[Date Index] [Thread Index] [Author Index]

Re: Working with Log

• To: mathgroup at smc.vnet.net
• Subject: [mg112117] Re: Working with Log
• From: Alexei Boulbitch <alexei.boulbitch at iee.lu>
• Date: Tue, 31 Aug 2010 04:17:00 -0400 (EDT)

Hi, Themis,
one idea may be as follows. Introduce two functions for expansion and to collect logarithms:

expandLog[expr_] := Module[{rule1, rule2, a, b, x},
   rule1 = Log[a_*b_] -> Log[a] + Log[b];
   rule2 = Log[a_^x_] -> x*Log[a];
   (expr /. rule1) /. rule2
   ];
collectLog[expr_] := Module[{rule1, rule2, a, b, x},
   rule1 = Log[a_] + Log[b_] -> Log[a*b];
   rule2 = x_*Log[a_] -> Log[a^x];
   (expr /. rule1) /. rule2 /. rule1 /. rule2
   ];

Then:


Log[Sin[x]*Cos[y]] // expandLog



Log[Cos[y]] + Log[Sin[x]]


Log[a + b] + Log[c + d] // collectLog

Log[(a + b) (c + d)]

Have fun, Alexei



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

-- 
Alexei Boulbitch, Dr. habil.
Senior Scientist
Material Development

IEE S.A.
ZAE Weiergewan
11, rue Edmond Reuter
L-5326 CONTERN
Luxembourg

Tel: +352 2454 2566
Fax: +352 2454 3566
Mobile: +49 (0) 151 52 40 66 44

e-mail: alexei.boulbitch at iee.lu

www.iee.lu

--

This e-mail may contain trade secrets or privileged, undisclosed or
otherwise confidential information. If you are not the intended
recipient and have received this e-mail in error, you are hereby
notified that any review, copying or distribution of it is strictly
prohibited. Please inform us immediately and destroy the original
transmittal from your system. Thank you for your co-operation.



__________ Information from ESET NOD32 Antivirus, version of virus signature database 5410 (20100830) __________

The message was checked by ESET NOD32 Antivirus.

http://www.eset.com