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MathGroup Archive 2007

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Re: Coaxing N[] to work

  • To: mathgroup at smc.vnet.net
  • Subject: [mg73398] Re: [mg73370] Coaxing N[] to work
  • From: "Igor C. Antonio" <igora at wolf-ram.com>
  • Date: Thu, 15 Feb 2007 05:03:50 -0500 (EST)
  • Organization: Wolfram Research, Inc.
  • References: <200702141020.FAA19038@smc.vnet.net>
  • Reply-to: igora at wolf-ram.com

p at dirac.org wrote:
> Sometimes N[,] doesn't appear to work.  Like here:
> 
> 
> x = {2.0, 3.0, 5.0};
> A = { {6.0, 2.0, 1.0}, {2.0, 3.0, 1.0}, {1.0, 1.0, 1.0} };
> For[ k=0, k<15, ++k,
>    lambda = x.A.x/(x.x);
>    y = LinearSolve[A,x];
>    x = y / Norm[y,Infinity];
> ]
> N[lambda, 30]
> 
> 
> The output is:
> 
>    Out[5]= 0.578933
> 
> I was expecting 30 digits.  Why did N[] ignore my request for 30 digits?

PLEASE read the documentation before posting to the newsgroup.  The very first 
line of the docs for N[] says:

"Unless numbers in expr are exact, or of sufficiently high precision, N[expr, n] 
may not be able to give results with nâ??digit precision."

In[1]:=
x={2/10,3/10,5/10};
A={{6/10,2/10,1/10},{2/10,3/10,1/10},{1/10,1/10,1/10}};
For[k=0,k<15,++k,lambda=x.A.x/(x.x);
   y=LinearSolve[A,x];
   x=y/Norm[y,Infinity];]
N[lambda,30]
Out[4]=
0.0578933385691052787623495851172



--
Igor C. Antonio
Wolfram Research, Inc.
http://www.wolfram.com

To email me personally, remove the dash.


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