Manipulation of copeland elections

  • Authors:

  • Piotr Faliszewski;Edith Hemaspaandra;Henning Schnoor

  • Affiliations:

  • AGH University of Science and Technology, Poland;Rochester Institute of Technology;Christian-Albrechts-Universität, Kiel, Germany

  • Venue:
  • Proceedings of the 9th International Conference on Autonomous Agents and Multiagent Systems: volume 1 - Volume 1
  • Year:
  • 2010

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Abstract

We resolve an open problem regarding the complexity of unweighted coalitional manipulation, namely, the complexity of Copelandα-manipulation for α ε {0, 1}. Copelandα, 0 ≤ α ≤ 1, is an election system where for each pair of candidates we check which one is preferred by more voters (i.e., we conduct a head-to-head majority contest) and we give one point to this candidate and zero to the other. However, in case of a tie both candidates receive α points. In the end, candidates with most points win. It is known [13] that Copelandα-manipulation is NP-complete for all rational α's in (0, 1) -- {0.5} (i.e., for all the reasonable cases except the three truly interesting ones). In this paper we show that the problem remains NP-complete for α ε {0, 1}. In addition, we resolve the complexity of Copelandα-manipulation for each rational α ε [0, 1] for the case of irrational voters.