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Re: exact integer from Log[ ]

• To: mathgroup at smc.vnet.net
• Subject: [mg58336] Re: [mg58321] exact integer from Log[ ]
• From: Bob Hanlon <hanlonr at cox.net>
• Date: Tue, 28 Jun 2005 21:56:36 -0400 (EDT)
• Reply-to: hanlonr at cox.net
• Sender: owner-wri-mathgroup at wolfram.com

Use FunctionExpand or FullSimplify

f=880*2^(1/12);

expr=FunctionExpand[Log[2^(1/12),f/440]]

13

expr=FullSimplify[Log[2^(1/12),f/440]]

13


Bob Hanlon

> 
> From: "Trevor Baca" <trevorbaca at gmail.com>
To: mathgroup at smc.vnet.net
> Date: 2005/06/28 Tue AM 05:13:22 EDT
> Subject: [mg58336] [mg58321] exact integer from Log[ ]
> 
> 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.
> 
>