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MathGroup Archive 1998

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Re: Re: warning for Round[Log[2]/Log[4]]

  • To: mathgroup at smc.vnet.net
  • Subject: [mg13955] Re: [mg13940] Re: warning for Round[Log[2]/Log[4]]
  • From: BobHanlon at aol.com
  • Date: Fri, 11 Sep 1998 15:06:45 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

As you indicated, the problem occurs because numerical values are used
in attempting to evaluate the ratio. However,

Log[2]/Log[4] // Simplify

Log[2]/Log[4]

%//FullSimplify

1/2

Consequently,

Round[Log[2]/Log[4]//FullSimplify]

0

Bob Hanlon

In a message dated 9/8/98 5:06:18 AM, bruck at math.usc.edu wrote:

>In article <6ssvo6$m9r at smc.vnet.net>,
>Wouter Meeussen  <eu000949 at pophost.eunet.be> wrote:
>>** this is not a bug report ***
>>  Windows 3.0.0.0.  
>>
>[That Mathematica didn't have enough precision to compute
Round[Log[2]/Log[4]]
>
>Hmmm.  Interesting.  Since Log[2]/Log[4] = 1/2, Mathematica can't decide
>whether to round up or down--it SHOULD apply round-to-even, to give
>zero, but it's trying to do this with just numerical values.
>
>As I recall, deciding when A == B is a profoundly difficult problem. 
>I'm not sure how else one could go about this.  (Oh, sure, the
>immediate problem could be fixed by applying a special rule for
>logarithms, but what about special functions for which the relations
>are still undiscovered?)


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