A Transformation Approach that Makes SPAI, PSAI and RSAI Procedures Efficient for Large Double Irregular Nonsymmetric Sparse Linear Systems

A sparse matrix is called double irregular sparse if it has at least one relatively dense column and row, and it is double regular sparse if all the columns and rows of it are sparse. The sparse approximate inverse preconditioning procedures SPAI, PSAI($tol$) and RSAI($tol$) are costly and even impractical to construct preconditioners for a large sparse nonsymmetric linear system with the coefficient matrix being double irregular sparse, but they are efficient for double regular sparse problems. Double irregular sparse linear systems have a wide range of applications, and 4.4\% of the nonsymmetric matrices in the Florida University collection are double irregular sparse. For this class of problems, we propose a transformation approach, which consists of four steps: (i) transform a given double irregular sparse problem into a small number of double regular sparse ones with the same coefficient matrix $\hat{A}$, (ii) use SPAI, PSAI($tol$) and RSAI($tol$) to construct sparse approximate inverses $M$ of $\hat{A}$, (iii) solve the preconditioned double regular sparse linear systems by Krylov solvers, and (iv) recover an approximate solution of the original problem with a prescribed accuracy from those of the double regular sparse ones. A number of theoretical and practical issues are considered on the transformation approach. Numerical experiments on a number of real-world problems confirm the very sharp superiority of the transformation approach to the standard approach that preconditions the original double irregular sparse problem by SPAI, PSAI($tol$) or RSAI($tol$) and solves the resulting preconditioned system by Krylov solvers.Comment: 20 pages, 4 figure

A Residual Based Sparse Approximate Inverse Preconditioning Procedure for Large Sparse Linear Systems

The SPAI algorithm, a sparse approximate inverse preconditioning technique for large sparse linear systems, proposed by Grote and Huckle [SIAM J. Sci. Comput., 18 (1997), pp.~838--853.], is based on the F-norm minimization and computes a sparse approximate inverse $M$ of a large sparse matrix $A$ adaptively. However, SPAI may be costly to seek the most profitable indices at each loop and $M$ may be ineffective for preconditioning. In this paper, we propose a residual based sparse approximate inverse preconditioning procedure (RSAI), which, unlike SPAI, is based on only the {\em dominant} rather than all information on the current residual and augments sparsity patterns adaptively during the loops. RSAI is less costly to seek indices and is more effective to capture a good approximate sparsity pattern of $A^{-1}$ than SPAI. To control the sparsity of $M$ and reduce computational cost, we develop a practical RSAI($tol$) algorithm that drops small nonzero entries adaptively during the process. Numerical experiments are reported to demonstrate that RSAI($tol$) is at least competitive with SPAI and can be considerably more efficient and effective than SPAI. They also indicate that RSAI($tol$) is comparable to the PSAI($tol$) algorithm proposed by one of the authors in 2009.Comment: 18 pages, 1 figur