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Re: A problem with FindRoot

  • To: mathgroup at smc.vnet.net
  • Subject: [mg83789] Re: A problem with FindRoot
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Fri, 30 Nov 2007 05:28:03 -0500 (EST)
  • Organization: The Open University, Milton Keynes, UK
  • References: <fim7qp$qqp$1@smc.vnet.net>

Vassilis Dimitrakas wrote:

> I'd like some help with respect to a problem I have with the FindRoot, 
> NSolve and FindInstance functions.
> My version of Mathematica is 5.2
> 
> I define the function f as follows:
> 
> f[x_]:={
> y/.FindRoot[y^3+1==x,{y,x}][[1]]
> }[[1]]
> 
> f[x] returns the root of the equation y^3 + 1 == x, i.e. the value 
> (x-1)^(1/3). f[x] has obviously a
> unique root at x=1.
> 
> If I now try to find f[x]'s root with FindRoot, for example like
> 
> FindRoot[f[x]==0,{x,3}]
> 
> Mathematica (v 5.2) returns error messages and no solution. The same 
> happens if I use instead NSolve
> or FindInstance. Can you guys explain why this happens and suggest a 
> remedy?

I bet that the error messages you get are a similar to "FindRoot::srect: 
"Value x in search specification {y,x} is not a number or array of numbers."

Add a condition to your definition of f so it is called only for numeric 
arguments.

In[1]:= Clear[f]
f[x_?NumberQ] := {y /. FindRoot[y^3 + 1 == x, {y, x}][[1]]}[[1]]
FindRoot[f[x] == 0, {x, 3}, AccuracyGoal -> 5]

Out[3]= {x -> 1.}

Regards,
-- 
Jean-Marc


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