Dynamically syndetic sets and the combinatorics of syndetic, idempotent filters (2408.12785v2)

Abstract: A subset of the positive integers is dynamically central syndetic if it contains the set of times that a point returns to a neighborhood of itself under a minimal transformation of a compact metric space. These sets are part of the highly-influential link between dynamics and combinatorics forged by Furstenberg and Weiss in the 1970's. Our main result is a characterization of dynamically central syndetic sets as precisely those sets that belong to syndetic, idempotent filters. Idempotent filters are combinatorial objects that abound in ergodic Ramsey theory but have been largely unnoticed and unexplored. We develop the algebra of these objects for the proof of the main theorem and with an eye toward future applications. The main result is best contextualized as a "global" analogue to the "local" characterization of Furstenberg's central sets as members of minimal, idempotent ultrafilters. It leads to a dual characterization of sets of topological pointwise recurrence, allowing us to answer a question of Glasner, Tsankov, Weiss, and Zucker. We draw numerous striking contrasts between pointwise recurrence and set recurrence, a topic with a long history in the subject and its applications, and answer four questions posed by Host, Kra, and Maass. We also show that the intersection of a dynamically central syndetic set with a set of pointwise recurrence must be piecewise syndetic, generalizing results of Dong, Glasner, Huang, Shao, Weiss, and Ye.