Flexible GMRES converges in two phases
Provides a theoretical understanding of FGMRES convergence behavior for numerical linear algebra practitioners.
Derived a sharp upper bound on FGMRES residuals, showing two-phase convergence depending on inner preconditioner tolerance, with the bound proven tight via an equality example.
We derive a sharp upper bound on the residuals produced by the flexible GMRES (FGMRES) method. The bound shows that FGMRES exhibits two phases of convergence depending on the residual tolerance of the inner preconditioner. For small tolerances, the convergence of FGMRES is practically geometric with a constant rate throughout, while for looser tolerances the two-phase behavior becomes more pronounced. We also show that the derived bound cannot be improved and construct an example for which it becomes an equality.