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Simplify[] shortcoming

  • To: mathgroup at
  • Subject: [mg21525] Simplify[] shortcoming
  • From: Gianluca Gorni <gorni at>
  • Date: Fri, 14 Jan 2000 02:43:40 -0500 (EST)
  • Sender: owner-wri-mathgroup at


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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