[petsc-users] Tensor product as matrix free method

[Jed Brown]

Mathis Friesdorf <mathisfriesdorf at gmail.com> writes:

> Thanks Jed! The libaries you are pointing to look very interesting indeed.
> For the particular implementation I have in mind, I was hoping to get away
> with something easier, as my time to work on this is a bit limited. All I
> really need is a way to construct a block-diagonal matrix where each block
> is given by the same matrix and all blocks are on the diagonal. Am I wrong
> to assume that this should be possible to implement with TAIJ?

That is exactly TAIJ.  How large are the diagonal blocks?  The
implementation is intended to be fast only for small sizes, though I
could make it good for large sizes as well.  In the special case of

  (I \otimes S) X

where I is the sparse identity and S is the dense diagonal block, this
involves interpreting X as a matrix rather than a vector and using dgemm.
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