Recurrence in topological dynamics

the long version

A recurrent point for the dynamical system of a map is a point whose orbit returns arbitrarily close to its initial position infinitely often. Different sorts of recurrence can be described by keeping track of just how frequently these returns occur. By considering the case when these return time sets are in some family of special subsets of the natural numbers - for example, the syndetic sets or IP sets - we obtain the concept of recurrence associated with the family. This groundbreaking volume is the first to elaborate the theory of set families as a tool for studying the phenomenon of recurrence. Though the theory is implicit in such seminal works as Hillel Furstenberg's Recurrence in Ergodic Theory and Combinatorial Number Theory (Princeton University Press, 1981), Ethan Akin's study expands upon the theory in detail and explicitly describes this useful piece of mathematical folklore.