Why matlab return me a complex number

Hi, Why >>(-27)^(1/3) return me a complex number rather than -3??? I am curious about this. Please help.

[David]

A similar post here but no answers. http://www.mathworks.com/matlabcentral/newsreader/view_thread/86495

Answer 1

[Andreas Goser]

From the documentation

doc power

"Note that for a negative value X and a non-integer value Y, if the abs(Y) is less than one, the power function returns the complex roots. To obtain the remaining real roots, use the nthroot function."

[David]

Thanks very much. I also found an useful answer here: http://www.mathworks.com/support/solutions/en/data/1-15M1N/?solution=1-15M1N

Answer 2

[Edu Santos]

If instead you do:

(abs(-27))^(1/3)

you may have the good absolute value with the bad sign.

Storing the sign in a term, for example like this:

-27/abs(-27)

we can arrive to the expected answer by doing:

-27/abs(-27)*(abs(-27))^(1/3)

=-3

A simpler way is to just do

nthroot(-27,3)

=-3

Answer 3

[Azzi Abdelmalek]

% By definition a^n=exp(n*log(a))
% also if a is a positive real number, log(-a)=log(a)+j*pi
a=-27;
n=1/3
out1=exp(n*log(a))
%the same as 
out2=a^n