[petsc-users] How does KSPSetNullSpace() work?

[Jed Brown]

On Mon, Jan 9, 2012 at 22:47, Mohamad M. Nasr-Azadani <mmnasr at gmail.com>wrote:

> It might be a naive question, but I am wondering how KSPSetNullSpace()
> works when it is passed to a linear system ksp context?
> Say, for instance, we have the simple case of Poisson equation solved in a
> square domain and Neumann boundary condition applied to all boundaries.
> Does it take the integral of the solution and set it to zero as an extra
> constraint?
>

It just projects out whatever you provide as a null space, so the Krylov
method effectively runs in the remaining subspace.
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