Hi all,

The question concerns que same subject as my previous post in this section, but I still didn't figure it out:

Let p be a prime number and a be an integer such that p does not divide a. Show that if $\displaystyle p > 2$ then $\displaystyle a^{\frac{p-1}{2}} \equiv 1 \ \ (mod \ p)$ or $\displaystyle a^{\frac{p-1}{2}} \equiv -1 \ \ (mod \ p)$.

Any tips or approach suggestions will be useful.

Thanks in advance,