Variations on a theme of rationality of cycles

We prove certain weak versions of some celebrated results due to Alexander Vishik comparing rationality of algebraic cycles over the function field of a quadric and over the base field. The original proofs use Vishik’s symmetric operations in the algebraic cobordism theory and work only in characteristic 0. Our proofs use the modulo 2 Steenrod operations in the Chow theory and work in any characteristic ≠ 2. Our weak versions are still sufficient for existing applications. In particular, Vishik’s construction of fields of u-invariant 2 ^r + 1, for r ≥ 3, is extended to arbitrary characteristic ≠ 2.     

The castelnuovo criterion of rationality

A new proof is provided for the rationality criterion for algebraic surfaces over an arbitrary base field, usingl-adic cohomologies.Translated from Matematicheskie Zametki, Vol.11, No. 1, pp. 27–32, January, 1972.     

Brauer characters and rationality

A finite group $G$ has no non-trivial rational-valued irreducible $p$ -Brauer characters if and only if $G$ has no non-trivial rational elements of order not divisible by $p$ .     

Stable rationality of certain PGLn-quotients

Summary Stable rationality of the field of matrix invariants M _n ×M _n ) ^PGL _n is proved forn=5 andn=7. In combination with existing results this shows that (V) ^PGL _n is stably rational wheneverV is an almost free representation ofPGL _n andn divides 420=2²·3·5·7.Oblatum 1-VII-1989 & 15-VI-1990 & 19-VII-1990Partially supported by the DFGResearch associate of the NFWO     

Fields of rationality of cusp forms

We prove bounded rational ergodicity for some discrepancy skew products whose rotation number has bad rational approximation. This is done by considering the asymptotics of associated affine random walks.     

Bounded rationality in multiobjective games

In this paper, we will establish the connections between bounded rationality and multiobjective games. We obtain some new results for robustness to ?$?$-equilibria and structural stability of multiobjective games and generalized multiobjective games.     

Complexity of games and bounded rationality

An attempt is made to propose a concept of limited rationality for choice junctions based on computability theory in computer science.

Starting with the observation that it is possible to construct a machine simulating strategies of each individual in society, one machine for each individual's preference structure, we identify internal states of this machine with strategies or strategic preferences. Inputs are possible actions of other agents in society thus society is effectively operating as a game generated by machines. The main result states that effective realization of game strategies bound by the “complexity of computing machines'.     

Frobenius groups and retract rationality

Let k$k$ be any field, G$G$ be a finite group acting on the rational function field k(_xg:g∈G)$k(x_g:g\in G)$ by h⋅_xg=_xhg$h\cdot x_g=x_{hg}$ for any h,g∈G$h,g\in G$. Define k(G)=k(_xg:g∈G)^G$k\left(G\right)=k{(x_g:g\in G)}^G$. Noether’s problem asks whether k(G)$k\left(G\right)$ is rational (= purely transcendental) over k$k$. A weaker notion, retract rationality introduced by Saltman, is also very useful for the study of Noether’s problem. We prove that, if G$G$ is a Frobenius group with abelian Frobenius kernel, then k(G)$k\left(G\right)$ is retract k$k$-rational for any field k$k$ satisfying some mild conditions. As an application, we show that, for any algebraic number field k$k$, for any Frobenius group G$G$ with Frobenius complement isomorphic to S_L2(_F5)$S{L}_2\left({\mathbb{F}}_5\right)$, there is a Galois extension field K$K$ over k$k$ whose Galois group is isomorphic to G$G$, i.e. the inverse Galois problem is valid for the pair (G,k)$(G,k)$. The same result is true for any non-solvable Frobenius group if k(_ζ8)$k\left({\zeta }_8\right)$ is a cyclic extension of k$k$.     

Bounded rationality and correlated equilibria

We study an interactive framework that explicitly allows for nonrational behavior. We do not place any restrictions on how players’ behavior deviates from rationality, but rather, on players’ higher-order beliefs about the frequency of such deviations. We assume that there exists a probability p such that all players believe, with at least probability p, that their opponents play rationally. This, together with the assumption of a common prior, leads to what we call the set of p-rational outcomes, which we define and characterize for arbitrary probability p. We then show that this set varies continuously in p and converges to the set of correlated equilibria as p approaches 1, thus establishing robustness of the correlated equilibrium concept to relaxing rationality and common knowledge of rationality. The p-rational outcomes are easy to compute, also for games of incomplete information. Importantly, they can be applied to observed frequencies of play for arbitrary normal-form games to derive a measure of rationality \(\overline{p}\) that bounds from below the probability with which any given player chooses actions consistent with payoff maximization and common knowledge of payoff maximization.     

Extended Chebyshev spaces in rationality

On a closed bounded interval, a given Extended Chebyshev space can be defined by means of generalised derivatives associated with systems of weight functions. Only recently we could identify all such systems, describing an iterative process to build them. In the present work, we interpret the first step of this process as the construction of rational spaces based on Extended Chebyshev spaces. This construction establishes an interesting symmetry between all Extended Chebyshev spaces “good for design” (i.e., all those which contain constants and which possess blossoms) and the rational spaces based on them (Extended Chebyshev spaces in rationality). In particular, this symmetry results in a very simple relation between the corresponding blossoms. A special case is obtained when considering polynomial spaces as examples of Extended Chebyshev spaces. The classical rational spaces then appear as examples of Extended Chebyshev spaces good for design, that is, possessing blossoms. This offers interesting new insights on the famous so-called rational Bézier curves.