further about PCComputeExplicitOperator()

Dave May dave.mayhem23 at gmail.com
Fri May 2 18:19:00 CDT 2008

Hi Barry,
   Does PCComputeExplicitOperator() really build B*A or A*B?
The above code appears to just be applying the preconditioner to the each
column of the identity matrix, with no reference to the original operator or
the preconditioner side.

I only wanted to clarify this fact as at one stage I wrote a function to
compute B*A or A*B as I had convinced myself that
PCComputeExplicitOperator() just assembled the inverse of the


On Sat, May 3, 2008 at 2:50 AM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>  Note also:
> PCComputeExplicitOperator - Computes the explicit preconditioned operator.
> this means it computes B*A or A*B (depending on left or right
> preconditioning). This beasty
> (unless you use Jacobi preconditioning) is always dense and it makes no
> sense to store in
> sparse format except for fun.
>   Barry
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