Fermats Little Theorem

$x^{n-1}\equiv 1\mathrm{mod}n$

n : Prime no.
x : +ve integer not divisible by n ($x\not\equiv \mathrm{mod}n$)

We get this from the fact that Φ(n) = n -1 when n is prime

$x^{\Phi (n)+1}\equiv x\mathrm{mod}n$ when GCD(x, n) = 1
$x^n\equiv x\mathrm{mod}n$