Null-Space Preconditioners for Saddle Point Systems

The null-space method is a technique that has been used for many years to reduce a saddle point system to a smaller, easier to solve, symmetric positive definite system. This method can be understood as a block factorization of the system. Here we explore the use of preconditioners based on incomplete versions of a particular null-space factorization and compare their performance with the equivalent Schur complement based preconditioners. We also describe how to apply the nonsymmetric preconditioners proposed using the conjugate gradient method (CG) with a nonstandard inner product. This requires an exact solve with the (1,1) block, and the resulting algorithm is applicable in other cases where Bramble--Pasciak CG is used. We verify the efficiency of the newly proposed preconditioners on a number of test cases from a range of applications.

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