       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>

```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= 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:=
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=
0.0578933385691052787623495851172

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

To email me personally, remove the dash.

```

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