Robust additive block triangular preconditioners for block two-by-two linear systems.

In this paper, a class of additive block triangular preconditioners are constructed for solving block two-by-two linear systems with symmetric positive (semi-)definite sub-matrices. Convergence analysis of the related splitting iteration method shows that it is almost unconditionally convergent and behaves problem independent with a convergence rate less than 0.5 under a practical parameter choice. Optimization of the preconditioned matrices, which have real and tight eigenvalue distributions, shows that it can result in an upper bound less than 2 for the condition number of the preconditioned matrices. Moreover, we also give a special consideration about the feasibility of the proposed preconditioner for solving more general problems with indefinite sub-matrices. Numerical experiments based on examples arising from complex symmetric linear systems and PDE-constrained optimization problems are presented to show the robustness and effectiveness of the proposed preconditioners compared with some other existing preconditioners. [ABSTRACT FROM AUTHOR]