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Re: Log manipulation

  • To: mathgroup at smc.vnet.net
  • Subject: [mg16850] Re: [mg16804] Log manipulation
  • From: Andrzej Kozlowski <andrzej at tuins.ac.jp>
  • Date: Thu, 1 Apr 1999 21:35:23 -0500
  • Sender: owner-wri-mathgroup at wolfram.com

On Tue, Mar 30, 1999, Pierre infelta <pierre.infelta at epfl.ch> wrote:

>I am sure this is a classic but I don't seem to find the answer.
>
>I am looking for the command that will do the opposite of powerExpand
>for the Log function.
>
>If I have 
>
>PowerExpand[Log[a*b]]
>I get 
>
>Log[a] +Log[b]
>
>Is there a command that does the opposite?
>
>Command[Log[a] + Log[b]]
>
>to get
>
>Log[a b]
>
>as an answer
>
>Thanks


I can't think of a built-in command that would do that but it is easy to
create one yourself. For example:


In[1]:=
CollectLogs[x_]:=Log[Simplify[E^x]]

In[2]:=
CollectLogs[Log[x]+Log[y]]
Out[2]=
Log[x y]

This also works with the difference instead of sum of logs:

In[3]:=
CollectLogs[Log[x]-Log[y]]
Out[3]=
    x
Log[-]
    y

and  with powers

In[4]:=
CollectLogs[n*Log[x]]
Out[4]=
     n
Log[x ]


Andrzej Kozlowski
Toyama International University
JAPAN
http://sigma.tuins.ac.jp/
http://eri2.tuins.ac.jp/



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