Orbits of Second Order Linear Recurrences over Finite Fields

Abstract: Let $Q$ be the matrix $\displaystyle \begin{pmatrix} a & b \ 1 & 0 \end{pmatrix}$ in $GL_2(\mathbb{F}_q)$ where $\mathbb{F}_q$ is a finite field, and let $G$ be the finite cyclic group generated by $Q$. We consider the action of $G$ on the set $\mathbb{F}_q \times \mathbb{F}_q$. In particular, we study certain relationships between the lengths of the non-trivial orbits of $G$, and their frequency of occurrence. This is done in part by investigating the order of elements of a product in an abelian group when the product has prime power order. For $q$ a prime and $b=1$, the orbits correspond to Fibonacci type linear recurrences modulo $q$ for different initial conditions. We also derive certain conditions under which the roots of the characteristic polynomial of $Q$ are generators of $\mathbb{F}_q^\times$. Examples are included to illustrate the theory.