{diag}(1,\gamma)$, $\alpha=1/\gamma$, and $\beta=(1-\gamma^{-1/2})^2$, every error-propagation block has spectral radius at most $1-\gamma^{-1/2}$, and equality occurs.