[petsc-users] Questions on CPR preconditioner

[Edoardo alinovi]

Hello petsc's friends

I have a curiosity about a sentence in the user guide about CPR
preconditioner:

*"**The Constrained Pressure Preconditioner (CPR) can be implemented
using PCCOMPOSITE
<https://petsc.org/release/manualpages/PC/PCCOMPOSITE/> with PCGALERKIN
<https://petsc.org/release/manualpages/PC/PCGALERKIN/>. CPR first solves
an *
*RAP subsystem, updates the residual on all variables (PCCompositeSetType
<https://petsc.org/release/manualpages/PC/PCCompositeSetType/>(pc,PC_COMPOSITE_MULTIPLICATIVE
<https://petsc.org/release/manualpages/PC/PCCompositeType/>)), and then
applies a simple ILU like preconditioner on all the variables.**"*

I am certainly lacking some background here, would you be able to
explain me a bit better how this is working? Do you have a minimal working
example in the tutorials?

Many thanks as always! :)
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