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MathGroup Archive 2004

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Re: How to substitute?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg45499] Re: How to substitute?
  • From: Bill Rowe <readnewsciv at earthlink.net>
  • Date: Sat, 10 Jan 2004 00:00:49 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

On 1/9/04 at 5:20 AM, stevebg at adelphia.net (Steve Gray) wrote:

> \!\(\((y\_0 - z\_3)\)\ \((\(-y\_1\) + z\_3)\)\ \((\(-y\_2\) + 
>         z\_3)\)\ \((\(-y\_3\) + z\_3)\)\)

>   I have output cells containing various expressions such as 
> (y[0]-z[1]) (y[1]-z[2]) (y[4] - z[0]), where I use [x] here to mean
> subscript. The integers can have any small values. For compactness of
> expression, I want to make substitutions which make the above
> expression look like Q[0,1]Q[1,2]Q[4,0]. There are many combinations
> of subscript pairs and I don't want to write explicit substitution
> rules for every possible combination. I tried doing /.(y[i]-z[j]) ->
> Q[i,j] , and /.(y[i_]-z[j_]) -> Q[i,j] but they don't work. 

These substitutions work fine when done properly. Try

(Subscript[y, 0] - Subscript[z, 3])*(-Subscript[y, 1] + 
    Subscript[z, 3])*(-Subscript[y, 2] + Subscript[z, 3])*
   (-Subscript[y, 3] + Subscript[z, 3]) /. 
  Subscript[y, i_] - Subscript[z, j_] -> 
   Subscript[Q, i, j]

(Subscript[z, 3] - Subscript[y, 1])*
  (Subscript[z, 3] - Subscript[y, 2])*
  (Subscript[z, 3] - Subscript[y, 3])*Subscript[Q, 0, 3]
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