Llull and Copeland Voting Computationally Resist Bribery and Control

Abstract

The only systems previously known to be resistant to all the standard control types were highly artificial election systems created by hybridization. We study a parameterized version of Copeland voting, denoted by Copeland^α, where the parameter αis a rational number between 0 and 1 that specifies how ties are valued in the pairwise comparisons of candidates. We prove that Copeland^{0.5}, the system commonly referred to as "Copeland voting," provides full resistance to constructive control, and we prove the same for Copeland^α, for all rational α, 0 < α< 1. Copeland voting is the first natural election system proven to have full resistance to constructive control. We also prove that both Copeland^1 (Llull elections) and Copeland^0 are resistant to all standard types of constructive control other than one variant of addition of candidates. Moreover, we show that for each rational α, 0 \leq α\leq 1, Copeland^αvoting is fully resistant to bribery attacks, and we establish fixed-parameter tractability of bounded-case control for Copeland^α. We also study Copeland^αelections under more flexible models such as microbribery and extended control and we integrate the potential irrationality of voter preferences into many of our results.

This 2008/9/28 version is the same as both the 2008/9/25 version at arxiv.org and the 2008/9/25 revision of URCS TR-2008-933, except the present version corrects a minor typo in the penultimate paragraph of Section 3