Many thanks, Wolfgang. Yes, I need to solve for the basis in each coarse cell separately, i.e., I have as many local fine meshes as coarse cells.

I think that's actually an important point you are making here, maybe inadvertently: You have as many *meshes* as there are coarse cells.

Now, just refining the global mesh a number of more times gives you another global mesh, but it does not give you a mesh for each coarse cell.

In my deal.II classes, I've had students solve problems like yours and they typically just build one new triangulation for each coarse mesh cell, then attach a DoFHandler to it, and solve problems on them. That is likely not very efficient if you want to solve real problems, but it's good enough for small testcases and it allows for easy parallelization using threads.

On the other hand, if you were to build a global DoFHandler on the refined mesh, you'd have to define what to do with those DoFs that lie outside the patch (that results from a single coarse mesh) you are currently considering. You'd likely want to set these to zero, but they're still there, and the resulting patch problem will be large. It's also much more difficult to do things in parallel because now you have one global (fine) mesh that you want to use for multiple things at the same time.



Do you think it would be possible to (ab)use the multigrid classes for that? As far as I understand there are dofhandlers on each level of the mesh. Can I get dofhandlers for all active subcells of say coarse cell number n in level 5?

No. The multigrid DoFHandler functionality considers each level as a whole. You can't pick and choose.

Best
 W.

--
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Wolfgang Bangerth          email:                 bange...@colostate.edu
                           www: http://www.math.colostate.edu/~bangerth/

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