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Re: A question about N[...]

  • To: mathgroup at smc.vnet.net
  • Subject: [mg89418] Re: A question about N[...]
  • From: Szabolcs Horvát <szhorvat at gmail.com>
  • Date: Mon, 9 Jun 2008 02:27:35 -0400 (EDT)
  • References: <g2fug6$2kg$1@smc.vnet.net>

wyelen at gmail.com wrote:
> Recently I came across a puzzling problem which I believed to be
> related to the function N.
> 
> My platform is Mathematica 6.0 for Microsoft Windows (32-bit). When
> calculating the following
> integral, I got different results from Integrate & NIntegrate:
> 
>           In[1]:= Integrate[BesselJ[0, 2.405 * r]^2, {r, 0, 20}]
> 
>           Out[1]= 0.
> 
>           In[2]:= NIntegrate[BesselJ[0, 2.405 * r]^2, {r, 0, 20}]
> 
>           Out[2]= 0.864755
> 
> Guessing a problem caused by numerical number 2.405, I rewrote it as
> an exact number:
> 
>           In[3]:= Integrate[BesselJ[0, (2 + 405/1000)*r]^2, {r, 0,
> 20}]
> 
>           Out[3]= 20*HypergeometricPFQ[{1/2, 1/2}, {1, 1, 3/2}, -
> (231361/100)]
> 
> then evaluated the numerical value, which was surprisingly still 0.:
> 
>           In[4]:= N[%]
> 
>           Out[4]= 0.
> 
> but evaluating with 6-digit precision gave the same result as
> NIntegrate:
> 
>           In[5]:= N[%%,6]
> 
>           Out[5]= 0.864755
> 
> In help page for N it said "N[expr] is equivalent to
> N[expr,MachinePrecision]", but evaluating with a
> approximate precision didn't gave 0.:
> 
>           In[6]:= N[MachinePrecision]
> 
>           Out[6]= 15.9546
> 
>           In[7]:= N[%3,15.9546]
> 
>           Out[7]= 0.8647551857405188
> 
> I wonder is this caused by the function N ,or whether I should just
> turn to another OS (say Linux) and things will go well.
> 

Looks like a bug in N/HypergeometricPFQ.

I'd just like to note that MachinePrecision should be used directly in N 
to evaluate a number with machine precision.  N[..., MachinePrecision] 
is not the same as N[..., 15.9546]


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