The Complexity of Fully Proportional Representation for Single-Crossing Electorates

TL;DR

This paper investigates the computational complexity of winner determination in single-crossing elections under two proportional representation rules, finding polynomial algorithms for some cases and NP-hardness for others, with special cases allowing efficient solutions.

Contribution

It demonstrates polynomial-time algorithms for Chamberlin--Courant's rule in single-crossing elections and introduces efficient algorithms for the egalitarian Monroe's rule in narcissistic cases.

Findings

Polynomial-time algorithm for Chamberlin--Courant's rule in single-crossing elections.

NP-hardness of Monroe's rule in single-crossing elections.

Efficient algorithm for egalitarian Monroe's rule in narcissistic single-crossing elections.

Abstract

We study the complexity of winner determination in single-crossing elections under two classic fully proportional representation rules---Chamberlin--Courant's rule and Monroe's rule. Winner determination for these rules is known to be NP-hard for unrestricted preferences. We show that for single-crossing preferences this problem admits a polynomial-time algorithm for Chamberlin--Courant's rule, but remains NP-hard for Monroe's rule. Our algorithm for Chamberlin--Courant's rule can be modified to work for elections with bounded single-crossing width. To circumvent the hardness result for Monroe's rule, we consider single-crossing elections that satisfy an additional constraint, namely, ones where each candidate is ranked first by at least one voter (such elections are called narcissistic). For single-crossing narcissistic elections, we provide an efficient algorithm for the egalitarian version of Monroe's rule.