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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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