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MathGroup Archive 2002

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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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