# Winning a Tournament by Any Means Necessary

# Winning a Tournament by Any Means Necessary

## Sushmita Gupta, Sanjukta Roy, Saket Saurabh, Meirav Zehavi

Proceedings of the Twenty-Seventh International Joint Conference on Artificial Intelligence

Main track. Pages 282-288.
https://doi.org/10.24963/ijcai.2018/39

In a tournament, $n$ players enter the competition. In each round, they are paired-up to compete against each other. Losers are thrown, while winners proceed to the next round, until only one player (the winner) is left. Given a prediction of the outcome, for every pair of players, of a match between them (modeled by a digraph $D$), the competitive nature of a tournament makes it attractive for manipulators. In the Tournament Fixing (TF) problem, the goal is to decide if we can conduct the competition (by controlling how players are paired-up) so that our favorite player $w$ wins. A common form of manipulation is to bribe players to alter the outcome of matches. Kim and Williams [IJCAI 2015] integrated such deceit into TF, and showed that the resulting problem is NP-hard when $\ell<(1-\epsilon)\log n$ alterations are possible (for any fixed $\epsilon>0$). For this problem, our contribution is fourfold. First, we present two operations that ``obfuscate deceit'': given one solution, they produce another solution. Second, we present a combinatorial result, stating that there is always a solution with all reversals incident to $w$ and ``elite players''. Third, we give a closed formula for the case where $D$ is a DAG. Finally, we present exact exponential-time and parameterized algorithms for the general case.

Keywords:

Agent-based and Multi-agent Systems: Computational Social Choice

Agent-based and Multi-agent Systems: Voting