Am 02.01.2021 um 17:28 schrieb Riccardo (Jack) Lucchetti:
On Sat, 2 Jan 2021, Sven Schreiber wrote:
Hi,
I was wondering what's an efficient way to calculate a (pre-)
multiplication with the duplication matrix.
(For a definition of what I mean see here:
https://en.wikipedia.org/wiki/Duplication_and_elimination_matrices
or here:
https://www.rdocumentation.org/packages/matrixcalc/versions/1.0-3/topics/duplication.matrix)
It's of course very easy to construct that matrix explicitly and then do
the multiplication, and I already have a function for that, but I
suspect it's not very efficient. (Similar to the commutation matrix,
where in the 'extra' addon we have a function called 'commute' which
achieves that without actually creating the interim matrix.)
Very much untested:
<hansl>
function matrix Dup(const matrix A)
scalar m = rows(A)
H = unvech(seq(1,m)')
ret = A[vec(H),]
return ret
end function
Hm, this cannot really work it seems, as the length of seq(1,m) for
arbitrary integer m will not be suitable for unvech-ing...
But I thought there is some known trick perhaps, it's really not very
important.
thanks
sven
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