Normal bases and irreducible polynomials

Abstract: Let $\mathbb{F}q$ denote the finite field of $q$ elements and $\mathbb{F}{q^n}$ the degree $n$ extension of $\mathbb{F}q$. A normal basis of $\mathbb{F}{q^n}$ over $\mathbb{F} _q$ is a basis of the form ${\alpha,\alpha^q,\dots,\alpha^{q^{n-1}}}$. An irreducible polynomial in $\mathbb{F} _q[x]$ is called an $N$-polynomial if its roots are linearly independent over $\mathbb{F} _q$. Let $p$ be the characteristic of $\mathbb{F} _q$. Pelis et al. showed that every monic irreducible polynomial with degree $n$ and nonzero trace is an $N$-polynomial provided that $n$ is either a power of $p$ or a prime different from $p$ and $q$ is a primitive root modulo $n$. Chang et al. proved that the converse is also true. By comparing the number of $N$-polynomials with that of irreducible polynomials with nonzero traces, we present an alternative treatment to this problem and show that all the results mentioned above can be easily deduced from our main theorem.