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Re: "Assuming"

  • To: mathgroup at smc.vnet.net
  • Subject: [mg85622] Re: "Assuming"
  • From: David Bailey <dave at Remove_Thisdbailey.co.uk>
  • Date: Fri, 15 Feb 2008 06:52:58 -0500 (EST)
  • References: <fp0m11$5u7$1@smc.vnet.net> <fp192b$gju$1@smc.vnet.net> <fp2khg$pka$1@smc.vnet.net>

> 
> Compare the following:
> 
> In[8]:= Assuming[x == 0, {x/x^2, x/x, x^2/x}]
> Out[8]= {1/x, 1, x}
> 
> In[9]:= x = 0; {x/x^2, x/x, x^2/x}
> Out[9]= {Indeterminate, Indeterminate, Indeterminate}
> 

Assuming supplies conditions for use by a Simplify (or similar) 
operations within it. It does not initiate a Simplify operation itself. 
Compare for example:

Assuming[x == 1, Simplify[x/x^2]]

Assuming[x == 1, x/x^2]

More generally, I think if Simplify[(a^2-1)(a-1)] returned a conditional 
expression (or did not evaluate at all, as in Simplify[Sqrt[x^2]] ), it 
would be practically useless in more complicated cases - A CAS is meant 
to be a practical tool.

David Bailey
http://www.dbaileyconsultancy.co.uk


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