The Monogenicity of Power-Compositional Characteristic Polynomials

Let $f(x)\in {\mathbb Z}[x]$ be monic of degree $N\ge 2$. Suppose that $f(x)$ is monogenic, and that $f(x)$ is the characteristic polynomial of the $N$th order linear recurrence sequence $\Upsilon_f:=(U_n)_{n\ge 0}$ with initial conditions \[U_0=U_1=\cdots =U_{N-2}=0 \quad \mbox{and} \quad U_{N-1}=1.\] Let $p$ be a prime such that $f(x)$ is irreducible over ${\mathbb F}_p$ and $f(x^p)$ is irreducible over ${\mathbb Q}$. We prove that $f(x^p)$ is monogenic if and only if $\pi(p^2)\ne \pi(p)$, where $\pi(m)$ denotes the period of $\Upsilon_f$ modulo $m$. These results extend previous work of the author, and provide a new and simple test for the monogenicity of $f(x^p)$. We also provide some infinite families of such polynomials.