[petsc-users] [MEF-QUAR] Re: Any changes in ML usage between 3.1-p8 -> 3.3-p6?

[Mark F. Adams]

> 
> 2) If you get "Indefinite PC" (I am guessing from using CG) it is because the preconditioner
>     really is indefinite (or possible non-symmetric). We improved the checking for this in one
>     of those releases.
> 
> AMG does not guarantee an SPD preconditioner so why persist in trying to use CG?
> 

AMG is positive if everything is working correctly.

Are these problems only semidefinite?  Singular systems can give erratic behavior.