Hi Abbas, thanks for the reply. I did debug with option A, but no error 
reported/no exception is thrown. To me, option A is the same as option B. 
However, the two generated system matrixs in the code are not the same. 

On Thursday, September 28, 2023 at 10:27:17 AM UTC-7 abbas.b...@gmail.com 
wrote:

> Did you try to run in Debug? If not give it a try and see if an exception 
> is thrown. 
>
> Abbas
>
> On Thursday, September 28, 2023 at 3:15:14 AM UTC+2 hitl...@gmail.com 
> wrote:
>
>> Hello all, 
>>
>>
>> Previously, I initialized my system matrix in this way (option A)
>>
>> DoFTools::make_flux_sparsity_pattern(dof_handler,
>>                                     dsp,
>>                                     cell_coupling,
>>                                     face_coupling);
>> constraints_newton_update.condense(dsp);
>> SparsityTools::distribute_sparsity_pattern(dsp,
>>                                    hp_index_set,
>>                                    mpi_communicator,
>>                                    hp_relevant_set);
>> system_matrix.reinit(hp_index_set,
>>                      hp_index_set,
>>                      dsp,
>>                      mpi_communicator);
>>
>> However, the generated system matrix is singular. I was stuck by the 
>> singular matrix problem for about 2 weeks.
>>
>>
>> Then, I initialized the matrix with option B, which generates a full rank 
>> matrix. Although, I don't see the difference between these 2 ways.  Can 
>> someone explain this to me? Thans in advance.
>>
>> DoFTools::make_flux_sparsity_pattern(dof_handler,
>>                                      dsp,
>>                                      cell_coupling,
>>                                      face_coupling);
>> constraints_newton_update.condense(dsp);
>> sparsity_pattern.copy_from(dsp);
>> system_matrix.reinit(hp_index_set,
>>                      sparsity_pattern,
>>                      mpi_communicator);
>>
>

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