[petsc-users] Basic question about local submatrix from DMComposite

[zakaryah]

I'm still working on implementing a homotopy solver for systems of
nonlinear equations.  It seems that everything is working in serial, but
I'm having at least one issue in parallel.

To form the (n+1) x (n+1) Jacobian, I use MatGetLocalSubMatrix, with the
ISs from the DMComposite.  One of the submatrices is (globally) 1 x n.  I
need to set one of its values, at global column j, to a particular value.
My current attempt to do this involves getting the local to global
mappings, with DMCompositeGetISLocalToGlobalMappings, then using
ISGlobalToLocalMappingsApply with global index j.  If the processor has a
locally mapped index, I call MatSetValuesLocal at the local index.

So far, this works, in that it seems the value is set at the appropriate
point.  My issue is that I must use ADD_VALUES, due to the nature of the
algorithm for the other submatrices.  Therefore, I need a way to ensure
that the correct value is set.  For example, if two processors map to the
same global index, then I need a way to check this and use local values
such that the sum is the correct global value.

I suspect that this has a simple solution - hopefully someone has some
hints.  Thanks!
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