Let $p$, $l$ be prime numbers, $n$ be positive integer. Find the number of irreducible element in $\mathbb {F_p}[x]$ with degree of $l^n$.
I have got the Gauss's formula $\dfrac{1}{n} *\displaystyle\sum_{d|n} µ(n/d)q^d$. But is there any easier method working out this problem?
Thank you!