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Re: Working with Log

• To: mathgroup at smc.vnet.net
• Subject: [mg112101] Re: Working with Log
• From: Peter Breitfeld <phbrf at t-online.de>
• Date: Tue, 31 Aug 2010 04:14:06 -0400 (EDT)
• References: <i5g0ku$grr$1@smc.vnet.net>

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 -- http://www.pBreitfeld.de