dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

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.

-- 
Anders

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