Increments of Random Partitions
Şerban Nacu · Combinatorics Probability Computing · 2006
For any partition of $\{1, 2, \ldots{,}\, n\}$ we define its increments $X_i, 1 \leq i \leq n$ by $X_i = 1$ if $i$ is the smallest element in the partition block that contains it, $X_i = 0$ otherwise. We prove that for partially exchangeable random partitions (where the probability of a partition depends only on its block sizes in order of appearance), the law of the increments uniquely determines the law of the partition. One consequence is that the Chinese Restaurant Process CRP( $\theta$ ) (the partition with distribution given by the Ewens sampling formula with parameter $\theta$ ) is the only exchangeable random partition with independent increments.