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MathGroup Archive 2006

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Re: Is -1^(2/5) really undefined in R?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg69160] Re: Is -1^(2/5) really undefined in R?
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Thu, 31 Aug 2006 04:39:30 -0400 (EDT)
  • Organization: The Open University, Milton Keynes, UK
  • References: <ed0uvq$po$1@smc.vnet.net> <ed3qqi$roj$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Jean-Marc Gulliet wrote:
> Ben wrote:
>> Is -1^(2/5) really undefined in R?
> 
> Yes.

Oops! Silly mistype of mine! Of course the line above should have read 
"No" rather than "Yes". I apologize for any head-scratching...

Jean-Marc

> 
>> Mathematica seems to think so, I guess since it looks like a negative square root, but
>>
>> -1^(2/5) = (-1^2)^(1/5) = 1^(1/5) = 1
>>
>> Is this correct mathematically?
> 
> No. See "College Algebra Tutorial 5: Rational Exponents" at 
> http://www.wtamu.edu/academic/anns/mps/math/mathlab/col_algebra/col_alg_tut5_ratexp.htm
> 
> In[12]:=
> ((-1)^(1/5))^2
> 
> Out[12]=
>      2/5
> (-1)
> 
> In[13]:=
> (-1)^(2/5)==((-1)^(1/5))^2
> 
> Out[13]=
> True
> 
> (-1)^(2/5)==((-1)^2)^(1/5)
> 
> Out[14]=
> False
> 
> HTH,
> Jean-Marc
> 


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