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From: Szabolcs Horvát on 1 Dec 2007 06:04
Jean-Marc Gulliet wrote:
> 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.}

I'd just like to add a small comment: In these situations it is more
correct to use NumericQ instead of NumberQ, otherwise expressions like
f[Sqrt[2]] will not evaluate. Sqrt[2] is not an "atomic" number in
Mathematica, but a compound expression. However, it does represent a
number in the mathematical sense, and N[Sqrt[2]] evaluates to a floating
point value. So NumberQ[Sqrt[2]] === False and NumericQ[Sqrt[2]] ===
True. Of course this does not make a difference if the function f is
only used inside FindRoot[].

--
Szabolcs

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