We explore the properties of nonpiecewise syndetic sets with positive upper density,
which we call
discordant, in countably infinite amenable (semi-)groups. Sets of this
kind are involved in many questions of Ramsey theory and manifest the
difference in complexity between the classical van der Waerden’s theorem
and Szemerédi’s theorem. We generalize and unify old constructions and
obtain new results about these historically interesting sets. Along the way,
we draw from various corners of mathematics, including classical Ramsey
theory, ergodic theory, number theory, and topological and symbolic dynamics.
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