  Let $f(x) \in \bbz[x]$ and consider the index divisibility set $D = \{n \in
\bbn : n \mid f^n(0)\}$. We present a number of properties of $D$ in the case
that $(f^n(0))_{n=1}^\infty$ is a rigid divisibility sequence, generalizing a
number of results of Chen, Stange, and the first author. We then study the
polynomial $x^d + x^e + c \in \bbz[x]$, where $d > e \ge 2$ and determine all
cases where this map has a finite index divisibility set.

Number Theory