English

Tournaments determined by three and five voters

Discrete Mathematics 2026-07-29 v1

Abstract

The Kemeny median problem asks for a linear order minimizing the total pairwise disagreement with mm given rankings of nn options; it is NP-hard for every even m4m \ge 4 and every odd m7m \ge 7, while m=3m = 3 and m=5m = 5 remain open. Weighting each arc of the majority tournament by its margin reduces the problem to minimum-weight feedback arc set (FAS). The fewest voters inducing a tournament is its McGarvey number, and its predictability α(T)\alpha^{*}(T) is the largest supermajority threshold at which TT is inducible. We refute three conjectures on inducibility. (i) In any tournament, every minimum FAS is a minimal hitting set of the directed 3-cycles, strengthening a theorem of Milosz, Hamel and Pierrot; both of their conjectures fail: the 3-cycle extension for all odd m5m \ge 5, and the equality FAS=HS3\mathrm{FAS} = \mathrm{HS}_3 at n=11n = 11. (ii) The threshold conjecture proposed by Shepardson and Tovey fails for m=3m = 3, exactly on the boundary (predictability =2/3= 2/3). (iii) For m=5m = 5 it fails strictly: the Paley tournament on 43 vertices, with predictability 181/301>3/5181/301 > 3/5, is not the majority of any 5 voters, making it the first explicit tournament of modest size beyond the reach of five voters.

Keywords

Cite

@article{arxiv.2607.26690,
  title  = {Tournaments determined by three and five voters},
  author = {Leonid Chindelevitch and Ararat Harutyunyan},
  journal= {arXiv preprint arXiv:2607.26690},
  year   = {2026}
}

Comments

29 pages, 7 figures