[firedrake] difference between matrix-action and form-action

[Eike Mueller]

Hi Lawrence,

ok, so if I apply the BCs to u before the multiplication and to v after the multiplication I would expect the result to be the same in both versions, but it doesn’t look like it.

Thanks,

Eike

> On 27 Apr 2015, at 12:19, Lawrence Mitchell <lawrence.mitchell at imperial.ac.uk> wrote:
> 
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> On 27/04/15 12:04, Eike Mueller wrote:
>> Dear firedrakers,
>> 
>> Assume I have assembled a UFL form in the HDiv space:
>> 
>> form_uu = [a UFL form] mat_uu = assemble(form_uu,bcs=bcs)
> 
> This assembles an operator with bcs applied, useable in a linear
> solver.  The rows /and/ columns corresponding to the BC nodes are the
> identity.
> 
>> then shouldn’t the following two experssions fo exactly the same
>> thing?
>> 
>> version A: with v.dat.vec as y: with u.dat.vec_ro as x: 
>> mat_uu.M.handle.mult(x,y)
> 
> So this computes I_{Gamma} + A_{Omega \ Gamma}, i.e. on the boundary
> nodes y is x elsewhere it's Ax.
> 
>> version B: v = self._mutilde._form_action(u)
> 
> This computes the action of A on u /without/ boundary conditions.
> What are you expecting to see?, Ax on the non-boundary condition
> nodes, bc-value on the boundary values?
> 
>> as fas as I can tell, only version B gives the correct result in my
>> case.
> 
> Lawrence
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