Newtonian dynamics in the plane corresponding to straight and cyclic motions on the hyperelliptic curve : Ergodicity, isochrony and fractals

We study the complexification of the one-dimensional Newtonian particle in a monomial potential. We discuss two classes of motions on the associated Riemann surface: the rectilinear and the cyclic motions, corresponding to two different classes of real and autonomous Newtonian dynamics in the plane. The rectilinear motion has been studied in a number of papers, while the cyclic motion is much less understood. For small data, the cyclic time trajectories lead to isochronous dynamics. For bigger data the situation is quite complicated; computer experiments show that, for sufficiently small degree of the monomial, the motion is generically isochronous with integer period, which depends in a quite sensitive way on the initial data. If the degree of the monomial is sufficiently high, computer experiments show essentially chaotic behavior. We suggest a possible theoretical explanation of these different behaviors. We also introduce a two-parameter family of two-dimensional mappings, describing the motion of the center of the circle, as a convenient representation of the cyclic dynamics; we call such a mapping the center map. Computer experiments for the center map show a typical multifractal behavior with periodicity islands. Therefore the above complexification procedure generates dynamics amenable to analytic treatment and possessing a high degree of complexity.