ufl team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Mon, May 05, 2008 at 10:45:25PM +0200, Martin Sandve Alnæs wrote:
> 2008/5/5 Anders Logg <logg@xxxxxxxxx>:
> > On Mon, May 05, 2008 at 03:34:41PM +0200, Martin Sandve Alnæs wrote:
> >> 2008/5/5 Anders Logg <logg@xxxxxxxxx>:
> >> > On Mon, May 05, 2008 at 03:05:24PM +0200, Martin Sandve Alnæs wrote:
> >> >> 2008/5/5 Anders Logg <logg@xxxxxxxxx>:
> >> >> > On Mon, May 05, 2008 at 02:02:39PM +0200, Martin Sandve Alnæs wrote:
> >> >> >> 2008/5/2 UFL <ufl@xxxxxxxxxx>:
> >> >> >> > One or more new changesets pushed to the primary ufl repository.
> >> >> >> > A short summary of the last three changesets is included below.
> >> >> >> >
> >> >> >> > changeset:   99:ab1f19b9eb5545605d2651bf95508956aa27ad0b
> >> >> >> > tag:         tip
> >> >> >> > user:        Anders Logg <logg@xxxxxxxxx>
> >> >> >> > date:        Fri May 02 23:16:10 2008 +0200
> >> >> >> > files:       ufl/basisfunctions.py ufl/finiteelement.py
> >> >> >> > description:
> >> >> >> > Implement recursive initialization of tensor elements.
> >> >> >> > Unit tests pass, but see what you think of it.
> >> >> >>
> >> >> >> I don't agree with this, we want to have e.g. symmetric
> >> >> >> second order tensor elements, which do not fit into this.
> >> >> >
> >> >> > In what way do they not fit?
> >> >> >
> >> >> > The flag is_symmetric() is still there.
> >> >> >
> >> >>
> >> >> For a symmetric tensor in 3D, there should be only 6 subelements,
> >> >> and this recursive initialization will create 9.
> >> >
> >> > Does that matter? Creating an UFL element is very lightweight so it
> >> > shouldn't matter that we have a few extra elements. The important
> >> > thing is for algorithms (and form compilers) to look at is_symmetric
> >> > and use that to optimize.
> >>
> >> I'm not sure, but I feel that this should closely match the resulting
> >> UFC element hierarchy that comes out at the other side.
> >
> > Then what should the hierarchy look like for a symmetric 2x2 tensor
> > element?
> >
> > We could do
> >
> >  [[e, e], [None, e]]
> >
> > if we like.
> >
> 
> Basically, yes, but I'm not sure what you mean with the []'s?

I mean that we have a mixed element which has two sub elements,
each consisting of two (scalar) elements.

> But if I interpret this correctly as Python list, I don't see the
> advantage of storing this kind of list with None values.

No, it's nested classes. The list notation was just shorthand (and
confusing).

> I'd rather store a single list of subelements, which can
> be handled like in any MixedElement most of the time,
> and store the subelement indices like:

How would you store a simple Taylor-Hood element that way?

I think the natural thing would be to have a MixedElement which has
two sub elements. The first one of these (for u) will be a
VectorElement (which is a special case of a MixedElement) and that
element will have a list of two scalar sub elements. The second one
(for p) will be a FiniteElement (a scalar).

Similarly, one can take a vector-valued BDM-element for u and a scalar
element for p and combine them into a MixedElement. In that case, each
of the two sub elements will be a FiniteElement, but one will be
vector-valued and one scalar.

-- 
Anders