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MathGroup Archive 2006

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Re: Mathematica bugs?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg67317] Re: [mg67301] Mathematica bugs?
  • From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
  • Date: Sun, 18 Jun 2006 05:13:04 -0400 (EDT)
  • References: <200606170836.EAA27990@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

On 17 Jun 2006, at 17:36, Yaroslav Bulatov wrote:

> When I run the following line
> x = Pi/4; For[i = 0, i < 56, i += 1, x = 2*Abs[x - 1/2]]; N[x]
> I get
> 3.
>
> But x should always stay between 0 and 1, why do I get 3?
>
> Also
> x = Pi/4; For[i = 0, i < 50, i += 1, x = 2*Abs[x - 1/2]]; N[Log[x]]
> gives me
> Indeterminate
>
> How can I get an indeterminate here?
>


Simply a very usual example of loss of precision due to your using  
machine precision arithmetic.

In[4]:=
x = Pi/4; For[i = 0, i < 56, i += 1, x = 2*Abs[x - 1/2]]; N[x,20]

Out[4]=
0.79387245267980382900

In[5]:=
Pi/4; For[i = 0, i < 50, i += 1, x = 2*Abs[x - 1/2]]; N[Log[x],20]

Out[5]=
-0.82266639946997263758

No bugs at all.

Andrzej Kozlowski


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