Re: Just another Mathematica "Gotcha"

• To: mathgroup at smc.vnet.net
• Subject: [mg120787] Re: Just another Mathematica "Gotcha"
• From: James Stein <mathgroup at stein.org>
• Date: Wed, 10 Aug 2011 06:46:02 -0400 (EDT)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com
• References: <201108091119.HAA15770@smc.vnet.net>

```By normal rules or ordinary interpretations, one does not expect these two
expressions to evaluate to each other:
h(g(f(y,z)))
g[h(f(y,z)))
If you accept a slight abuse of notation,
the above pair is equivalent to following pair:
f(y,z) // g // h
f(y,z) // h // g
which are pretty much what what you have written
(where 'f' is 'Series', 'g' is 'Normal', and 'h' is 'ReplaceAll', etc.).

On Tue, Aug 9, 2011 at 4:19 AM, AES <siegman at stanford.edu> wrote:

> Seems as if the following two expression should yield the same output
> -- seems that way to me anyway -- but they don't.  I'll hide the
> actual outputs down below so Mathematica gurus (or "ordinary users")
> can make their predictions as to which one does what.
>
> In[1]:= Series[a+(b1+b2)x,{x,0,1}] //Normal /.{b2->0}
>
> In[2]:= Series[a+(b1+b2)x,{x,0,1}] /.{b2->0} //Normal
>
> My conclusions:
>
> 1)  By any normal rules of interpretation or ordinary interpretations
> of these statements, they both should do the same same thing.
>
> 2)  This is just another Mathematica "Gotcha" -- and not a
> particularly forgivable one.
>
>
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> Out[1]= a+(b1+b2) x
>
> Out[2]= a+b1 x
>
>

```

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