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MathGroup Archive 2004

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Re: Problem with eval. of neg. cube root of neg. #

  • To: mathgroup at smc.vnet.net
  • Subject: [mg49920] Re: [mg49894] Problem with eval. of neg. cube root of neg. #
  • From: Murray Eisenberg <murray at math.umass.edu>
  • Date: Fri, 6 Aug 2004 03:09:37 -0400 (EDT)
  • Organization: Mathematics & Statistics, Univ. of Mass./Amherst
  • References: <200408051321.JAA05879@smc.vnet.net>
  • Reply-to: murray at math.umass.edu
  • Sender: owner-wri-mathgroup at wolfram.com

Try

   Needs["Miscellaneous`RealOnly`"]
   Plot[x^(1/3)*(x + 4), {x, -10, 10}]

and see, e.g.,

  http://hilbert.math.hr/arhive/mathgroup/1999/01/0051.html

for an explanation.


Josh wrote:
> I am having trouble plotting the following function:
> 
> Plot[x^(1/3)*(x + 4), {x, -10, 10}]
> 
> Mathematica won't plot this function for negative x, although it is
> obviously defined for negative x. It seems to be evaluating the
> negative part of this function to imaginary numbers for some odd
> reason.If I do:
> 
> f[x_] := (x^(1/3))*(x + 4)
> 
> and then:
> 
> f[-5] // N
> 
> I get:
> 
> -0.854988 - 1.48088 \[ImaginaryI]
> 
> when the correct answer is the negative cube root of negative 5, which
> is approximately - (-1.70998) = 1.70998
> 
> I can send a copy of the notebook that shows where this is happening
> to anyone who requests it...
> 
> Can anyone explain what is going on here? Is this a bug or am I
> missing something?
> 
> Thanks in advance for any help ...
> 
> 

-- 
Murray Eisenberg                     murray at math.umass.edu
Mathematics & Statistics Dept.
Lederle Graduate Research Tower      phone 413 549-1020 (H)
University of Massachusetts                413 545-2859 (W)
710 North Pleasant Street            fax   413 545-1801
Amherst, MA 01003-9305


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