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Re: Summing above diagonal in a tensor

  • To: mathgroup at smc.vnet.net
  • Subject: [mg112624] Re: Summing above diagonal in a tensor
  • From: Ray Koopman <koopman at sfu.ca>
  • Date: Thu, 23 Sep 2010 04:23:02 -0400 (EDT)
  • References: <i79hv2$oin$1@smc.vnet.net>

On Sep 20, 11:05 pm, Alice Lesser <alice.les... at bwin.org> wrote:
> Hi group,
>
> Summing the elements of a matrix which are above the diagonal is easy:
>
> m = Table[Subscript[a, x, y], {x, 1, 3}, {y, 1, 3}]
> Total[UpperTriangularize[m, 1], 2]
>
> gives
> a_1,2 + a_1,3 + a_2,3
> of course.
>
> If I instead have a 4-dim tensor
>
> t = Table[ Subscript[a, x, y, z, q], {x, 1, 3}, {y, 1, 3}, {z, 1, 3}, {q, 1, 3}]
>
> is there a nifty way to get the sum of the elements which are above
> the diagonal in the outer matrix and above the diagonal in the submatrices?
> In other words I'm looking for the sum
>
> a_1,2,1,2 + a_1,2,1,3 + a_1,2,2,3 + a_1,3,1,2 + a_1,3,1,3 + a_1,3,2,3+ a_2, 3,1,2 + a_2,3,1,3 + a_2,3,2,3
>
> UpperTriangularize only seems to work on matrices, not tensors.
> I could of course solve this with a few ugly for-loops, but it would be nice to know if there is a better way.
>
> Thanks,
> Alice

This is probably a little more efficient than my first suggestion:

Tr @ Extract[t, Flatten/@Tuples[Subsets[{1,2,3},{2}],2] ]


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