[petsc-users] solving singular system

[Benjamin Sanderse]

Hi guys,

I am trying to solve a singular matrix that results from the discretization of a Poisson equation with Neumann boundary conditions. In this case the null space consists of a constant vector.
According to the manual MatNullSpaceCreate should be used to construct the null space. Since the constant functions are not needed when providing basis vectors, I am wondering what I should put as basis vectors? My code is now:

PetscInt zero=0

MatNullSpaceCreate(PETSC_COMM_WORLD,PETSC_TRUE,zero,???,&nsp);
KSPSetNullSpace(ksp,nsp);

If anybody knows what I should put at the question marks, that would be of great help. Thanks!

Ben