nth root of matrix

?sign(a)*abs(a)^(1/3)
#????????? [,1]
#[1,] -1.000000
#[2,] -1.259921
#[3,] -1.442250
A.K. 




----- Original Message -----
From: Sachinthaka Abeywardana <sachin.abeywardana at gmail.com>
To: "r-help at r-project.org" <r-help at r-project.org>
Cc: 
Sent: Tuesday, July 2, 2013 11:11 PM
Subject: [R] nth root of matrix

Hi all,

I want to do the following:

a=matrix(c(-1,-2,-3))
a^(1/3) #get 3rd root of numbers[,1]

[1,]? NaN
[2,]? NaN
[3,]? NaN


All I get is NaNs, what is the proper way of doing this? Would like to
retain the fact that it is a matrix if possible (not a requirement
though).


Thanks,

Sachin

??? [[alternative HTML version deleted]]

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Hi all,

I want to do the following:

a=matrix(c(-1,-2,-3))
a^(1/3) #get 3rd root of numbers[,1]

[1,]  NaN
[2,]  NaN
[3,]  NaN


All I get is NaNs, what is the proper way of doing this? Would like to
retain the fact that it is a matrix if possible (not a requirement
though).

?complex

 a=matrix(c(-1+0i,-2+0i,-3+0i))


David Winsemius
Alameda, CA, USA

On Jul 2, 2013, at 8:11 PM, Sachinthaka Abeywardana wrote:

Hi all,

I want to do the following:

a=matrix(c(-1,-2,-3))
a^(1/3) #get 3rd root of numbers[,1]

[1,]  NaN
[2,]  NaN
[3,]  NaN


All I get is NaNs, what is the proper way of doing this? Would like to
retain the fact that it is a matrix if possible (not a requirement
though).

?complex

  a=matrix(c(-1+0i,-2+0i,-3+0i))

I tried that.  The problem is that there are 3 different cube 
roots in the complex plane, and a^(1/3) only gives one of them.  See 
Wikipedia, "roots of unity" or the examples in the help file for 
"newton_raphson {elliptic}".


       I assume that Sachinthaka wants the real roots.  Try the following:


n <- 3 # n must be an odd integer for this to work
a=matrix(c(-1,-2,-3))
as <- sign(a)
ab <- abs(a)
cr <- as*(ab^(1/n))
 > cr
           [,1]
[1,] -1.000000
[2,] -1.259921
[3,] -1.442250
cr^n


       Hope this helps.  Spencer Graves

David Winsemius
Alameda, CA, USA

______________________________________________
R-help at r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.