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Some results on the Flynn–Poonen–Schaefer conjecture

Published online by Cambridge University Press:  11 August 2021

Shalom Eliahou
Affiliation:
Laboratoire de Mathématiques Pures et Appliquées Joseph Liouville (LMPA), Université du Littoral Côte d’Opale, UR 2597, F-62100 Calais, France, and CNRS, FR2037, France e-mail: eliahou@univ-littoral.fr
Youssef Fares*
Affiliation:
LAMFA, CNRS-UMR 7352, Université de Picardie, 80039 Amiens, France, and CNRS, FR2037, France

Abstract

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.

Type
Article
Copyright
© Canadian Mathematical Society 2021

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