[petsc-dev] MetSetBlockSize issue

[Jed Brown]

On Wed, Apr 25, 2012 at 19:46, Mark F. Adams <mark.adams at columbia.edu>wrote:

> I think we shouldn't try to encode the near-null space being sparse. For
> elasticity, we would only reduce the 18 matrix entries to 12, which is only
> about break-even on storage since the dense null space can use fewer column
> indices (this is ignoring the dense case also being more efficient).
>
>
> Its worse than that in aggregation MG, I think, because P is not the null
> space but Q in a QR decomposition of the rigid body modes ... that might
> still have some sparsity, not sure.
>

Well, the translational modes are position-independent, so they should
retain sparsity with a standard implementation of QR (in the sense that the
zeros will remain very close to 0 so that you could discard them, though I
don't think it's even worth trying to do that).
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