A dynamical analogue of a question of fermat

Official URL: https://dx.doi.org/10.7146/math.scand.a-142342

Given a quadratic polynomial with rational coefficients, we investigate the existence of consecutive squares in the orbit of a rational point under the iteration of the polynomial. We display three different constructions of 1-parameter quadratic polynomials with orbits containing three consecutive squares. İn addition, we show that there exists at least one polynomial of the form x2 + c with a rational point whose orbit under this map contains four consecutive squares. This can be viewed as a dynamical analogue of a question of Fermat on rational squares in arithmetic progression. Finally, assuming a standard conjecture on exact periods of periodic points of quadratic polynomials over the rational field, we give necessary and sufficient conditions under which the orbit of a periodic point contains only rational squares.