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IWOCA 2011: Combinatorial Algorithms pp 287-299

# The Rand and Block Distances of Pairs of Set Partitions

• Frank Ruskey
• Jennifer Woodcock
Conference paper
Part of the Lecture Notes in Computer Science book series (LNCS, volume 7056)

## Abstract

The Rand distance of two set partitions is the number of pairs {x,y} such that there is a block in one partition containing both x and y, but x and y are in different blocks in the other partition. Let R(n,k) denote the number of distinct (unordered) pairs of partitions of n that have Rand distance k. For fixed k we prove that R(n,k) can be expressed as $$\sum_j c_{k,j} {n \choose j} B_{n-j}$$ where c k,j is a non-negative integer and B n is a Bell number. For fixed k we prove that there is a constant K n such that $$R(n,{n \choose 2}-k)$$ can be expressed as a polynomial of degree 2k in n for all n ≥ K n . This polynomial is explicitly determined for 0 ≤ k ≤ 3.

The block distance of two set partitions is the number of elements that are not in common blocks. We give formulae and asymptotics based on N(n), the number of pairs of partitions with no blocks in common. We develop an O(n) algorithm for computing the block distance.

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## Copyright information

© Springer-Verlag Berlin Heidelberg 2011

## Authors and Affiliations

• Frank Ruskey
• 1
• Jennifer Woodcock
• 1
1. 1.Dept. of Computer ScienceUniversity of VictoriaCanada

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