Deriving average productivity from a CES production function Following the work of Raurich et al. (2012) I got stuck trying to derive the average productivity starting from the following CES production function:

$$Y=A\left [ \alpha K^{\frac{\sigma -1}{\sigma }}+(1-\alpha)(AL)^{\frac{\sigma -1}{\sigma }} \right ]^{\frac{\sigma}{\sigma-1 }}$$

The result they get is: 
$$\left (\frac{Y}{AL}  \right )^{\frac{1-\sigma }{\sigma }}=\frac{1}{\alpha \left ( \frac{K}{AL} \right )^{\frac{\sigma-1 }{\sigma }}+(1-\alpha )}$$

Is there anyone who can help me with this?
This is the solution to your difficulty. By the way, which text are you following?