## 2017.12: A New Analysis of Iterative Refinement and its Application to Accurate Solution of Ill-Conditioned Sparse Linear Systems

2017.12:
Erin Carson and Nicholas J. Higham
(2017)
*A New Analysis of Iterative Refinement and its Application to
Accurate Solution of Ill-Conditioned Sparse Linear Systems.*

*This is the latest version of this eprint.*

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## Abstract

Iterative refinement is a long-standing technique for improving the accuracy of a computed solution to a nonsingular linear system $Ax = b$ obtained via LU factorization. It makes use of residuals computed in extra precision, typically at twice the working precision, and existing results guarantee convergence if the matrix $A$ has condition number safely less than the reciprocal of the unit roundoff, $u$. We identify a mechanism that allows iterative refinement to produce solutions with normwise relative error of order $u$ to systems with condition numbers of order $u^{-1}$ or larger, provided that the update equation is solved with a relative error sufficiently less than $1$. A new rounding error analysis is given and its implications are analyzed. Building on the analysis, we develop a GMRES-based iterative refinement method (GMRES-IR) that makes use of the computed LU factors as preconditioners. GMRES-IR exploits the fact that even if $A$ is extremely ill conditioned the LU factors contain enough information that preconditioning can greatly reduce the condition number of $A$. Our rounding error analysis and numerical experiments show that GMRES-IR can succeed where standard refinement fails, and that it can provide accurate solutions to systems with condition numbers of order $u^{-1}$ and greater. Indeed in our experiments with such matrices---both random and from the University of Florida Sparse Matrix Collection---GMRES-IR yields a normwise relative error of order $u$ in at most $3$ steps in every case.

Item Type: | MIMS Preprint |
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Uncontrolled Keywords: | ill-conditioned linear system, iterative refinement, multiple precision, mixed precision, rounding error analysis, backward error, forward error, GMRES, preconditioning |

Subjects: | MSC 2000 > 15 Linear and multilinear algebra; matrix theory MSC 2000 > 65 Numerical analysis |

MIMS number: | 2017.12 |

Deposited By: | Nick Higham |

Deposited On: | 26 July 2017 |

### Available Versions of this Item

- A New Analysis of Iterative Refinement and its Application to
Accurate Solution of Ill-Conditioned Sparse Linear Systems (deposited 26 July 2017)
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