University of Technology, The Netherlands. On the Convergence of GMRES with

We consider the solution of large and sparse linear systems of equations by GM-RES. Due to the appearance of unfavorable eigenvalues in the spectrum of the coeffi-cient matrix, the convergence of GMRES may hamper. To overcome this, a deflated variant of GMRES can be used, which treats those unfavorable eigenvalues effec-tively. In the literature, several deflated GMRES variants are applied successfully to various problems, while a theoretical justification is often lacking. In contrast to deflated CG, the convergence of deflated GMRES seems to be harder to analyze and to understand. This paper presents some new theoretical insights into deflated GMRES based on A-invariant deflation subspaces. Fundamental results regarding the convergence of deflated GMRES are proved in order to show the effectiveness and robustness of this method. Numerical experiments are provided to illustrate the theoretical results and to show some further properties of deflated GMRES. Consequently, practical variants of deflated GMRES from the literature can be better understood based o