# Saving Flops in LU Based Shift-and-Invert Strategy

1 GRAND-LARGE - Global parallel and distributed computing
LRI - Laboratoire de Recherche en Informatique, LIFL - Laboratoire d'Informatique Fondamentale de Lille, UP11 - Université Paris-Sud - Paris 11, Inria Saclay - Ile de France, CNRS - Centre National de la Recherche Scientifique : UMR8623
Abstract : The shift-and-invert method is very efficient in eigenvalue computations, in particular when interior eigenvalues are sought. This method involves solving linear systems of the form $(A-\sigma I)z=b$. The shift $\sigma$ is variable, hence when a direct method is used to solve the linear system, the LU factorization of $(A-\sigma I)$ needs to be computed for every shift change. We present two strategies that reduce the number of floating point operations performed in the LU factorization when the shift changes. Both methods perform first a preprocessing step that aims at eliminating parts of the matrix that are not affected by the diagonal change. This leads to $43\%$ and $50\%$ flops savings respectively.
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Reports

Cited literature [23 references]

https://hal.inria.fr/inria-00286417
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Submitted on : Tuesday, June 10, 2008 - 9:52:06 AM
Last modification on : Sunday, June 26, 2022 - 11:48:18 AM
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• HAL Id : inria-00286417, version 2

### Citation

Laura Grigori, Desire W. Nuentsa, Hua Xiang. Saving Flops in LU Based Shift-and-Invert Strategy. [Research Report] RR-6553, INRIA. 2008, pp.15. ⟨inria-00286417v2⟩

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