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A problem with FindRoot

• To: mathgroup at smc.vnet.net
• Subject: [mg83728] A problem with FindRoot
• From: Vassilis Dimitrakas <vdimitrakas at googlemail.com>
• Date: Thu, 29 Nov 2007 06:24:03 -0500 (EST)

Hi All,

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?

Thanks,

Vassilis

• Follow-Ups:
  • Re: A problem with FindRoot
    • From: Murray Eisenberg <murray@math.umass.edu>