Fwd: matrices without diagonals...

[Matthew Knepley]

I may be wrong, but I think a lot of AIJ methods need the diagonal. Do we think
it is feasible to relax this? I would be more for filling the diagonal
with zeros
automatically.

   Matt

---------- Forwarded message ----------
From: Todd Munson <tmunson at mcs.anl.gov>
Date: Oct 30, 2007 9:18 AM
Subject: matrices without diagonals...
To: Barry Smith <bsmith at mcs.anl.gov>
Cc: Matthew Knepley <knepley at mcs.anl.gov>



Barry,

We discussed this yesterday and I want to come to a conclusion before I go
and change any code.  The problem is that some of the matrix I have do not
have diagonal entries...in fact both the rows and columns do not have any
nonzeros.  Rather than PETSc terminating gracefully and telling me the
problem, I get a segmentation violation and a core dump.

There are two questions:

   1) Which functions require a diagonal?
   2) Should these functions check for the diagonal and abort with a useful
      error message?

My feeling is that the diagonal is required only in a few cases and it may
not even be necessary, but I've been known to be wrong before.  Most of
the code should work without requiring diagonal entries, which is why it
should only check for them when necessary.

Todd.



-- 
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