Mathematica 9 is now available
Services & Resources / Wolfram Forums
-----
 /
MathGroup Archive
2005
*January
*February
*March
*April
*May
*June
*July
*August
*September
*October
*November
*December
*Archive Index
*Ask about this page
*Print this page
*Give us feedback
*Sign up for the Wolfram Insider

MathGroup Archive 2005

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

Search the Archive

Re: exact integer from Log[ ]

  • To: mathgroup at smc.vnet.net
  • Subject: [mg58337] Re: exact integer from Log[ ]
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Tue, 28 Jun 2005 21:56:37 -0400 (EDT)
  • Organization: The Open University, Milton Keynes, England
  • References: <d9r4o7$54g$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Trevor Baca wrote:
> The question is how to get the exact integer 13 when we let
> 
>   f = 880*2^(1/12)
> 
> and let
> 
>   expr = Log[2^(1/12), f/440]
> 
> with no numerics (only exact integers) in both expressions.
> 
> It seems that expr should already equal the integer 13 exactly, yet
> evaluating expr gives
> 
>   (12*Log[2*2^(1/12)])/Log[2]
> 
> or
> 
>   Log[8192]/Log[2]
> 
> with Simplify[expr], which, while clearly 13 to the eye, still isn't
> exactgly 13 on the page.
> 
> So how do you get 13 back out of expr, preferably without recourse to
> numerics?
> 
> 
> Trevor.
> 
Hi Trevor,

If I remember correctly, *Simplify* does very few attempt to manipulate 
exponents. So, try one of the following commands

In[24]:=
FullSimplify[expr]

Out[24]=
13

In[25]:=
PowerExpand[expr]

Out[25]=
13

In[26]:=
FunctionExpand[expr]

Out[26]=
13

Best regards,
/J.M.


  • Prev by Date: Re: Common factors in a list
  • Next by Date: Re: Common factors in a list
  • Previous by thread: Re: exact integer from Log[ ]
  • Next by thread: ListInterpolation