some corrections:
> I want to implement the two-level Schwarz method. Let us denote P as the
> one-level
> Schwarz projection and P_0 as the coarse projection. If I want the hybrid
> form of
> preconditioned system as follows:
>
> (I-P_0) P (I-P_0)
this should be
I - (I-P_0) (I-P) (I-P_0)
in general case. Only when P_0 is exact solve, we have the precedent form.
> which is suitable: PCMG or PCComposite? It seems to me that PCComposite is
> good.
> But PCMG is not suitable because it can only implement the following form:
>
> (I-P) P_0 (I-P)
this should be
I - (I-P) (I-P_0) (I-P)
since P is not a projection.
