### Abstract

We consider the manipulability of tournament rules for round-robin tournaments of n competitors. Specifically, n competitors are competing for a prize, and a tournament rule r maps the result of all (n2) pairwise matches (called a tournament, T) to a distribution over winners. Rule r is Condorcet-consistent if whenever i wins all n - 1 of her matches, r selects i with probability 1. We consider strategic manipulation of tournaments where player j might throw their match to player i in order to increase the likelihood that one of them wins the tournament. Regardless of the reason why j chooses to do this, the potential for manipulation exists as long as Pr[r(T) = i] increases by more than Pr[r(T) = j] decreases. Unfortunately, it is known that every Condorcetconsistent rule is manipulable [1]. In this work, we address the question of how manipulable Condorcet-consistent rules must necessarily be-by trying to minimize the difference between the increase in Pr[r(T) = i] and decrease in Pr[r(T) = j] for any potential manipulating pair. We show that every Condorcet-consistent rule is in fact 1/3-manipulable, and that selecting a winner according to a random single elimination bracket is not α-manipulable for any α > 1/3. We also show that many previously studied tournament formats are all 1/2-manipulable, and the popular class of Copeland rules (any rule that selects a player with the most wins) are all in fact 1-manipulable, the worst possible. Finally, we consider extensions to match-fixing among sets of more than two players.

Original language | English (US) |
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Title of host publication | 8th Innovations in Theoretical Computer Science Conference, ITCS 2017 |

Editors | Christos H. Papadimitriou |

Publisher | Schloss Dagstuhl- Leibniz-Zentrum fur Informatik GmbH, Dagstuhl Publishing |

ISBN (Electronic) | 9783959770293 |

DOIs | |

State | Published - Nov 1 2017 |

Event | 8th Innovations in Theoretical Computer Science Conference, ITCS 2017 - Berkeley, United States Duration: Jan 9 2017 → Jan 11 2017 |

### Publication series

Name | Leibniz International Proceedings in Informatics, LIPIcs |
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Volume | 67 |

ISSN (Print) | 1868-8969 |

### Other

Other | 8th Innovations in Theoretical Computer Science Conference, ITCS 2017 |
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Country | United States |

City | Berkeley |

Period | 1/9/17 → 1/11/17 |

### All Science Journal Classification (ASJC) codes

- Software

### Keywords

- Condorcet-consistent
- Non-manipulability
- Strategyproofness
- Tournament design

## Cite this

*8th Innovations in Theoretical Computer Science Conference, ITCS 2017*(Leibniz International Proceedings in Informatics, LIPIcs; Vol. 67). Schloss Dagstuhl- Leibniz-Zentrum fur Informatik GmbH, Dagstuhl Publishing. https://doi.org/10.4230/LIPIcs.ITCS.2017.35