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MathGroup Archive 2005

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Re: Why are the functions different?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg55652] Re: [mg55609] Why are the functions different?
  • From: Sseziwa Mukasa <mukasa at jeol.com>
  • Date: Fri, 1 Apr 2005 05:36:28 -0500 (EST)
  • References: <200503310624.BAA15126@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

On Mar 31, 2005, at 1:24 AM, Helge Stenstroem wrote:

> How are these two functions (f0 and f1) different? f0 can be plotted,
> but f1 cannot.
>
> f1[x_] := Normal[Series[Tanh[x], {x, 0, 5}]]
> f0[x_] := x - x^3/3 + (2*x^5)/15
>
> When evaluated like this:
>  f0[x]
>  f1[x]
> they look the same.
>
> The following gives error messages if f0 is replaced by f1.
>  Plot[{Tanh[z], f0[z]}, {z, -1, 1}]
>

Because x is evaluated before the Series and Normal in f1.  Redefine f1 
as

f1[y_] := Block[{x}, Normal[Series[Tanh[x], {x, 0, 5}]] /.{x->y}]

and it will work as you expect.

Regards,

Ssezi



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