[petsc-users] Nullspace for a coupled system of equations

[Thomas Witkowski]

I want to solve some (weakly) coupled system of equations of the 
following form:

A  B     u
         .     =   .....
0  C     v


so, C is the discrete Laplacian and A and B are some more complicated 
operators (I make use of linear finite elements). All boundary 
conditions are periodic, so the unknown v is determined only up to a 
constant. A and B contain both the identity operator, so u is fixed. Now 
I want to solve the system on the whole (there are reasons to do it in 
this way!) and I must provide information about the nullspace to the 
solver. When I am right, to provide the correct nullspace I must solve 
one equation with A. Is there any way in PETSc to circumvent the problem?

Thomas