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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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