English

When you come at the kings you best not miss

Combinatorics 2022-09-28 v2 Discrete Mathematics

Abstract

A tournament is an orientation of a complete graph. We say that a vertex xx in a tournament T\vec T controls another vertex yy if there exists a directed path of length at most two from xx to yy. A vertex is called a king if it controls every vertex of the tournament. It is well known that every tournament has a king. We follow Shen, Sheng, and Wu (SIAM J. Comput., 2003) in investigating the query complexity of finding a king, that is, the number of arcs in T\vec T one has to know in order to surely identify at least one vertex as a king. The aforementioned authors showed that one always has to query at least Ω(n4/3)\Omega(n^{4/3}) arcs and provided a strategy that queries at most O(n3/2)O(n^{3/2}). While this upper bound has not yet been improved for the original problem, Biswas et al. (Frontiers in Algorithmics, 2017) proved that with O(n4/3)O(n^{4/3}) queries one can identify a semi-king, meaning a vertex which controls at least half of all vertices. Our contribution is a novel strategy which improves upon the number of controlled vertices: using O(n4/3polylogn)O(n^{4/3} \operatorname{polylog} n) queries, we can identify a (12+217)(\frac{1}{2}+\frac{2}{17})-king. To achieve this goal we use a novel structural result for tournaments.

Keywords

Cite

@article{arxiv.2209.12082,
  title  = {When you come at the kings you best not miss},
  author = {Oded Lachish and Felix Reidl and Chhaya Trehan},
  journal= {arXiv preprint arXiv:2209.12082},
  year   = {2022}
}