A local-global principle in the dynamics of quadratic polynomials

Abstract: Let $K$ be a number field, $f\in K[x]$ a quadratic polynomial, and $n\in{1,2,3}$. We show that if $f$ has a point of period $n$ in every non-archimedean completion of $K$, then $f$ has a point of period $n$ in $K$. For $n\in{4,5}$ we show that there exist at most finitely many linear conjugacy classes of quadratic polynomials over $K$ for which this local-global principle fails. By considering a stronger form of this principle, we strengthen global results obtained by Morton and Flynn-Poonen-Schaefer in the case $K=\mathbf Q$. More precisely, we show that for every quadratic polynomial $f\in\mathbf Q[x]$ there exist infinitely many primes $p$ such that $f$ does not have a point of period 4 in the $p$-adic field $\mathbf Q_p$. Conditional on knowing all rational points on a particular curve of genus 11, the same result is proved for points of period 5.