RE: How to substitute?

• To: mathgroup at smc.vnet.net
• Subject: [mg45489] RE: [mg45474] How to substitute?
• From: "David Park" <djmp at earthlink.net>
• Date: Sat, 10 Jan 2004 00:00:36 -0500 (EST)
• Sender: owner-wri-mathgroup at wolfram.com

```Steve,

expr = (Subscript[y, 0] - Subscript[z, 3])*
(-Subscript[y, 1] + Subscript[z, 3])*
(-Subscript[y, 2] + Subscript[z, 3])*
(-Subscript[y, 3] + Subscript[z, 3])

expr //. {Subscript[y, i_] - Subscript[z, j_] ->
Q[i, j], -Subscript[y, i_] + Subscript[z, j_] ->
-Q[i, j]}
-Q[0, 3] Q[1, 3] Q[2, 3] Q[3, 3]

(I used Bobby Treat's neat copy palette to copy and paste these
expressions.)

David Park

From: Steve Gray [mailto:stevebg at adelphia.net]
To: mathgroup at smc.vnet.net

\!\(\((y\_0 - z\_3)\)\ \((\(-y\_1\) + 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.
Any suggestions will be gratefully received.

```

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