Dense packing crystal structures of physical tetrahedra

We present a method for discovering dense packings of general convex hard particles and apply it to study the dense packing behavior of a one-parameter family of particles with tetrahedral symmetry representing a deformation of the ideal mathematical tetrahedron into a less ideal, physical, tetrahedron and all the way to the sphere. Thus, we also connect the two well studied problems of sphere packing and tetrahedron packing on a single axis. Our numerical results uncover a rich optimal-packing behavior, compared to that of other continuous families of particles previously studied. We present four structures as candidates for the optimal packing at different values of the parameter, providing an atlas of crystal structures which might be observed in systems of nano-particles with tetrahedral symmetry

Proactive and reactive control by the medial frontal cortex

Adaptive behavior requires the ability to flexibly control actions. This can occur either proactively to anticipate task requirements, or reactively in response to sudden changes. Recent work in humans has identified a network of cortical and subcortical brain region that might have an important role in proactive and reactive control. However, due to technical limitations, such as the spatial and temporal resolution of the BOLD signal, human imaging experiments are not able to disambiguate the specific function(s) of these brain regions. These limitations can be overcome through single-unit recordings in non-human primates. In this article, we describe the behavioral and physiological evidence for dual mechanisms of control in response inhibition in the medial frontal cortex of monkeys performing the stop signal or countermanding task

Stock Repurchases And False Signals

Each year many firms repurchase shares of their common stock. Research evidence shows that when firms announce the repurchase of common stock, their share prices typically rise.  Numerous studies attribute these increases to a signaling effect.  But some firms that announce their intention of repurchasing shares of common stock either repurchase no shares at all or repurchase fewer shares than initially announced. Although the practice of firms intentionally announcing the repurchase of more shares than they expect to repurchase is illegal, the expected increase in share prices may give firms an incentive to make such false announcements. This study surveys top financial executives to learn the extent that firms repurchase fewer shares than announced, identify the reasons for this activity, and learn how managers view this activity.  We surveyed 642 firms that conducted common stock repurchases from January 1998 to September 1999.  Based on 218 responses, we find that while managers are uncertain about the legality of this activity, they believe that the intentional repurchase of fewer shares than announced is unethical, sends a false signal to the market, and damages the firm’s credibility with its stockholders.  Managers also believe that firms repurchasing fewer shares than announced should publicly reveal both the reason for not repurchasing all shares and the amount by which the repurchase fell short of the firm’s announced intentions.  Despite these beliefs, managers report that repurchasing fewer shares than announced is a common practice

Atividades experimentais e computacionais como recursos instrucionais que se complementam : um estudo exploratório no ensino de eletromagnetismo em física geral

Neste trabalho relatamos um estudo exploratório que teve como objetivo o levantamento de proposições norteadoras para embasar um estudo mais amplo, que visa investigar a integração entre atividades computacionais e experimentais no ensino de Eletromagnetismo em Física Geral como meio de propiciar condições favoráveis para uma aprendizagem significativa (Ausubel, 2003) e vivência com atividades práticas que propiciem aos alunos uma visão adequada de modelos teóricos (Bunge, 1974). Trabalhamos com 32 alunos matriculados na disciplina de Física III (Eletromagnetismo) do curso de Física da UFRGS, no 1° semestre de 2008. Os resultados mostram que as atividades integradas tornam os alunos mais ativos e participativos em comparação com as tradicionais aulas de laboratório, principalmente em discussões sobre questões conceituais e no auxílio a seus parceiros de grupo

Partially distributed outer approximation

This paper presents a novel partially distributed outer approximation algorithm, named PaDOA, for solving a class of structured mixed integer convex programming problems to global optimality. The proposed scheme uses an iterative outer approximation method for coupled mixed integer optimization problems with separable convex objective functions, affine coupling constraints, and compact domain. PaDOA proceeds by alternating between solving large-scale structured mixed-integer linear programming problems and partially decoupled mixed-integer nonlinear programming subproblems that comprise much fewer integer variables. We establish conditions under which PaDOA converges to global minimizers after a finite number of iterations and verify these properties with an application to thermostatically controlled loads and to mixed-integer regression

Consequences of the H-Theorem from Nonlinear Fokker-Planck Equations

A general type of nonlinear Fokker-Planck equation is derived directly from a master equation, by introducing generalized transition rates. The H-theorem is demonstrated for systems that follow those classes of nonlinear Fokker-Planck equations, in the presence of an external potential. For that, a relation involving terms of Fokker-Planck equations and general entropic forms is proposed. It is shown that, at equilibrium, this relation is equivalent to the maximum-entropy principle. Families of Fokker-Planck equations may be related to a single type of entropy, and so, the correspondence between well-known entropic forms and their associated Fokker-Planck equations is explored. It is shown that the Boltzmann-Gibbs entropy, apart from its connection with the standard -- linear Fokker-Planck equation -- may be also related to a family of nonlinear Fokker-Planck equations.Comment: 19 pages, no figure