Counting the number of $2$-periodic integral points of a discrete dynamical system with applications from arithmetic statistics, IV

Abstract: In this follow-up paper, we inspect a surprising relationship between the set of $2$-periodic points of a polynomial map $\varphi_{d, c}$ defined by $\varphi_{d, c}(z) = z^d + c$ for all $c, z \in \mathbb{Z}$ and the coefficient $c$, where $d>2$ is an integer. As in the previous articles, we again wish to study here counting problems that are inspired by exciting advances of Bhargava-Shankar-Tsimerman and their collaborators on $2$-torsion point-counting in arithmetic statistics, and also by Hutz's conjecture and Panraksa's work on $2$-periodic point-counting in arithmetic dynamics. In doing so, we then first prove that for any given prime $p\geq 3$, the average number of distinct $2$-periodic integral points of any polynomial map $\varphi_{p, c}$ modulo $p$ is either zero or unbounded as $c$ tends to infinity. Furthermore, inspired by a conjecture of Hutz on $2$-periodic rational points of any polynomial map $\varphi_{p-1, c}$ for any given prime $p\geq 5$ in arithmetic dynamics, we then also prove that the average number of distinct $2$-periodic integral points of any $\varphi_{p-1, c}$ modulo $p$ is equal to $1$ or $2$ or $0$ as $c$ tends to infinity. Finally, as in the previous follow-up articles, we then also apply here density and number field-counting results from arithmetic statistics, and consequently obtain again several counting and statistical results on the irreducible monic integer polynomials and on the algebraic number fields arising naturally in our dynamical setting.