Chebyshev-Exact Acceleration under Hessian Variation, II: Computer-Assisted Variational Lower Bound

We study two-step recurrences with positive step sizes and nonnegative momentum coefficients whose terminal residual is the Chebyshev minimax polynomial on a prescribed spectral interval $[μ,L]$. These recurrences have the same terminal error on a fixed quadratic, but their first-order responses to