ufl team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Tue, May 06, 2008 at 10:26:27AM +0200, Martin Sandve Alnæs wrote:
> 2008/5/6 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?
> 
> Irrelevant. A Taylor-Hood element is not a TensorElement.

No, it's not irrelevant if a TensorElement should be a subclass of
MixedElement (which I think it should since it is a special case).

> > 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.
> 
> I completely agree with that for a general MixedElement.
> 
> I just don't agree that a TensorElement should be
> represented as a nested hierarchy of MixedElements,
> since this will not be simple as intended for symmetric elements.
> 
> Actually, I'm still not quite comfortable with VectorElement meaning
> "vector of element" instead of "vector valued element", and I don't
> think that matches what people would expect. Our different opinions
> here may be related to this.

It's very convenient if one can take any scalar element and use that
to create a vector-valued element, and it's also convenient if one can
have a single base class (MixedElement) which takes care of it all
and have VectorElement and TensorElement as special cases.

-- 
Anders