MEASURABLE REALIZATIONS OF ABSTRACT SYSTEMS OF CONGRUENCES
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Published:2020
Issue:
Volume:8
Page:
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ISSN:2050-5094
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Container-title:Forum of Mathematics, Sigma
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language:en
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Short-container-title:Forum of Mathematics, Sigma
Author:
CONLEY CLINTON T.,MARKS ANDREW S.,UNGER SPENCER T.
Abstract
An abstract system of congruences describes a way of partitioning a space into finitely many pieces satisfying certain congruence relations. Examples of abstract systems of congruences include paradoxical decompositions and
$n$
-divisibility of actions. We consider the general question of when there are realizations of abstract systems of congruences satisfying various measurability constraints. We completely characterize which abstract systems of congruences can be realized by nonmeager Baire measurable pieces of the sphere under the action of rotations on the
$2$
-sphere. This answers a question by Wagon. We also construct Borel realizations of abstract systems of congruences for the action of
$\mathsf{PSL}_{2}(\mathbb{Z})$
on
$\mathsf{P}^{1}(\mathbb{R})$
. The combinatorial underpinnings of our proof are certain types of decomposition of Borel graphs into paths. We also use these decompositions to obtain some results about measurable unfriendly colorings.
Publisher
Cambridge University Press (CUP)
Subject
Computational Mathematics,Discrete Mathematics and Combinatorics,Geometry and Topology,Mathematical Physics,Statistics and Probability,Algebra and Number Theory,Theoretical Computer Science,Analysis
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