[petsc-users] Use of PCApplyBAorAB with PC_SYMMETRIC side

Jed Brown jed at jedbrown.org
Mon Feb 17 09:03:10 CST 2014

Sylvain Mercier <sylvainmercier85 at gmail.com> writes:

> Hello everyone,
> I want to use PCApplyBAorAB symmetrically with a preconditioner from an ICC
> factorization.
> However, I don't understand why the result is not the same if I use a
> natural or a RCM ordering during the factorization.

Uh, ICC is not the same in RCM versus natural ordering.  Incomplete
factorization is _highly_ order-dependent.

> This problem does not occur if I use this routine with PC_LEFT or PC_RIGHT

What do you mean "does not occur"?  You should expect that changing
factorization ordering will always change the preconditioner, though
some cases could cancel.
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