On Thu, Jun 5, 2025 at 2:27 AM Frank Bramkamp <bramk...@nsc.liu.se> wrote:
> Thanks for the response,
>
>
> I refer to the routine src/mat/impls/baij/seq/dgefa6.c
>
>
> /*
> Inverts 6 by 6 matrix using gaussian elimination with partial
> pivoting.
>
> Used by the sparse factorization routines in
> src/mat/impls/baij/seq
>
> This is a combination of the Linpack routines
> dgefa() and dgedi() specialized for a size of 6.
>
> */
>
>
> which implements the the computation of an inverse for a dense 6x6 block.
> I think this routine is used in the ILU numeric factorization to compute
> the inverse diagonal block
> when we later apply ILU preconditioning where we need the inverse.
>
> And there it looks we use a direct LU solve and not triangular solves ?!
>
> In this context I am not sure if something is missing at the end or not
> regarding the inverse pivoting ?!
>
We do not think so, but there is a simple test for this. You can construct
a small block diagonal matrix with
blocksize 6, and apply it to the vector [1, 2, 3, 4, 5, 6 ...]. Use that
vector as the RHS for a solve with
preonly/ILU. That should get the exact inverse and output the original
vector. If the vector is permuted, then
we indeed have a bug.
Thanks!
Matt
> Greetings, Frank
>
>
>
>
>
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