ffc team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Fri, Jan 29, 2010 at 10:10:37AM +0000, Garth N. Wells wrote:

> > Sub domains seem to be very different, but the other two cases just
> > seem to be a matter of some dofs being "active" and the other zeroed
> > out. This is what Marie suggested yesterday, that a restricted element
> > only considers a subset of the dofs of some given element.
> >
>
> Sounds good.
>
> > The thing I don't understand yet is the selection of which dofs should
> > be active. If we think of the case with restriction to facets, then
> > the element needs to be restricted to different facets depending on
> > which facet we are integrating over, or are we always mapping one
> > specific facet of the reference cell to the current facet?
> >
> It works the same way as the DG elements, just the internal dofs are
> thrown away, which is the latter if the above, right?
>
> > Say we have P1 elements in 2D which have 3 dofs. Then we could
> > restrict that element to the dofs on the first facet (facet 0). These
> > dofs are then labeled 1 and 2. But sometimes a facet in the mesh will
> > correspond to the edge between 0 and 1 or 0 and 2.
> >
>
> We don't restrict to individual facets, but to all facts of a cell.

That makes sense, but one thing still confuses me. Say that we have a
P1 element and restrict it to facets. Then all dofs are on the facets
so the result of the restriction is just a new P1 element. Same for P2
where the result again is a new P2 element. For P3, the result is P3
element minus just one dof. So does this make much difference for
other than very high degree elements?

--
Anders

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