On Poonen's Conjecture Concerning Rational Preperiodic Points of Quadratic Maps

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The purpose of this note is give some evidence in support of conjectures of Poonen, and Morton and Silverman, on the periods of rational numbers under the iteration of quadratic polynomials. In particular, Poonen conjectured that there are at most 9 periodic points defined over the rational numbers for any map in the family x² + c for c rational. We verify this conjecture for c values up to height 10⁸. For quadratic number fields, we provide evidence that the upper bound on the exact period of Q-rational periodic point is 6.