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

  • To: mathgroup at
  • Subject: [mg13963] Re: warning for Round[Log[2]/Log[4]]
  • From: bruck at (Ronald Bruck)
  • Date: Fri, 11 Sep 1998 15:06:52 -0400
  • Organization: University of Southern California, Los Angeles, CA
  • References: <6ssvo6$> <6t2l44$>
  • Sender: owner-wri-mathgroup at

In article <6t2l44$47o at>, Ronald Bruck <bruck at> wrote:
>In article <6ssvo6$m9r at>,
>Wouter Meeussen  <eu000949 at> wrote:
>>** this is not a bug report ***
>>  Windows  
>[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?)

I've had an interesting e-mail discussion with...  Hmmm, I suppose the
ethics of the internet prohibit me from saying whom :-(  To summarize:

  The error message is generated by Round[Log[2]/Log[4]], not the //N
  This is because Mathematica attempts to apply purely-numeric
  which can't work of course; after it's tried 66 decimal digits of
  it punts (returning the correct answer, but how much confidence can
  put in that?).

  This is because Mathematica doesn't attempt to simplify the
  term.  In general, it's NOT safe to replace Log[z^2] by 2 Log[z],
  this is NOT TRUE (branch problems) in the complex plane.  But surely
  Mathematica knows that Log[a b] = Log[a] + Log[b] for POSITIVE reals
a, b.

  And it does; FullSimplify does the job:


  returns 0.  You have to put the FullSimplify INSIDE the Round, of
  otherwise you get the error message.

  It's interesting that FullSimplify of Log[z^2] returns Log[z^2], of
  Log[z^2]/Log[z] returns Log[z^2]/Log[z], and of Log[4]/Log[2] returns

  This seems to me a reasonable compromise between generality and
  Simplify computations which involve Numbers; don't simplify those
  involve general variables.  But why couldn't this be incorporated into

--Ron Bruck

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