Solved by tom in post #6 
              
              
                Well, -(378**(1/3))is a root, too, so I’d start with that. 
If not, you can precompute -1**(1/3) to (0.5+0.866j) and “outer” multiply 378**(1/3) (or whatever outcome you have by a two-tensor tensor([0.5, 0.866]) if it is negative and tensor([1.0, 0]) if the mean is positive. 
Best regards 
Thomas