ffc team mailing list archive

[Kristian Oelgaard <k.b.oelgaard@xxxxxxxxx>]

On 18 June 2010 12:05, Marie Rognes <meg@xxxxxxxxx> wrote:
> On 18. juni 2010 11:38, Kristian Oelgaard wrote:
>> On 18 June 2010 01:44, Marie Rognes <meg@xxxxxxxxx> wrote:
>>
>>> On 17. juni 2010 15:44, Mehdi wrote:
>>>
>>> On Wed, 2010-06-16 at 18:04 +0200, Kristian Oelgaard wrote:
>>>
>>>
>>>
>>> Mehdi and I discussed this a bit, one way to get around this in FFC is to
>>> let
>>> VectorElement accept a FiniteElement as argument, then you can do
>>>
>>> element = VectorElement(V + Q)
>>>
>>> and still be dimension independent.
>>>
>>> Or in UFL we can tweak the '+' operator, such that enriching a
>>> VectorElement means enriching each of the components of 'self' with
>>> the components of 'other'. For this to work the dimension of the two
>>> vector elements must of course be identical but I guess that will
>>> always be the case, otherwise we throw an error.
>>>
>>>
>>> I will go for this option. This allows us to have simpler code and
>>> preserves accessing to the sub-elements of enriched mixed element.
>>>
>>>
>>>
>>> How do you plan on handling elements such as the following (relevant in
>>> connection with the PEERS element for linear elasticity) with this approach?
>>>
>>> V = FiniteElement("RT", "triangle", 1)
>>> Q = VectorElement("B", "triangle", 3)
>>> W = V + Q
>>>
>> I was planning on throwing an error :)
>>
>
>
> Please don't :)
>
>> I did not know that one would ever want to enrich a scalar element
>> with a vector bubble function,
>
> Since RT is a vector-valued element, enriching it with a vector bubble
> function
> is well-defined.

Ha, I completely missed the 'RT', that's what you get for working with
'CG' only :)
Now it makes a lot more sense to me.

> Strictly speaking, my example is not the enrichment of one UFL VectorElement
> with another VectorElement. So, I guess you could overload + for
> VectorElement (and TensorElement) only. However, that would make

Yes, that's what we had in mind, then instead of an error we just
return an EnrichedElement, then it's up to the user to make sure that
the enrichment makes sense. I haven't looked at the code in detail
now, but maybe there are other things we need to check for.

>    V = FiniteElement("CG", "triangle", 1)
>    V = V*V
>    B = VectorElement("B", "triangle", 3)
>    W = V + B
>
> and
>
>    V2 = VectorElement("CG", "triangle", 1)
>    W = V2 + B
>
> behave differently, which I imagine could be rather confusing.

They do?
I would expect both instances to end up meaning
W = MixedElement([FiniteElement('CG', 'triangle', 1) + B]*2)

or can you spell it out for me?

>
>> but since this appears to be the case
>> the only two options, as I see it, are:
>>
>> 1) do nothing such that one will have to do
>>
>>   P1 = FiniteElement("Lagrange", tetrahedron, 1)
>>   B = FiniteElement("Bubble", tetrahedron, 3)
>>   Q = FiniteElement("CG", triangle, 1)
>>   Mini = MixedElement([P1 + B], [P1 + B], [P1 + B])*Q
>>
>> manually.
>>
>> 2) have VectorElement accept FiniteElementBase as argument to allow
>>
>>   Mini = VectorElement(P1 + B)*Q
>>
>> which will reduce the labour of defining elements and still be
>> independent of dimensions.
>>
>>
>
>
> Option 2) sounds neat and useful in other contexts too :)

We of course still do this even if we decide on option 1).

Kristian

>
>> We could of course try to 'guess' what the user wants but I don't
>> think that is a good idea.
>>
>>
>
>
> Agree.
>
> --
> Marie
>
>
>> Kristian
>>
>>
>>> v = TestFunction(W)
>>> u = TrialFunction(W)
>>> a = dot(v, u)*dx
>>>
>>> --
>>> Marie
>>>
>>>
>>>
>>>
>>> Mehdi
>>>
>>>
>>> Kristian
>>>
>>>
>>>
>>> Garth
>>>
>>>
>>>
>>> Kristian
>>>
>>>
>>>
>>> Garth
>>>
>>>
>>>
>>>
>>>
>>> --
>>> Marie
>>>
>>>
>>>
>>>  Mehdi
>>>
>>>
>>>
>>>
>>> --
>>> Marie
>>>
>>>
>>>
>>>
>>>
>>> Mehdi
>>>
>>>
>>>
>>>
>>>
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>
>