negative indices

Cristian Tibirna ctibirna at
Thu Aug 14 09:56:19 CDT 2008

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?

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

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