Unable to find functional inverse of power function

for example, I have a function , and the inverse function of x is : or it can be written to

But when I use MATLAB, it throws a warning: Unable to find functional inverse.

code:

syms x theta
f = x^theta;
g = finverse(f, x);

Answer 1

[David Hill]

There is an inverse for some theta's but most theta's will produce imaginary results for negative values of x.

syms x 
f=x.^3;
g = finverse(f);%this works but other values of theta will not previde a pure inverse function

[Jun Liang]

thanks, Does this mean that finverse is invalid when imaginary values may appear in the function?