dolfin team mailing list archive

["Garth N. Wells" <gnw20@xxxxxxxxx>]

Anders Logg wrote:
> On Mon, Sep 15, 2008 at 02:45:58PM +0100, Garth N. Wells wrote:
> > Anders Logg wrote:
> > > On Mon, Sep 15, 2008 at 11:21:59AM +0100, Garth N. Wells wrote:
> > > > DOLFIN wrote:
> > > > > One or more new changesets pushed to the primary dolfin repository.
> > > > > A short summary of the last three changesets is included below.
> > > > >
> > > > > changeset:   4754:23602808c60413cb8faffca818e7a8c04527d3ec
> > > > > tag:         tip
> > > > > user:        Anders Logg <logg@xxxxxxxxx>
> > > > > date:        Sun Sep 14 19:46:40 2008 +0200
> > > > > files:       demo/pde/elasticity/python/demo.py
> > > > > description:
> > > > > Use symmetric gradient in variational form in elasticity demo
> > > > In particular reason for this? It is simpler (and still correct) to use 
> > > > the gradient.
> > > >
> > > > Garth
> > > To make the form and the matrix symmetric. I showed the demo to a
> > > friend (in computational mechanics) and he insisted that we replace
> > > grad(v) by epsilon(v).
> > He's wrong :). When you take the inner product A : B, where B is 
> > symmetric, the inner product only 'sees' the symmetric part of A. It's a 
> > classic exercise and you can prove it by writing out the indices.
> >
> > Garth
>
> I don't understand how that makes it wrong. My point (and maybe his)
> is that since B is symmetric, we may replace A by its symmetric part
> (as you say easy to see by writing out the indices). So, both
> epsilon(v) and grad(v) are correct.

They are both correct (I never said they they aren't). Quoting your 
earlier email

"I showed the demo to a friend (in computational mechanics) and he 
insisted that we replace grad(v) by epsilon(v)."

which is not correct. We can use grad(v) *or* epsilon(v). Either way, 
the matrix will be symmetric. grad(v) leads to a slightly small .h file 
and I would expect is easier for FFC.

Garth



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