Title:

Stability and numerical errors in the Nbody problem

Despite the wide acceptance that errors incurred in numerical solutions to Nbody systems grow exponentially, most research assumes that the statistical results of these systems are reliable. However, if one is to accept that the statistical results of Nbody solutions are reliable, it is important to determine if there are any systematic statistical errors resulting from the incurred growth of errors in individual solutions. In this thesis we consider numerical solutions to the 3body problem in which one of the bodies escapes the system. It is shown for a particular 3body con guration, known as the Sitnikov problem, that the mean lifetime of the system is dependent on the accuracy of the numerical integration. To provide a theoretical explanation of the phenomenon, an approximate Poincar´e map is developed whose dynamics on a particular surface of section is shown to be similar to the dynamics of the Sitnikov Problem. In fact there is a set on which the approximate Poincar´e map is topologically equivalent, like the Sitnikov Problem, to the shift map on the set of biin nite sequences. The structure of the escape regions on the surface of section form a cantor setlike structure whose boundary can more easily be delineated using the approximate Poincar´e map than for the Sitnikov problem. Further it is shown that numerical errors destroy escape regions and can cause orbits to migrate to a region in which escape is faster. Finally, a relationship between the Lyapunov time, tl, and the lifetime, td, of the 3body problem is discussed. Firstly, the Sitnikov problem and the approximate Poincar´e map of the Sitnikov problem both exhibit a twopart power law relationship beween tl and td like that for a particular case of the general 3body problem. Further, it is demonstrated that large perturbations to the energy of the escaping body in uences the relationship between tl and td for small tl. Finally, it is shown that the approximate Poincar´e map yields a theoretical explanation of the phenomenon based on the structure of the escape regions the orbits traverse before escape.
