Hey everybody, 
I have a problem with initializing a Sparsematrix in a parallel program.
 am using a parallel::distributed::triangulaton (as in the Step40 tutorial) 
on 
the Navier Stokes Projection solver from the step35 tutorial.
The problem arises in the function 

NavierStokesProjection<dim>::initialize_gradient_operator()
{
{
DynamicSparsityPattern 
<https://www.dealii.org/current/doxygen/deal.II/classDynamicSparsityPattern.html>
 
dsp(dof_handler_velocity.n_dofs(), dof_handler_pressure.n_dofs());
DoFTools::make_sparsity_pattern 
<https://www.dealii.org/current/doxygen/deal.II/group__constraints.html#ga38d88a1a559e9fc65d60f3e168921ba5>
 
(dof_handler_velocity, dof_handler_pressure, dsp);
sparsity_pattern_pres_vel.copy_from (dsp);
}

where I have to use two different FE-spaces, one for the pressure and one 
for the velocity.
To perform this in parallel I used the following code (according to the 
sparsematrices in step40)

{
DynamicSparsityPattern 
dsp(locally_relevant_dofs_vel.size(),locally_relevant_dofs_pres.size());
DoFTools::make_sparsity_pattern (dof_handler_velocity, 
dof_handler_pressure, dsp); 
sparsity_pattern_pres_vel.copy_from (dsp);
}

This code compilates fine, but when Running it with MPI, it stops in the 
"make_sparsity_pattern" line with the error code
"this->is_artificial() == false
Additional information:
Cant asks for Dofs on articial cells."

Does somebody know how I can fix this? 

My last try was to use the Indexsets instead of their size, when declaring 
dsp.
But unfortunately DynamicSparsityPattern can only take on IndexSet.


Thanks a lot

Gabriel




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