Yuxuan Yang

This book provides a systematic introduction and treatment of non-integrable flat systems. More precisely, it develops analogues of the well-known Kronecker-Weyl equidistribution theorem for various non-integrable flat systems. The distribution properties of half-infinite geodesics in non-integrable flat dynamical systems are investigated, with particular emphasis on density and uniformity on various flat surfaces, as well as generalizations to higher dimensions and related problems. The approach to addressing some of these uniformity questions combines traditional ergodic theory methods, such as the Birkhoff ergodic theorem, with tools from number theory and new ideas outside the field of ergodic theory. Several non-trivial results on 3-dimensional flat systems are included, expanding upon the extensive literature on 2-dimensional non-integrable flat dynamical systems, which often focuses on plane-specific methods that do not readily adapt to 3-dimensional systems. New approaches are developed to address these challenges. A systematic study of a class of dissipative systems is also presented, where the flow is neither time-reversible nor measure-preserving. The overwhelming majority of the material in this book represents previously unpublished research.

The subject of this monograph is to describe orbits of slowly chaotic motion. The study of geodesic flow on the unit torus is motivated by the irrational rotation sequence, where the most outstanding result is the Kronecker-Weyl equidistribution theorem and its time-quantitative enhancements, including superuniformity. Another important result is the Khinchin density theorem on superdensity, a best possible form of time-quantitative density. The purpose of this monograph is to extend these classical time-quantitative results to some non-integrable flat dynamical systems. The theory of dynamical systems is on the most part about the qualitative behavior of typical orbits and not about individual orbits. Thus, our study deviates from, and indeed is in complete contrast to, what is considered the mainstream research in dynamical systems. We establish non-trivial results concerning explicit individual orbits and describe their long-term behavior in a precise time-quantitative way. Our non-ergodic approach gives rise to a few new methods. These are based on a combination of ideas in combinatorics, number theory, geometry and linear algebra. Approximately half of this monograph is devoted to a time-quantitative study of two concrete simple non-integrable flat dynamical systems. The first concerns billiard in the L-shape region which is equivalent to geodesic flow on the L-surface. The second concerns geodesic flow on the surface of the unit cube. In each, we give a complete description of time-quantitative equidistribution for every geodesic with a quadratic irrational slope.