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MathGroup Archive 2004

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Sum question and general comment

  • To: mathgroup at smc.vnet.net
  • Subject: [mg50638] Sum question and general comment
  • From: Steve Gray <stevebg at adelphia.net>
  • Date: Wed, 15 Sep 2004 01:49:43 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

I don't want to overload the group with my questions, so I only post after
not being able to find the answer in the Help or at the site. Part of the
problem of course is that it isn't clear how to state the question so that
I can look it up*. Anyway, the current question has to do with Sum and
similar "indexed" operations:

I find no way to do, for example, 
"Sum over i=1 to 100 except i!= 23 and 36", etc., or Sum over values belonging to a list, such as 
"Sum over i (belonging to) {1,2,3,5,7,8,21}", etc., or
"Sum ( i=1 to 10) Sum (0ver j=1 to 10 but j !=i)", etc. (this can be awkwardly done with j=1 to i-1
and j=i+1 to 10)
In some cases there can be workarounds using things like
(1- KroneckerDelta[i,j]), etc., but these can get complicated and obscure. 
I would have thought that Mathematica could do operations like these directly,
but ??. Thank you for any information.

Steve Gray

* Someone who makes major progress on the problem of letting users communicate with a computer in
ordinary, appropriate technical "people" language will have big success. Currently in almost all
software one must ask using exactly the right terms. (I realize that Microsoft and others are trying
to make progress here, but it's negligible so far in my opinion.) Part of the answer would be a
greatly expanded index, compiled knowing what terms people are likely to use for their questions.


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