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Published online by Cambridge University Press: 11 August 2021
For $c \in \mathbb {Q}$, consider the quadratic polynomial map
$\varphi _c(z)=z^2-c$. Flynn, Poonen, and Schaefer conjectured in 1997 that no rational cycle of
$\varphi _c$ under iteration has length more than
$3$. Here, we discuss this conjecture using arithmetic and combinatorial means, leading to three main results. First, we show that if
$\varphi _c$ admits a rational cycle of length
$n \ge 3$, then the denominator of c must be divisible by
$16$. We then provide an upper bound on the number of periodic rational points of
$\varphi _c$ in terms of the number s of distinct prime factors of the denominator of c. Finally, we show that the Flynn–Poonen–Schaefer conjecture holds for
$\varphi _c$ if
$s \le 2$, i.e., if the denominator of c has at most two distinct prime factors.