# [R] cube root

David Winsemius dwinsemius at comcast.net
Sun Apr 17 10:25:40 CEST 2011

```Hey :)

In addition, although (-2)^3 = -8, (-8)^(1/3) != -2, mathematically. A
fractional power of a negative number is complex.

_Not_ in addition. The citation of FAQ 7.33 did not address the correct
issue since the sequence was enclosed in parens. Yours is the only correct
of negative numbers one can get further approaches by searching the
archives.

--
David.

And this can be
obtained properly in R:

> (-8+0i)^(1/3)
[1] 1+1.732051i

Cheers,

Tsjerk

>> This is some interesting:
>>
>>> -8^(1/3)
>>
>> [1] -2
>>
>>> x=(-8:8)
>>
>>> y=x^(1/3)
>>
>>> y
>>
>>  [1]      NaN      NaN      NaN      NaN      NaN      NaN      NaN
>> NaN
>> 0.000000 1.000000
>>
>> [11] 1.259921 1.442250 1.587401 1.709976 1.817121 1.912931 2.000000
>> So, can anybody explain this?! (Why is x[1]^(1/3)=y[1]=NaN, but
>> -8^(1/3)=-2?)
>>
>
> Operator precedence.
>
> See R FAQ 7.33 Why are powers of negative numbers wrong?
>
>
> Berend
>

Tsjerk Wassenaar wrote:
>
> Hey :)
>
> In addition, although (-2)^3 = -8, (-8)^(1/3) != -2, mathematically. A
> fractional power of a negative number is complex. And this can be
> obtained properly in R:
>
>> (-8+0i)^(1/3)
> [1] 1+1.732051i
>
> Cheers,
>
> Tsjerk
>
>>> This is some interesting:
>>>
>>>> -8^(1/3)
>>>
>>> [1] -2
>>>
>>>> x=(-8:8)
>>>
>>>> y=x^(1/3)
>>>
>>>> y
>>>
>>>  [1]      NaN      NaN      NaN      NaN      NaN      NaN      NaN
>>> NaN
>>> 0.000000 1.000000
>>>
>>> [11] 1.259921 1.442250 1.587401 1.709976 1.817121 1.912931 2.000000
>>> So, can anybody explain this?! (Why is x[1]^(1/3)=y[1]=NaN, but
>>> -8^(1/3)=-2?)
>>>
>>
>> Operator precedence.
>>
>> See R FAQ 7.33 Why are powers of negative numbers wrong?
>>
>>
>> Berend
>>
>>
>> --
>> View this message in context:
>> http://r.789695.n4.nabble.com/cube-root-tp3455020p3455027.html
>> Sent from the R help mailing list archive at Nabble.com.
>>
>> ______________________________________________
>> R-help at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>
>
>
> --
> Tsjerk A. Wassenaar, Ph.D.
>
> post-doctoral researcher
> Molecular Dynamics Group
> * Groningen Institute for Biomolecular Research and Biotechnology
> * Zernike Institute for Advanced Materials
> University of Groningen
> The Netherlands
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

Tsjerk Wassenaar wrote:
>
> Hey :)
>
> In addition, although (-2)^3 = -8, (-8)^(1/3) != -2, mathematically. A
> fractional power of a negative number is complex. And this can be
> obtained properly in R:
>
>> (-8+0i)^(1/3)
> [1] 1+1.732051i
>
> Cheers,
>
> Tsjerk
>
>>> This is some interesting:
>>>
>>>> -8^(1/3)
>>>
>>> [1] -2
>>>
>>>> x=(-8:8)
>>>
>>>> y=x^(1/3)
>>>
>>>> y
>>>
>>>  [1]      NaN      NaN      NaN      NaN      NaN      NaN      NaN
>>> NaN
>>> 0.000000 1.000000
>>>
>>> [11] 1.259921 1.442250 1.587401 1.709976 1.817121 1.912931 2.000000
>>> So, can anybody explain this?! (Why is x[1]^(1/3)=y[1]=NaN, but
>>> -8^(1/3)=-2?)
>>>
>>
>> Operator precedence.
>>
>> See R FAQ 7.33 Why are powers of negative numbers wrong?
>>
>>
>> Berend
>>
>>
>> --
>> View this message in context:
>> http://r.789695.n4.nabble.com/cube-root-tp3455020p3455027.html
>> Sent from the R help mailing list archive at Nabble.com.
>>
>> ______________________________________________
>> R-help at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>
>
>
> --
> Tsjerk A. Wassenaar, Ph.D.
>
> post-doctoral researcher
> Molecular Dynamics Group
> * Groningen Institute for Biomolecular Research and Biotechnology
> * Zernike Institute for Advanced Materials
> University of Groningen
> The Netherlands
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help