Daily Archives: December 8, 2019

2019-22 Prime divisors of polynomial iterates

Let \(f = X^n + a_{n-1}X^{n-1} + \dots + a_0\in \mathbb{Z}[X]\) be a polynomial with integer coefficients, and let \(m\in \mathbb{Z}\).

Consider the sequence \[f_0,f_1,f_2,\dots \]

where \(f_0:=m\), and \(f_i:=f(f_{i-1})\) for all \(i\ge 1\).

Let \(S:=\{p\in \mathbb{P}: p \text{ divides } f_i \text{ for some } i\ge 0\}\) be the set of prime divisors of the sequence \(f_0,f_1,f_2,\dots\). 

Assume that \(S\) is finite, but \(\{f_i\mid i\ge 0\}\) is infinite. Show that \(f=X^n\). 

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