On Gauss Periods

Abstract

Let $q$ be a prime power, and let $r=nk+1$ be a prime such that $r\nmid q$, where $n$ and $k$ are positive integers. Under a simple condition on $q$, $r$ and $k$, a Gauss period of type $(n,k)$ is a normal element of $\Bbb F_{q^n}$ over $\Bbb F_q$; the complexity of the resulting normal basis of $\Bbb F_{q^n}$ over $\Bbb F_q$ is denoted by $C(n,k;q)$. Recent works determined $C(n,k;q)$ for $k\le 7$ and all qualified $n$ and $q$. In this paper, we show that for any given $k>0$, $C(n,k;q)$ is given by an explicit formula except for finitely many primes $r=nk+1$ and the exceptional primes are easily determined. Moreover, we describe an algorithm that allows one to compute $C(n,k;q)$ for the exceptional primes $r=nk+1$. The numerical results of the paper cover $C(n,k;q)$ for $k\le 20$ and all qualified $n$ and $q$.