Symmetries of Random Partitions

Abstract

This paper is motivated by a recent result of Pitman and Yakubovich stating that a partially exchangeable, stationary (PES) random partition of $\mathbb{N}$ is exchangeable. This echoes an earlier theorem of Kallenberg on the equivalence of spreadability (contractability) and exchangeability for infinite partitions. We revise the hierarchy of symmetries with a focus on partitions of finite sets $[n]$, and ask about the extent to which these relaxed symmetry types differ from exchangeability, framing the question in terms of the geometry of the polytope of distributions defined by the symmetry constraints. We show that single-orbit exchangeable partitions remain extreme among spreadable and PES distributions, and that $n=5$ is the first case where non-exchangeable extreme partitions occur, causing the associated polytopes to deviate from a simplex structure.