       Re: Why is this so?

• To: mathgroup at smc.vnet.net
• Subject: [mg15311] Re: [mg15304] Why is this so?
• From: "Kevin J. McCann" <kevinmccann at Home.com>
• Date: Fri, 8 Jan 1999 04:15:04 -0500
• Sender: owner-wri-mathgroup at wolfram.com

```Yes, but that is not the only possible answer.  Try

N[(-125)^(1/3)]

and you get

2.5+4.33013 I

which is why Plot gets upset.

E.g.

NRoots[x^3 == -125,x]

gives

x==-5.||x==2.5 - 4.33013 I||x==2.5 +4.33013 I

Mathematica picks the one it does, I suspect, because it is the first
one you come to in the complex plane if you start at the x-axis (phi=0)
and crank around CCW.

Kevin

which is one of the 3 roots.
-----Original Message-----
From: Chester Lin <chester at nicco.sscnet.ucla.edu> To:
mathgroup at smc.vnet.net
Subject: [mg15311] [mg15304] Why is this so?

>The following in/out does not make sense to me:
>
>Clear[f, x]
>f[x_] := x^(1/3)
>Plot[f[x], {x, -125, 125}]
>
>Plot::plnr : f[x] is not a machine-size real number at x = -125..
>Plot::plnr : f[x] is not a machine-size real number at x = -114.858.
>Plot::plnr : f[x] is not a machine-size real number at x = -103.798.
>General::stop :
> Further output of Plot::plnr will be suppressed during this
>calculation.
>
>Isn't it true that (-125)^(1/3) == -5?
>
>Why do I get this strange result?
>
>I am using Mathematica 3.01 for Students on Macintosh.
>
>Thanks for any info.
>
>Chester Lin
>chester at nicco.sscnet.ucla.edu
>
>
>

```

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