Let X be a series of negative numbers. Then the command

series Y = X^(1/3)

(or any other "1/oddnumber" fractional power)

returns a "Warning: pow: Domain error" message, overwrites series "Y" and returns a "no valid values" if one attempts to view the series.

Why is this happening, considering that X^(1/3) is a real number even if X is a negative number?

Now, consider series "Z" which has both positive and negative values. We want to compute

W = Z^(1/3)

Given a recent new feature in gretl, the sign function  sgn(), I guess we can avoid the obvious conditional if-else statement, and compute W  by writing

W = sgn(Z) * (abs(Z)^(1/3))

that covers also the case of Z taking an exact zero value.

Is this the proper/efficient way?

-- 
Alecos Papadopoulos PhD
Athens University of Economics and Business
web: alecospapadopoulos.wordpress.com/
skype:alecos.papadopoulos