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MathGroup Archive 2000

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Re: Simplify[] shortcoming

  • To: mathgroup at smc.vnet.net
  • Subject: [mg21554] Re: Simplify[] shortcoming
  • From: "Allan Hayes" <hay at haystack.demon.co.uk>
  • Date: Sat, 15 Jan 2000 02:03:59 -0500 (EST)
  • References: <85mlo9$1vc@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Gianluca,

This doesn't explain the higher leaf count in your results from Simplify and
FullSimplify, but:

Simplify[Apart[expr]]

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

And if you know that x-y is the problem:

Collect[expr, x - y]

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


Allan
---------------------
Allan Hayes
Mathematica Training and Consulting
Leicester UK
www.haystack.demon.co.uk
hay at haystack.demon.co.uk
Voice: +44 (0)116 271 4198
Fax: +44 (0)870 164 0565



"Gianluca Gorni" <gorni at dimi.uniud.it> wrote in message
news:85mlo9$1vc at smc.vnet.net...
>
> 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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