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

• To: mathgroup at smc.vnet.net
• Subject: [mg49935] Re: [mg49894] Problem with eval. of neg. cube root of neg. #
• From: Bob Hanlon <hanlonr at cox.net>
• Date: Fri, 6 Aug 2004 03:10:02 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

```Plot[Sign[x]*Abs[x]^(1/3)*(x + 4), {x, -10, 10}, PlotRange -> All];

Bob Hanlon

>
> From: pootleguard-mathgroup at yahoo.com (Josh)
To: mathgroup at smc.vnet.net
> Date: 2004/08/05 Thu AM 09:21:47 EDT
> To: mathgroup at smc.vnet.net
> Subject: [mg49935] [mg49894] Problem with eval. of neg. cube root of neg. #
>
> 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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