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Re: Gross Bug in Simplify
*To*: mathgroup at smc.vnet.net
*Subject*: [mg32598] Re: [mg32585] Gross Bug in Simplify
*From*: Andrzej Kozlowski <andrzej at platon.c.u-tokyo.ac.jp>
*Date*: Thu, 31 Jan 2002 01:45:21 -0500 (EST)
*Sender*: owner-wri-mathgroup at wolfram.com
I can't see why you claim this is a bug and that it is caused by
> Simplify misparses expressions m + n f[__], where m and n are
> numeric, as (m+n)
> f[__]
It seems to me that what you are seeing is just a special case of the
following:
In[1]:=
z/:z^v_=z;
In[2]:=
Simplify[1-z]
Out[2]=
0
This seems to me entirely correct, since z==1 is the only complex number
with the property that z^(anything)==z. Thus it would appear that your
function f[z__] ought to have the value 1 for all arguments. This is
consistent with all your outputs. Maybe I am missing your point, but
mathematically at least there appears to be nothing wrong here.
On Wednesday, January 30, 2002, at 05:19 PM, Alan Mason wrote:
> There is a destructive bug in Mathematica (v4.1, win2000) in which
> Simplify
> misparses expressions m + n f[__], where m and n are numeric, as (m+n)
> f[__]. One can only wonder how many other rule combinations, with or
> without Simplify, are also faulty.
> The following example is a stripped-down version of some rules for f =
> Kronecker delta, which explains why the upvalue rule is needed. It is
> specifically this rule that seems to be causing the problem with
> Simplify.
> However, it's not the upvalue itself -- the problem remains if Power is
> Unprotected and the rule is replaced by Power[f[u__], v_] :> f[u].
>
> In[1]:=
> Clear[f];
> Simplify[1-f[4]]
> f[u__]^v_ ^:= f[u];
> Simplify[{1-f[4], 1 - 7 f[4], 1 + f[4], 1 - x f[4]}]
>
> Out[2]=
> 1-f[4]
>
> Out[4]=
> {0,-6 f[4],2 f[4],1-x f[4]}
>
> Doesn't WRI test all its built-in functions by running special
> diagnostic
> code on them round the clock? It's hard to imagine how a bug like this
> could have gone undetected!
>
> Alan
>
>
>
>
Andrzej Kozlowski
Toyama International University
JAPAN
http://platon.c.u-tokyo.ac.jp/andrzej/
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