negative indices

Matthew Knepley knepley at
Thu Aug 14 10:58:45 CDT 2008

On Thu, Aug 14, 2008 at 9:56 AM, Cristian Tibirna
<ctibirna at> wrote:
> On Tuesday, 12 August 2008, Matthew Knepley wrote:
>> If you ignore entries for the rows and columns associated with BCs, but do
>> not
>> eliminate them from the ordering, do you remember to put something on the
>> diagonal of the Jacobian?
> Thank you for the heads up. Indeed my initial intention was to eliminate the
> lines completely from the matrix structure (i.e. I preallocate zero non-zero
> entries for these lines), but after checking my code more attentively, I
> found that was failing to do it correctly because of a subtle error.
> Unfortunately, even after fixing this error and thus eliminating the lines
> completely, I can't manage to do this Dirichlet elimination work. This time,
> the LU preconditioner complains that the matrix has an empty row (which I
> thought should be permitted).
> I examined the aij.c code a bit and could tell (to my humble understanding)
> that using matrices with completely empty rows should work for iterative
> solvers -- at least the MatMult method does seem to ignore empty lines
> completely -- but I didn't do actual tests yet.
> Shouldn't it also be possible to eliminate lines completely even when using
> direct solving?

I am not sure I understand. You would like to have a row in the matrix which has
only zeros (items not filled in are implicitly)? This would mean a
singular matrix,
and thus LU fails. When I do this, I eliminate these rows from the
matrix and rhs.


> --
> Cristian Tibirna                                (1-418-) 656-2131 / 4340
>  Laval University - Quebec, CAN ...
>  Research professional at GIREF ... ctibirna at

What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which
their experiments lead.
-- Norbert Wiener

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