Rigorous Shadowing of Numerical Solutions of Ordinary Differential Equations by Containment Wayne Brian Hayes
Doctor of Philosophy Graduate Department of Computer Science University of Toronto 2001An exact trajectory of a dynamical system lying close to a numerical trajectory is called ashadow. We present a general-purpose method for proving the existence of finite-time shadows of numerical ODE integrations of arbitrary dimension in which some measure of hyperbolicity is present and there is either 0 or 1 expanding modes, or 0 or 1 contracting modes. Much of the rigor is provided automatically by interval arithmetic and validated ODE integration software that is freely available. The method is a generalization of a previously published containment process that was applicable only to two-dimensional maps. We extend it to handle maps of arbitrary dimension with the above restrictions, and finally to ODEs. The method involves building n-cubes around each point of the discrete numerical trajectory through which the shadow is guaranteed to pass at appropriate times. The proof consists of two steps: first, the rigorous computational verification of an inductive containment property; and second, a simple geometric argument showing that this property implies the existence of a shadow. The computational step is almost entirely automated and easily adaptable to any ODE problem.The method allows for the rescaling of time, which is a necessary ingredient for successfully shadowing ODEs. Finally, the method is local, in the sense that it builds the shadow inductively, requiring information only from the most recent integration step, rather than more global information typical of several other methods. The method produces shadows of comparable length and distance to all currently published results.iii
AcknowledgementsOur strength as a species comes from our ability to communicate with each other. Very few feats, scholarly or otherwise, can be accomplished in a vacuum. Without the ideas, help, and challenges from professional collegues, and without the warmth, care, and compassion of friends and family, our work would be non-existent or meaningless.On the professional side, I thank my committee. Their doors were always open, both for professional consultation, and for occasional personal discussion. The guidance of my supervisor, Ken Jackson, both around obstacles and out of dead ends, is much appreciated. On many occasions Wayne Enright provided much-needed guidance when my understanding of accuracy and stability issues went awry, and his keen eye for precise use of terms tightened my presentation in several places in the thesis. Tom Fairgrieve's experience with nonlinear chaotic systems helped me to view the larger picture, and his probing questions always made me think carefully about what was important. Ted Shepherd's practical experience in numerical techniques for physics problems and deep understanding of classical mechanics provided me with an even wider view than I would have ever thought possible. I cannot t...