[petsc-users] Preconditioning of laplace equation with boomeramg

[Thomas Witkowski]

I'm about to write a preconditioner for a 2x2 block matrix coming from  
the FEM discretization (2D, equidistance mesh) of a 4th order PDE. The  
(0,0) block is the discrete laplace with Neumann boundary conditions.  
I want to precondition this block by using some amg iterations. Just  
for tests, the preconditioner should solve in each step the neumann  
problem with some rhs using a gmres solver with hypre/boomeramg  
preconditioner up to a given tolerance. But in most iterations it ends  
up with DIVERGED_DTOL. What could be the reason for? Is AMG is linear  
preconditioner? The constant null space of the matrix is set via  
KSPSetNullSpace.

Thomas