The "No Justice in the Universe" phenomenon: why honesty of effort may not be rewarded in tournaments Preprint, 2018

In 2000 Allen Schwenk, using a well-known mathematical model of matchplay tournaments in which the probability of one player beating another in a single match is fixed for each pair of players, showed that the classical single-elimination, seeded format can be "unfair" in the sense that situations can arise where an indisputibly better (and thus higher seeded) player may have a smaller probability of winning the tournament than a worse one. This in turn implies that, if the players are able to influence their seeding in some preliminary competition, situations can arise where it is in a player's interest to behave "dishonestly", by deliberately trying to lose a match. This motivated us to ask whether it is possible for a tournament to be both honest, meaning that it is impossible for a situation to arise where a rational player throws a match, and "symmetric" - meaning basically that the rules treat everyone the same - yet unfair, in the sense that an objectively better player has a smaller probability of winning than a worse one. After rigorously defining our terms, our main result is that such tournaments exist and we construct explicit examples for any number n >= 3 of players. For n=3, we show (Theorem 3.6) that the collection of win-probability vectors for such tournaments form a 5-vertex convex polygon in R^3, minus some boundary points. We conjecture a similar result for any n >= 4 and prove some partial results towards it.

doubly monotonic matrix

fairness

convex polytope

honesty

symmetry

Tournaments

Författare

Peter Hegarty

Chalmers, Matematiska vetenskaper, Algebra och geometri

Anders Martinsson

Eidgenössische Technische Hochschule Zürich (ETH)

Edvin Wedin

Chalmers, Matematiska vetenskaper, Algebra och geometri

Ämneskategorier

Filosofi

Diskret matematik

Fundament

Grundläggande vetenskaper

2022-08-11