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Re: Sum of Products

  • To: mathgroup at smc.vnet.net
  • Subject: [mg126679] Re: Sum of Products
  • From: Ray Koopman <koopman at sfu.ca>
  • Date: Wed, 30 May 2012 05:25:51 -0400 (EDT)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • References: <jq266k$6jv$1@smc.vnet.net>

On May 29, 2:51 am, "Harvey P. Dale" <h... at nyu.edu> wrote:
>         If I have a list and want to sum the products of each possible
> grouping of two elements in the list, this program will do that:
>
>                 Total[Times @@@ Subsets[{a, b, c}, {2}]]
>
> Is there any other, shorter, built-in Mathematica object that will
> produce the same result?
>
>         Thanks.
>
>         Harvey

T = Transpose (* shorter ;) *);
x = {a,b,c,d,e};
SameQ[
  Total[Times @@@ Subsets[x,{2}]],
  Dot @@ T @ Subsets[x,{2}],
  (Tr@x^2 - x.x)/2
//Expand]

True



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