compute cubic root with negative argument

[Christopher Kappe <kappe@...>]

Hello,

I just noticed that it is not possible to compute the cubic root of a 
negative number. See e.g.
print( math.pow( -27, 1/3 ) ) --> -nan instead of -3
The same is true for the ^ operator (not surprising).

This probably stems from the fact that the result of the n-th root of a 
negative number is not real if n is even (e.g. sqrt(-1) = (-1)^(1/2) = 
i). However, (-3)^3 = -27 and likewise should hold that (-27)^(1/3) = -3.
This works by the way with Google (try googling f(x) = x^(1/3) and you 
will see the graph of the function also for the negative x-axis).

If such a correct behavior is not easy to implement in the generic 
math.pow function, I would suggest offering a math.cbrt function as 
exists e.g. in C, C++ or Octave.

The least one should do, is document the fact that exponentiation only 
works for a non-negative base argument.

Regards,
Christopher