Some subgroups of a finite field and their applications for obtaining explicit factors

Let $\mathcal{S}_q$ denote the group of all square elements in the multiplicative group $\mathbb{F}_q^*$ of a finite field $\mathbb{F}_q$ of odd characteristic containing $q$ elements. Let $\mathcal{O}_q$ be the set of all odd order elements of $\mathbb{F}_q^*$. Then $\mathcal{O}_q$ turns up as a subgroup of $\mathcal{S}_q$. In this paper, we show that $\mathcal{O}_q=\langle4\rangle$ if $q=2t+1$ and, $\mathcal{O}_q=\langle t\rangle$ if $q=4t+1$, where $q$ and $t$ are odd primes. This paper also gives a direct method for obtaining the coefficients of irreducible factors of $x^{2^nt}-1$ in $\mathbb{F}_q[x]$ using the information of generator elements of $\mathcal{S}_q$ and $\mathcal{O}_q$, when $q$ and $t$ are odd primes such that $q=2t+1$ or $q=4t+1$.