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Borel Combinatorics of Abelian Group Actions (2401.13866v1)

Published 25 Jan 2024 in math.LO and math.CO

Abstract: We study the free part of the Bernoulli action of $\mathbb{Z}n$ for $n\geq 2$ and the Borel combinatorics of the associated Schreier graphs. We construct orthogonal decompositions of the spaces into marker sets with various additional properties. In general, for Borel graphs $\Gamma$ admitting weakly orthogonal decompositions, we show that $\chi_B(\Gamma)\leq 2\chi(\Gamma)-1$ under some mild assumptions. As a consequence, we deduce that the Borel chromatic number for $F(2{\mathbb{Z}n})$ is $3$ for all $n\geq 2$. Weakly orthogonal decompositions also give rise to Borel unlayered toast structures. We also construct orthogonal decompositions of $F(2{\mathbb{Z}2})$ with strong topological regularity, in particular with all atoms homeomorphic to a disk. This allows us to show that there is a Borel perfect matching for $F(2{\mathbb{Z}n})$ for all $n\geq 2$ and that there is a Borel lining of $F(2{\mathbb{Z}2})$.

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