  Let $\mathbb{F}_q$ be the finite field with $q$ elements, where $q$ is a
power of a prime $p$. Recently, a particular action of the group
$\mathrm{GL}_2(\mathbb F_q)$ on irreducible polynomials in $\mathbb F_q[x]$ has
been introduced and many questions concerning the invariant polynomials have
been discussed. In this paper, we give a natural extension of this action on
the polynomial ring $\mathbb F_q[x_1, \ldots, x_n]$ and study the algebraic
properties of the invariant elements.
