Invariant Rational Functions, Linear Fractional Transformations and Irreducible Polynomials over Finite Fields

For a subgroup of $PGL(2,q)$ we show how some irreducible polynomials over ${\mathbb{F}}_q$ arise from the field of invariant rational functions. The proofs rely on two actions of $PGL(2,F)$, one on the projective line over a field $F$ and the other on the rational function field $F(x)$. The invariant functions in $F(x)$ are used to show that regular patterns exist in the factorization of certain polynomials into irreducible polynomials. We use some results about group actions and the orbit polynomial, whose proofs are included. An unusual connection to the conjugacy classes of $PGL(2,q)$ is shown. At the end of the paper we present an alternative approach, using Lang’s theorem on algebraic groups.