On amenability of group algebras, I

We study amenability of algebras and modules (based on the notion of almost-invariant finite-dimensional subspace), and apply it to algebras associated with finitely generated groups. We show that a group G is amenable if and only if its group ring G is amenable for some (and therefore for any) field . Similarly, a G-set X is amenable if and only if its span X is amenable as a G-module for some (and therefore for any) field .     

A model of fads,fashions, and group formation

I develop a model of consumer behavior where agents purchase goods in order to signify personal characteristics. Agents purchase goods in order to imitate agents similar to them and agents they want to emulate. Depending on parameter values of consumer preferences the model generates stable groups, fads, and fashion cycles, or a mixture of both. The model is unique to the economic literature on fads in that the extinction of fads occurs endogenously in the model. © 2004 Wiley Periodicals, Inc. Complexity 9: 51–61, 2004     

Cocycles,extensions of group actions,and bundle representations

Let Y be an ergodic Borel G space. Conditions are determined in order for an extension X of Y to be a skew product extension of Y. These conditions depend on the behavior of the unitary bundle representation of the G space Y induced by the extension and on the existence of extensions of the G space X with certain appropriate properties.     

Graph puzzles,homotopy, and the alternating group

The so-called 15-puzzle may be generalized to a puzzle based on an arbitrary graph. We consider labelings or colorings of the vertices and the operation of switching one distinguished label with a label on an adjacent vertex. Starting from a given labeling, iterations of this operation allow one to obtain all, or exactly half, of the labelings on a non-separable graph (with the polygons and one other graph as exceptions).     

Quivers, Floer cohomology, and braid group actions

We consider the derived categories of modules over a certain family () of graded rings, and Floer cohomology of Lagrangian intersections in the symplectic manifolds which are the Milnor fibres of simple singularities of type We show that each of these two rather different objects encodes the topology of curves on an -punctured disc. We prove that the braid group acts faithfully on the derived category of -modules, and that it injects into the symplectic mapping class group of the Milnor fibers. The philosophy behind our results is as follows. Using Floer cohomology, one should be able to associate to the Milnor fibre a triangulated category (its construction has not been carried out in detail yet). This triangulated category should contain a full subcategory which is equivalent, up to a slight difference in the grading, to the derived category of -modules. The full embedding would connect the two occurrences of the braid group, thus explaining the similarity between them.

     

Social comparison,group composition,and incentive provision

This article analyzes the optimal composition of groups of arbitrary size according to the social preferences of group members in a situation of moral hazard. We focus on rivalry, pure self-interest and altruism to show that in a situation where every group member compares his or her remuneration to the average remuneration of all members in the same group, ideally there should be as many extreme altruists as there are extreme rivals. If, by contrast, every member of the group compares his or her own remuneration to that of a focal agent, ideally, this focal agent should be a strong altruist, while all other group members should be highly competitive. Our results show that it is important to foster heterogeneity in the social preferences of the members of a group in order to maximize the profitability of the firm.     

Toroidal orbifolds, gerbes and group cohomology

In this paper we compute the integral cohomology of certain semi-direct products of the form , arising from a linear action on the -torus, where is a finite group. The main application is the complete calculation of torsion gerbes for six-dimensional examples arising in string theory.

     

Combinatorics,Bethe Ansatz,and representations of the symmetric group

Techniques developed in the realms of the quantum method of the inverse problem are used to analyze combinatorial problems (Young diagrams and rigged configurations).Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova AN SSSR, Vol. 155, pp. 50–64, 1986.     

Quantum group actions, twisting elements, and deformations of algebras

We construct twisting elements for module algebras of restricted two-parameter quantum groups from factors of their R-matrices. We generalize the theory of Giaquinto and Zhang to universal deformation formulas for catagories of module algebras and give examples arising from R-matrices of two-parameter quantum groups.     

The group PSL(2,q) is not the multiplication group of a loop

ABSTRACT

We introduce the notion of weak transitivity for torsion-free abelian groups. A torsion-free abelian group G is called weakly transitive if for any pair of elements x, y ∈ G and endomorphisms ?, ψ ∈ End(G) such that x? = y, yψ = x, there exists an automorphism of G mapping x onto y. It is shown that every suitable ring can be realized as the endomorphism ring of a weakly transitive torsion-free abelian group, and we characterize up to a number-theoretical property the separable weakly transitive torsion-free abelian groups.     

Cutting sequences, regular polygons, and the Veech group

We describe the cutting sequences associated to geodesic flow on regular polygons, in terms of a combinatorial process called derivation. This work is an extension of some of the ideas and results in Smillie and Ulcigrai’s recent paper, where the analysis was made for the regular octagon. It turns out that the main structural properties of the octagon generalize in a natural way.     

Every finite solvable group with a unique element of order two,except the quaternion group,has a symmetric sequencing

All finite solvable groups that have symmetric sequencings are characterized. Let G be a finite solvable group. It is shown that G has a symmetric sequencing if and only if G has a unique element of order two and is not the quaternion group. All finite groups with a unique element of order two such that the order of the group is not divisible by three are solvable and thus, except for the quaternion group, have symmetric sequencings. A crucial step used in the proof of these facts is a construction showing that if a finite group H has a normal subgroup C of odd order such that H/C admits a 2-sequencing, then H admits a 2-sequencing. The results of this article can be viewed as generalizing a theorem of Gordon about Abelian groups and as extending the idea of a starter, suitably modified, to a large class of groups of even order by showing the existence of the required object. © 1993 John Wiley & Sons, Inc.