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MathGroup Archive 2003

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Re: Log[Exp[r]] <> r?

  • To: mathgroup at
  • Subject: [mg39279] Re: Log[Exp[r]] <> r?
  • From: "Kevin J. McCann" <KevinMcCann!kjm at>
  • Date: Sun, 9 Feb 2003 04:50:17 -0500 (EST)
  • References: <b22jou$6bh$>
  • Sender: owner-wri-mathgroup at

It is because the log is a multivalued function. I believe that Log gives
the principle branch, i.e. -Pi < Arg[Log[z]] < Pi; equivalently, the branch
cut is chosen to go from 0 to -Infinity (real). This may be illustrated by


which returns 2.0+1.5708I. it does not return 2+4.5*Pi*I, rather 2+0.5*Pi*I
which is in the principle branch.


"Jonathan Greenberg" <greenberg at> wrote in message
news:b22jou$6bh$1 at
> Hi there, why is it when I enter Log[Exp[r]] (or use Simplify or
> FullSimplify) I don't get r?  If I do Log[Exp[2]] I get 2!
> --j
> --
> Jonathan Greenberg
> Graduate Group in Ecology, U.C. Davis
> AIM: jgrn307 or jgrn3007
> MSN: jgrn307 at or jgrn3007 at

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