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MathGroup Archive 2004

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Re: Problem with Mathematica driving me nuts

  • To: mathgroup at smc.vnet.net
  • Subject: [mg46804] Re: [mg46791] Problem with Mathematica driving me nuts
  • From: Daniel Lichtblau <danl at wolfram.com>
  • Date: Tue, 9 Mar 2004 04:30:47 -0500 (EST)
  • References: <200403080910.EAA10442@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

benwoodward.com wrote:
> FindRoot[x^2 == 4x - 4, {x, 1}]
> 
> Out[4]=
> {x -> 1.99902}
> 
> In[15]:=
> FindRoot[x^2 - 4*x + 4, {x, 3}]
> 
> Out[15]=
> {x -> 2.00098}
> 
> When the root is clearly two.
> Is Mathematica using Newton's Method like a Ti-92?
> Even if so, why wont it give a more accurate answer?
> I've tried N[%,30] but it doesn't do anything.
> I'm new to Mathematica coming over from a Ti-92, so everything is
> frustrating right now.
> Thanks.
> 

I assume you are using a version 4.something, as version 5 appears to do 
better (i.e. it gives x->2.). The code below was run in version 4.2.

In[9]:=  FindRoot[x^2 == 4x - 4, {x, 1}, AccuracyGoal->14, 
MaxIterations->200]

Out[9]= {x -> 2.}

InputForm will reveal that it is really only correct to 7 or so places, 
which is appropriate because you are at a double root (so accuracy will 
be roughly twice that, as requested).

Note that the Automatic setting for AccuracyGoal will try to attain 6 
decimal places in version 4. We see below that this was in fact 
accomplished.

In[13]:= rt = FindRoot[x^2 - 4*x + 4, {x, 3}]
Out[13]= {x -> 2.00098}

In[14]:= Log[10, x^2 - 4*x + 4 /. rt]
Out[14]= -6.0206

The other example gives a similar result.


Daniel Lichtblau
Wolfram Research



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