How does calling FormSystemMatrix change bilinear forms for future use? Does calling Finalize() prevent further operations?

[j-signorelli]

Hi,

I am new to MFEM and have a question regarding ParBilinearForm's, in reference to #1811.

As discussed in that issue, ParBilinearForm::FormSystemMatrix(...) is not just a factory method for generating matrices with eliminated essential DOFs, but does have an effect on future usage of the bilinear form. If I wanted to generate a HypreParMatrix using a bilinear form, and then later on multiply that same bilinear form with a primal vector to get a dual vector, it does not seem possible as from the issue referenced above.

Is this because once one calls ParBilinearForm::Finalize(), as is done in ParBilinearForm::FormSystemMatrix(...), then any further operations like ParBilinearForm::Mult() are no longer allowed?

Additionally, assuming the above is true, if I call ParBilinearForm::Update() as I would need to for time-dependent coefficients in bilinear form integrators, does this "unfinalize" the bilinear form?

Thank you for any help.