[petsc-dev] attempt at understanding PetscFEM

[Geoffrey Irving]

On Mon, Nov 18, 2013 at 4:39 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
> Geoffrey Irving <irving at naml.us> writes:
>
>> The seems to be a typo in the abstract:
>>
>>     This effect typically limits order to at most *quadratic*, despite
>> the favorable accuracy and stability properties offered by *quadratic*
>> and higher order discretization
>
> I guess it sounds funny, but quadratic is the crossover point: too
> expensive according to many people, but required by others.
>
>> The index notation was the whole point, incidentally, since it
>> transparently defines the order of array dimensions in the code.
>
> Okay, that is a convention that I think you got right, but there are
> vectorization reasons to change it in some settings, so if you're going
> to build something significant, it would be worth hedging against index
> ordering changes.

For what it's worth, I had actually gotten the convention wrong.
According to my document, g3 in the FEM case looked like

    g3(a,b,c,d) = d(P(a,b))/d(F(c,d))

but it's actually

    g3(a,b,c,d) = d(P(a,c))/d(F(b,d))

Geoffrey