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 Out= 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 Out= 0 I also want to apply rules based on c-a=b In:= c-a /. n1_. c+n2_. a /; n1==-n2 -> n1 b Out= 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 Out= -a-b+c 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. Thanks. James