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Re: log expression not simplified

  • To: mathgroup at smc.vnet.net
  • Subject: [mg115698] Re: log expression not simplified
  • From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
  • Date: Tue, 18 Jan 2011 05:53:23 -0500 (EST)

On 17 Jan 2011, at 11:41, Praeceptor wrote:

> Sorry, can anybody help me understand why
>
> Simplify[Log[a/b]/Log[b/a], Assumptions -> {a > 0, b > a}]
>
> doesn't reduce to -1 ??
> Thanks (it's for a simple Carnot cycle calculation...)!
>

Simplify can verify that this is indeed the case

Simplify[Log[a/b]/Log[b/a] == -1,
 Assumptions -> {a > 0, b > a}]

True

However, it is hard to get it to simplify the expression to -1 because the only way to do so seems to involve temporarily increasing the default complexity. Thus, to get it to work we need a craftily designed custom complexity function, like, for example, this one:

f[expr_] := -2 Count[expr, _Log, {0, Infinity}] + LeafCount[expr]

This function "rewards" Simplify for expanding logs and, in this case, the "incentive" works:

Simplify[Log[a/b]/Log[b/a], Assumptions -> {a > 0, b > a},
 ComplexityFunction -> f]

-1

But trying to find the right function is clearly not worth the effort; a much better way is to use PowerExpand with Assumptions followed by Simplify:

PowerExpand[Log[a/b]/Log[b/a],
  Assumptions -> {a > 0, b > a}] // Simplify

-1

(Using PowerExpand without Assumptions does not guarantee that the answer is correct).

Andrzej Kozlowski


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