Why doesn't this rule work?
- To: mathgroup at smc.vnet.net
- Subject: [mg32447] Why doesn't this rule work?
- From: James Jennings <djeimz at no.spam.megaseattle.com>
- Date: Mon, 21 Jan 2002 02:54:59 -0500 (EST)
- Sender: owner-wri-mathgroup at wolfram.com
Suppose I want to simplify expressions linear in a, b, and c where a+b=c
-- something of this sort.
In:= a+b /. a+b -> c
In:= a+b+c /. a+b -> c
Out= 2 c
With more complicated coefficients I can use:
In:= -2 a - 2 b /. n_. a+n_. b -> n c
Out= -2 c
In:= -a-b+c /. n_. a+n_. b -> n c
I also want to apply rules based on c-a=b
In:= c-a /. n1_. c+n2_. a /; n1==-n2 -> n1 b
The problem comes with I apply the above rule to longer expressions.
In:= c-a-b /. n1_. c+n2_. a /; n1==-n2 -> n1 b
Why didn't that work? I had thought that since Plus is Orderless, my
rule ought to be applied to all pieces of my expression, but it doesn't
appear to be.
Before someone suggests that I should just set c = a+b and be done with
it, my actual problem involved 12 objects like a, b, and c, where 16
distinct triplets add up like a+b=c. I'm looking for rules that will
keep my expressions as short as possible, even if they aren't unique.
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