[petsc-users] Null space of discrete Laplace with periodic boundary conditions

[Thomas Witkowski]

I discretize the Laplace operator (using finite element) on the unit 
square equipped with periodic boundary conditions on all four edges. Is 
it correct that the null space is still constant? I wounder, because 
when I run the same code on a sphere (so a 2D surface embedded in 3D), 
the resulting matrix is non-singular. I thought, that both cases should 
be somehow equal with respect to the null space?

Thomas