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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: [mg49923] RE: [mg49894] Problem with eval. of neg. cube root of neg. #
  • From: "David Park" <djmp at earthlink.net>
  • Date: Fri, 6 Aug 2004 03:09:42 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Josh,

Needs["Miscellaneous`RealOnly`"]

Plot[x^(1/3)*(x + 4), {x, -10, 10}]; 

Or use Ted Ersek's SwitchableRealOnly package from MathSource.

Quit[]
Needs["Miscellaneous`SwitchableRealOnly`"]

RealOnlyOn[]
Plot[x^(1/3)*(x + 4), {x, -10, 10}]; 

David Park
djmp at earthlink.net
http://home.earthlink.net/~djmp/ 

From: Josh [mailto:pootleguard-mathgroup at yahoo.com]
To: mathgroup at smc.vnet.net

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 ...




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