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arXiv · 0809.4484

Llull and Copeland Voting Computationally Resist Bribery and Control

Piotr Faliszewski·Edith Hemaspaandra·Lane A. Hemaspaandra·Joerg Rothe
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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.

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Piotr Faliszewski, Edith Hemaspaandra, Lane A. Hemaspaandra, Joerg Rothe. 2008-09-28. Llull and Copeland Voting Computationally Resist Bribery and Control. https://arxiv.org/abs/0809.4484

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