On Thu, 10 Aug 2017, Renato Poli wrote:

Now I am trying to solve the Poisson equation (as if there were no
storage and no compressibility in the media) and to plot the results
in my GUI.

As I understood from your notes, something like that is going to work - right?
   | foreach e=element in mesh
   |    foreach n=node in e
   |         dof = n.dof_number(1,0,0)

Assuming that you're trying to get dof indices for the *first*
variable in the *second* System in your EquationSystems, yes.  If
you've still just got one System then you want dof_number(0,0,0).

   |         node_pressure = system.solution(dof)

In serial, close - solution is a UniquePtr so you need
(*system.solution)(dof) to use it.  In parallel,
(*system.solution)(dof) is undefined behavior (or an assertion falure
in dbg/devel modes) unless that node is owned by the processor calling
it.  Using system.current_solution(dof) gets you (the last cached copy
of) ghosted node DoF values too.  If you want remote DoF values in
parallel you have to sync or serialize them yourself.

How to use the shape function to interpolate inside the element?

There's about 5 different ways to do that, I'm afraid, and tradeoffs
for flexibility vs efficiency vs brevity.  I build an FEMContext in
most cases, but if you're not writing speed-critical code then
System::point_value(variable_number, point, element) is the easiest
solution.

If you allow me (sorry to bother you this much):

No problem, but the price is that we keep discussion on-list, because:

I didn't understand completely the parallelization strategy of libmesh.
Do you have any readings to suggest?

No great ones.  I'm afraid that just reading through some of the
example codes as a tutorial and then searching through
https://sourceforge.net/p/libmesh/mailman/search/?mail_list=libmesh-users
with questions is how most of our users get up to speed.
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Roy
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