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MathGroup Archive 2005

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Re: //N bug, but WHY?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg58648] Re: //N bug, but WHY?
  • From: "Jens-Peer Kuska" <kuska at informatik.uni-leipzig.de>
  • Date: Tue, 12 Jul 2005 05:21:32 -0400 (EDT)
  • Organization: Uni Leipzig
  • References: <data3n$mec$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Hi,
if you use N[] the expression is converted into 
machine precision numbers,
these numbers have only 16 digits and if you make 
calculations with
numbers that need more than 16 digits you get 
errors.
Try
Block[{$MaxExtraPrecision = 1000},
(N[#1, 32] & )[Cosh[(43*Pi)/Sqrt[2]] +
(1 - Cosh[43*Sqrt[2]*Pi])*Csch[43*Sqrt[2]*Pi]*
Sinh[(43*Pi)/Sqrt[2]]]]

Regards
  Jens

"symbio" <symbio at has.com> schrieb im Newsbeitrag 
news:data3n$mec$1 at smc.vnet.net...
> Evaluating (using //N) two exact same 
> expressions, gives wrong answer unless
> fullsimplify is used first, I spent 2 days on a 
> problem thinking my answer
> was wrong, but turned out Mathematica 5 was 
> giving me buggy answers, I
> debugged it to this point, but WHY in the world 
> is this happening?  Please
> help!!!
>
> cut and paste below to see the test case:
>
> In[243]:=
> \!\(\(Cosh[\(43\ \[Pi]\)\/\@2] + \((1 - Cosh[43\ 
> \@2\ \[Pi]])\)\ Csch[
>            43\ \@2\ \[Pi]]\ Sinh[\(43\ 
> \[Pi]\)\/\@2] // FullSimplify\) //
>    N\[IndentingNewLine]
>  Cosh[\(43\ \[Pi]\)\/\@2] + \((1 - Cosh[43\ \@2\ 
> \[Pi]])\)\ Csch[
>          43\ \@2\ \[Pi]]\ Sinh[\(43\ 
> \[Pi]\)\/\@2] // N\)
> Out[243]=
> \!\(6.551787517854307`*^-42\)
> Out[244]=
> \!\(\(-1.9342813113834067`*^25\)\)
>
>
>
> thanks,
> 



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