Copeland voting: ties matter

We study the complexity of manipulation for a family of election systems derived from Copeland voting via introducing a parameter that describes how ties in head-to-head contests are valued. We show that the thus obtained problem of manipulation for unweighted Copeland elections is NP-complete even if the size of the manipulating coalition is limited to two. Our result holds for all rational values of such that 0 < < 1 except for = 1 2 . Since it is well known that manipulation via a single voter is easy for Copeland, our result is the first one where an election system originally known to be vulnerable to manipulation via a single voter is shown to be resistant to manipulation via a coalition of a constant number of voters. We also study the complexity of manipulation for Copeland for the case of a constant number of candidates. We show that here the exact complexity of manipulation often depends closely on the : Depending on whether we try to make our favorite candidate a ...

• Type: Conference
• Year: 2008
• Where: ATAL
• Authors: Piotr Faliszewski, Edith Hemaspaandra, Henning Schnoor
• Keywords: ATAL 2008, Constant Number, Election System, Intelligent Agents, Unweighted Copeland Elections