Hi Wolfgang,
 

> I would start by asking where the nonzero entries are going to lie. What 
> kind of operator do you plan on implementing for which you don't know 
> where the nonzeros are? 
>

I want to multiply this matrix with a PETSc vector (which is also stored in 
parallel).
When I initialize the PETSc sparsity matrix, I don't know where the 
nonzeros are.
Then I loop over the locally owned cells on each processor, and guadually 
know for each row of the matrix the locations and values of the nonzeros. 
Do you mean that I can't directly use the PETSc_sparsity_matrix.add() 
function to insert nonzero values into the matrix without telling the 
sparsity pattern the nonzeros locations?

*If I have to tell the sparsity pattern where the nonzeros are manually, 
what are the necessary procedures and functions I should follow and use 
(since the matrix is not a system matrix, I can't use the 
DoFTools::make_sparsity_pattern function with an dofhandler as input)?*

*For example, for row 0, how can I tell the sparsity pattern the elements 
at (0,3) and (0,5) arenonzeros? I search the documentation, and find maybe 
I should use dynamic_spasity_pattern? I am so confused.*

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