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[Kristian B. Ølgaard <question158285@xxxxxxxxxxxxxxxxxxxxx>]

Question #158285 on DOLFIN changed:
https://answers.launchpad.net/dolfin/+question/158285

Kristian B. Ølgaard proposed the following answer:
On 20 May 2011 09:11, James Avery <question158285@xxxxxxxxxxxxxxxxxxxxx> wrote:
> Question #158285 on DOLFIN changed:
> https://answers.launchpad.net/dolfin/+question/158285
>
> James Avery posted a new comment:
> Hi Anders,
>
> Thanks a lot! That works. However, a quick follow-up question: It seems
> to me that it should be possible to compute the derivatives of a finite-
> element function f in a more efficient manner, since we have f's
> expansion in terms of basis functions that can be differentiated,
> although I have not been able to find out how these are accessed in
> DOLFIN.

You can get the finite element of the function and call the function
evaluate_basis_derivatives on that.

Kristian

> Is it strictly necessary to assemble and solve a new linear system to
> obtain a gradient? The reason I'm asking is, that the bottleneck in my
> application is due to the matrices describing the linear systems (with
> mesh size of up to a couple million cells). Thus assembling a matrix (or
> storing a precalculated one) and solving a linear system is a fairly
> large operation. Is there perhaps a shortcut for just computing the
> function derivatives with DOLFIN?

This is what the project() function in PyDOLFIN does and you're right, that
it might not be efficient for some applications.

Kristian

> Thank you again for your help!
>
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