Voting on Cyclic Orders, Group Theory, and Ballots

TL;DR

This paper explores voting methods for cyclic orders, using group theory to analyze ballots and aggregation procedures, providing characterizations for small cases and insights into symmetry-respecting voting rules.

Contribution

It applies the representation theory of the symmetric group to analyze and characterize voting procedures on cyclic orders, a novel approach in social choice theory.

Findings

Characterized voting procedures for n=4 cyclic orders

Analyzed symmetry-respecting ballots using group theory

Provided insights for n=5 cases

Abstract

A cyclic order may be thought of informally as a way to seat people around a table, perhaps for a game of chance or for dinner. Given a set of agents such as $\{A,B,C\}$, we can formalize this by defining a cyclic order as a permutation or linear order on this finite set, under the equivalence relation where $A\succ B\succ C$ is identified with both $B\succ C\succ A$ and $C\succ A\succ B$. As with other collections of sets with some structure, we might want to aggregate preferences of a (possibly different) set of voters on the set of possible ways to choose a cyclic order. However, given the combinatorial explosion of the number of full rankings of cyclic orders, one may not wish to use the usual voting machinery. This raises the question of what sort of ballots may be appropriate; a single cyclic order, a set of them, or some other ballot type? Further, there is a natural action of the group of permutations on the set of agents. A reasonable requirement for a choice procedure would be to respect this symmetry (the equivalent of neutrality in normal voting theory). In this paper we will exploit the representation theory of the symmetric group to analyze several natural types of ballots for voting on cyclic orders, and points-based procedures using such ballots. We provide a full characterization of such procedures for two quite different ballot types for $n=4$, along with the most important observations for $n=5$.