Secret-Sharing Matroids need not be Algebraic

We combine some known results and techniques with new ones to show that there exists a non-algebraic, multi-linear matroid. This answers an open question by Matus (Discrete Mathematics 1999), and an open question by Pendavingh and van Zwam (Advances in Applied Mathematics 2013). The proof is constructive and the matroid is explicitly given

One against all in the fictitious play process

There are only few "positive" results concerning multi-person games with the fictitious play property, that is, games in which every fictitious play process approaches the set of equilibria. In this paper we chararcterize classes of multi-person games with the fictitious play property. We consider an (n+1) player game {0,1,2,...,n} based on n two-person sub-games. In each of these sub-games player 0 plays against one of the other players. Player 0 is regulated, so that he must choose the same strategy in all n sub-games. we show that if all sub-games are either zero-sum ganes, weighted potential games, or games with identical payoff functions, then the fictitious play property holds for the associated game.

Sequential Two-Prize Contests

We study two-stage all-pay auctions with two identical prizes. In each stage, players compete for one prize. Each player may win either one or two prizes. We analyze the equilibrium strategies where players’ marginal values for the prizes are either declining or incliningMulti-prize contests, All-pay auctions

BEST-OF-THREE ALL-PAY AUCTIONS

We study a three-stage all-pay auction with two players in which the ?rst player to win two matches wins the best-of-three all-pay auction. The players have values of winning the contest and may have also values of losing, the latter depending on the stage in which the contest is decided. It is shown that without values of losing, if players are heterogenous (they have di¤erent values) the best-of-three all-pay auction is less competitive (the di¤erence between the players?probabilities to win is larger) as well as less productive (the players?total expected e¤ort is smaller) than the one-stage all-pay auction. If players are homogenous, however, the productivity and obviously the competitiveness of the best-of-three all-pay auction and the one-stage all-pay auction are identical. These results hold even if players have values of losing that do not depend on the stage in which the contest is decided. However, the best-of-three all-pay auction with di¤erent values of losing over the contest?s stages may be more productive than the one-stage all-pay auction.

Commutator maps, measure preservation, and T-systems

Let G be a finite simple group. We show that the commutator map $a : G \times G \to G$ is almost equidistributed as the order of G goes to infinity. This somewhat surprising result has many applications. It shows that for a subset X of G we have $a^{-1}(X)/|G|^2 = |X|/|G| + o(1)$, namely $a$ is almost measure preserving. From this we deduce that almost all elements $g \in G$ can be expressed as commutators $g = [x,y]$ where x,y generate G. This enables us to solve some open problems regarding T-systems and the Product Replacement Algorithm (PRA) graph. We show that the number of T-systems in G with two generators tends to infinity as the order of G goes to infinity. This settles a conjecture of Guralnick and Pak. A similar result follows for the number of connected components of the PRA graph of G with two generators. Some of our results apply for more general finite groups, and more general word maps. Our methods are based on representation theory, combining classical character theory with recent results on character degrees and values in finite simple groups. In particular the so called Witten zeta function plays a key role in the proofs.Comment: 28 pages. This article was submitted to the Transactions of the American Mathematical Society on 21 February 2007 and accepted on 24 June 200