ffc team mailing list archive

[Martin Sandve Alnæs <martinal@xxxxxxxxx>]

On Tue, Jul 7, 2009 at 5:52 PM, Kristian
Oelgaard<k.b.oelgaard@xxxxxxxxxx> wrote:
> Quoting Martin Sandve Alnæs <martinal@xxxxxxxxx>:
>
>> On Wed, Jun 17, 2009 at 5:15 PM, Anders Logg<logg@xxxxxxxxx> wrote:
>> > On Wed, Jun 17, 2009 at 03:32:03PM +0200, UFL wrote:
>> >> One or more new changesets pushed to the primary ufl repository.
>> >> A short summary of the last three changesets is included below.
>> >>
>> >> changeset:   908:ef256e35e417096ae9b9d3a7e915e27e6971ae3f
>> >> tag:         tip
>> >> user:        "Martin Sandve Alnæs <martinal@xxxxxxxxx>"
>> >> date:        Wed Jun 17 15:32:01 2009 +0200
>> >> files:       test/newtests/test_demos/test_demos.py
>> test/newtests/test_elements/test_elements.py
>> test/newtests/test_exprbasics/test_construction.py ufl/__init__.py
>> ufl/exproperators.py ufl/finiteelement.py ufl/function.py
>> >> description:
>> >> Added ElementRestriction class with notation "Vr = V[dx(k)]".
>> >
>> > What does this mean? What is V?
>> >
>> > --
>> > Anders
>>
>> The restriction of a function space to a domain.
>> Obviously the use of the measure "dx" is an
>> abuse of concepts, but it's convenient to reuse it.
>>
>> Consider:
>>
>> V1 = FiniteElement("CG", cell, 1)
>> V0 = FiniteElement("DG", cell, 0)
>> V = V1 + V0[ds(2)]
>> u, lamda = TrialFunctions(V)
>>
>> Here lamda lives only on boundary 2.
>>
>> Of course, this is useless without support in the rest of
>> FEniCS, so it's only a proof of concept for input syntax.
>>
>> Feel free to suggest alternative syntax and naming.
>
> First of all, any ideas on how to implement support for this in say FFC?

No. It will probably require non-trivial extensions to
UFC and DOLFIN as well, and seems like a fairly
complicated design task to get it general enough.

> In relation to this new functionality.
> If we want to restrict an element, not to a particular subdomain, but to the
> exterior of each element, i.e., only consider the dofs that live on facets,
> would that also fit in this framework?
> Should the syntax for this be V[ds + dS]?
> But then what should the syntax be for only considering dofs that live
> internally on the cell?
>
> Or, if V is defined on 'triangle', then one could do
> V['interval'] to get the facet dofs, and V['triangle'] to get the interior dofs.
> It would even be possible to do V['vertex']. If we also allow this syntax we can
> restrict elements with respect to either Cells or Measure.
> Could this work?
>
> How can this be implemented in FFC? In the case of V['interval'], we could get
> the finite element from FIAT as usual, and then find all dofs NOT on the facet
> and dump them in a list of dofs that should be ignored when generating all the
> code.
>
> Kristian

The use of Measure to represent a domain didn't feel
right to begin with. Seems like it's not general enough
anyway, so I suggest designing a separate abstraction
for description of subdomains or subsets of dofs.

My guess is that this task will require some
design iterations to make it good.

I won't be around to see this through though,
since I'm beginning in another job in august.

Martin