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Re: FullSimplify again ...

  • To: mathgroup at
  • Subject: [mg59160] Re: [mg59138] FullSimplify again ...
  • From: Andrzej Kozlowski <akoz at>
  • Date: Sun, 31 Jul 2005 01:30:36 -0400 (EDT)
  • References: <>
  • Sender: owner-wri-mathgroup at

On 30 Jul 2005, at 07:25, Detlef Müller wrote:

> Hello.
> I happened to type the following lines:
> In[15]:= f=FullSimplify[Product[(5-i)/5,{i,1,n}]]
> Out[15]= 0
> In[17]:= Product[(5-i)/5,{i,1,n}]/.(n->4)
> Out[17]= 24/625
> Strange, isn't it?
> Version Number:
> Platform: X
> Greetings,
>    Detlef

Well, I suspect this is a "legacy bug", dating to the way Mathematica  
used to work before the Assumptions mechanism was introduced. The  
point is:
what should FullSimplify do with

Pochhammer[-4, n]?

(of course the same holds for other negative integers instead of -4)

For n <=4 this non zero and for n>4 it is 0.

FullSimplify, however, always gives



Before the assumption mechanism was indroduced in v.4 of Mathematica,  
Simplify or FullSimplify would often introduce their own implicit  
assumptions, presumably the ones that the user was "most likely" to  
have intended. This was not entirely unreasonable in those days:  
after all one woudl expect that a sensible user would not ask  
Mathematica to evaluate something wanting to get his own input  
returned back to him! Since the assumptions mechanism was introduced  
this behaviour was largely eliminated but it seems that some cases  
managed to escape scrutiny and survived.

Andrzej Kozlowski

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