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Re: Working with Log

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  • Subject: [mg112101] Re: Working with Log
  • From: Peter Breitfeld <phbrf at>
  • Date: Tue, 31 Aug 2010 04:14:06 -0400 (EDT)
  • References: <i5g0ku$grr$>

Themis Matsoukas wrote:

PowerExpand treats its expressions assuming all variables are positive.
Normally Log[ab]==Log[a]+Log[b] is not always tru, e.g.

0=Log[(-1)(-1)]!=Log[-1]+Log[-1]==2Pi I

So you have to declare the variables as positives (if your domains are right):

In[1]:= Simplify[Log[a]+Log[b], Assumptions->a>0&&b>0]
Out[1]= Log[ab]

> I can use PowerExapnd to expand a Log:
> PowerExpand[Log[a b]]
> Log[a] + Log[b]
> How can I do the opposite, i.e. combine Log[a] + Log[b] into Log[a b]?
> Thanks
> Themis

Peter Breitfeld, Bad Saulgau, Germany --

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