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ralna / spral

Sparse Parallel Robust Algorithms Library
https://ralna.github.io/spral/
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num_neg #147

Closed bharswami closed 11 months ago

bharswami commented 11 months ago

Can you please check if the number of negative pivots (NUM_NEG) is being computed correctly?

jfowkes commented 11 months ago

Could you be more specific please? To which code/test are you referring to?

bharswami commented 11 months ago

In fortran code ssids.f90 subroutine: spral_ssids_factor_ptr32 The return variable cinform%NUM_NEG, after deeper function calls, stores the number of negative pivots. Just wanted confirmation that it is being computed correctly.

jfowkes commented 11 months ago

As far as we are aware yes.

bharswami commented 11 months ago

What does the number of negative pivots actually refer to? Mathematically?

jfowkes commented 11 months ago

As the documentation states (see https://www.numerical.rl.ac.uk/spral/doc/latest/C/ssids.html):

int num_neg Number of negative eigenvalues of the matrix D after factorize phase.

i.e. the number of negative eigenvalues of D in the LDLT factorisation.