RE: What does power operator ^ returns?

[Richardson, Anthony]

There may be a more efficient way, but 

  sign(A) .* abs(A)  .^ (1/3)

will give you what you want when A is a matrix.  
(This is the same cube root most calculators return.)

Tony Richardson

-----Original Message-----
From: Guido Walter Pettinari [mailto:address@hidden 
Sent: Monday, October 19, 2009 11:42 AM
To: address@hidden
Subject: What does power operator ^ returns?

Hi all!

I need to compute the cubic root of many negative numbers. My problem  
is real-valued, hence I am not interested in the imaginary solutions.  
However, if I use the power operator '^', Octave returns just the  
first imaginary solution. The same applies when doing any odd root.

Example:
octave> a=-8
a = -8
octave> a^(1/3)
ans = 1.000000000000000e+00 + 1.732050807568877e+00i

How can I make octave to return the real solution, which in my example  
is just -2? I do not want just the absolute value (i.e. abs(a^(1/3)),  
since it does not preserve the sign.  I could do with a check on the  
sign, but it would be inefficient.

I am using Octave 3.2.3 on Mac Os X 10.6.

Thank you very much,

Guido
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