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MathGroup Archive 2000

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Re: Simplify[] shortcoming

  • To: mathgroup at smc.vnet.net
  • Subject: [mg21546] Re: [mg21525] Simplify[] shortcoming
  • From: Andrzej Kozlowski <andrzej at tuins.ac.jp>
  • Date: Sat, 15 Jan 2000 02:03:53 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

It seems that Matheamtica just fails to "see" that it can cancel x-y.
Curiously one can get the desired answer in the following way:

expr = (3*x + (x - y)*Log[x] + (x - y)*Log[-1 + y/x])/(x - y);

In[7]:=
Simplify[expr /. (x - y) -> u] /. u -> (x - y)

Out[7]=
 3 x                      y
----- + Log[x] + Log[-1 + -]
x - y                     x



> From: Gianluca Gorni <gorni at dimi.uniud.it>
To: mathgroup at smc.vnet.net
> Date: Fri, 14 Jan 2000 02:43:40 -0500 (EST)
> To: mathgroup at smc.vnet.net
> Subject: [mg21546] [mg21525] Simplify[] shortcoming
> 
> 
> Hello!
> 
> I have run into a simple expression for which both Simplify and
> FullSimplify cannot see an easy simplification:
> 
> expr = (3*x + (x - y)*Log[x] + (x - y)*Log[-1 + y/x])/(x - y);
> 
> You can see that the factors (x-y) cancel out with the denominator.
> So you can write expr as
> 
> (3*x)/(x - y) + Log[x] + Log[-1 + y/x]
> 
> which has a LeafCount of 21, while expr scored 34. Nevertheless,
> Simplify[] leaves expr as it was.
> 
> FullSimplify[expr] does collect (x-y) in the numerator, but still
> the LeafCount is 29.
> 
> My version is Mathematica 4 on MacOS.
> 
> Gianluca Gorni
> 
> 



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