[petsc-users] Handling singular matrix in KSP and ASM

[Amneet Bhalla]

Hi Folks,

We are dealing with a singular matrix that arises from Stokes' equation
with Dirichlet velocity BCs. The way we are handling it is to pass
nullspace info to KSP, i.e, vector of {u,p} = {0,1}, and letting the matrix
to be singular. However, we are using ASM as the preconditioner and when
the subpc covers the entire domain (which can be the case in the
preconditioning step of our MG algorithm), LU decomposition fails. We are
thinking of pinning one of the pressure DOFs to zero explicitly, so that we
can avoid singular matrix for the LU decomposition. Is there another method
of doing it, which avoids explicit pinning down of an arbitrary pressure
DOF?

Thanks,
--Amneet

-- 
--Amneet
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