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Improves the bound estimation

Nicolas Tancogne-Dejean requested to merge eigensolver/fix_bounds_finding into main

Description

Improves the bound estimation

In particular this commit does the following:

  • fixes a bug related to calling lalg_eigensolve after matrix_norm2. The tridiagonal matrix was destroyed and this was leading to a wrong lower bound. Fixing this seems to help a bit with the convergence.

  • The calculation of the lower bound was a upper limit and not a lower limit. This is fixed by removing the norm of the residual.

  • Tighter bounds are now obtained using the formula (2.6) in Y. Zhou, R.-C. Li / Linear Algebra and its Applications 435 (2011) 480–493

News snippet

Minor bugfix for the Chebyshev filtering.

Checklist

  • I have checked that my code follows the Octopus coding standards
  • I have added tests for all the new features added in this request.

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