A Stress/Displacement Virtual Element Method for Plane Elasticity Problems

The numerical approximation of 2D elasticity problems is considered, in the framework of the small strain theory and in connection with the mixed Hellinger-Reissner variational formulation. A low-order Virtual Element Method (VEM) with a-priori symmetric stresses is proposed. Several numerical tests are provided, along with a rigorous stability and convergence analysis

Who Creates Jobs? Small vs. Large vs. Young

The view that small businesses create the most jobs remains appealing to policymakers and small business advocates. Using data from the Census Bureau Business Dynamics Statistics and Longitudinal Business Database, we explore the many issues at the core of this ongoing debate. We find that the relationship between firm size and employment growth is sensitive to these issues. However, our main finding is that once we control for firm age there is no systematic relationship between firm size and growth. Our findings highlight the important role of business startups and young businesses in U.S. job creation.

Collapse of Primordial Clouds

We present here studies of collapse of purely baryonic Population III objects with masses ranging from $10M_\odot$ to $10^6M_\odot$. A spherical Lagrangian hydrodynamic code has been written to study the formation and evolution of the primordial clouds, from the beginning of the recombination era ($z_{rec} \sim 1500$) until the redshift when the collapse occurs. All the relevant processes are included in the calculations, as well as, the expansion of the Universe. As initial condition we take different values for the Hubble constant and for the baryonic density parameter (considering however a purely baryonic Universe), as well as different density perturbation spectra, in order to see their influence on the behavior of the Population III objects evolution. We find, for example, that the first mass that collapses is $8.5\times10^4M_\odot$ for $h=1$, $\Omega=0.1$ and $\delta_i={\delta\rho / \rho}=(M / M_o)^{-1/3}(1+z_{rec})^{-1}$ with the mass scale $M_o=10^{15}M_\odot$. For $M_o=4\times10^{17}M_\odot$ we obtain $4.4\times10^{4}M_\odot$ for the first mass that collapses. The cooling-heating and photon drag processes have a key role in the collapse of the clouds and in their thermal history. Our results show, for example, that when we disregard the Compton cooling-heating, the collapse of the objects with masses $>8.5\times10^4M_\odot$ occurs earlier. On the other hand, disregarding the photon drag process, the collapse occurs at a higher redshift.Comment: 10 pages, MN plain TeX macros v1.6 file, 9 PS figures. Also available at http://www.iagusp.usp.br/~oswaldo (click "OPTIONS" and then "ARTICLES"). MNRAS in pres

Collapse of Primordial Clouds II. The Role of Dark Matter

In this article we extend the study performed in our previous article on the collapse of primordial objects. We here analyze the behavior of the physical parameters for clouds ranging from $10^7M_\odot$ to $10^{15}M_\odot$. We studied the dynamical evolution of these clouds in two ways: purely baryonic clouds and clouds with non-baryonic dark matter included. We start the calculations at the beginning of the recombination era, following the evolution of the structure until the collapse (that we defined as the time when the density contrast of the baryonic matter is greater than $10^4$). We analyze the behavior of the several physical parameters of the clouds (as, e.g., the density contrast and the velocities of the baryonic matter and the dark matter) as a function of time and radial position in the cloud. In this study all physical processes that are relevant to the dynamical evolution of the primordial clouds, as for example photon-drag (due to the cosmic background radiation), hydrogen molecular production, besides the expansion of the Universe, are included in the calculations. In particular we find that the clouds, with dark matter, collapse at higher redshift when we compare the results with the purely baryonic models. As a general result we find that the distribution of the non-baryonic dark matter is more concentrated than the baryonic one. It is important to stress that we do not take into account the putative virialization of the non-baryonic dark matter, we just follow the time and spatial evolution of the cloud solving its hydrodynamical equations. We studied also the role of the cooling-heating processes in the purely baryonic clouds.Comment: 8 pages, MN plain TeX macros v1.6 file, 13 PS figures. Also available at http://www.iagusp.usp.br/~oswaldo (click "OPTIONS" and then "ARTICLES"). MNRAS in pres

Scalar Spectrum from a Dynamical Gravity/Gauge model

We show that a Dynamical AdS/QCD model is able to reproduce the linear Regge trajectories for the light-flavor sector of mesons with high spin and also for the scalar and pseudoscalar ones. In addition the model has confinement by the Wilson loop criteria and a mass gap. We also calculate the decay amplitude of scalars into two pion in good agreement to the available experimental data.Comment: Presented in the 4th International Workshop on Astronomy and Relativistic Astrophysic

Signals of high cognitive abilities by teachers: structural validaty of the Scale of Cognitive Ability and of Learning

As nomeações pelos professores são muitas vezes utilizadas na sinalização das crianças sobredotadas complementando as avaliações realizadas através dos testes psicológicos de inteligência. Contudo nem sempre essa identificação é isenta de viés, melhorando a qualidade quando os instrumentos de identificação usados pelos professores apresentam boas características métricas de validade e fiabilidade. O objetivo deste estudo foi testar a estrutura empírica da Escala de Habilidades Cognitivas e de Aprendizagem (EHC/A) pertencente à BISAS-T (Bateria de Instrumentos para a Sinalização de Alunos Sobredotados e Talentosos; Almeida, Oliveira, & Melo, 2000), ou seja, procurou-se avaliar a qualidade do ajustamento da sua estrutura fatorial numa amostra de alunos do 6º ano de escolaridade. Esta escala foi construída com base no modelo teórico de Renzulli (1976) sendo formada por 16 itens respondidos numa escala de tipo likert de 6 pontos. A amostra contou com 132 alunos do 6º ano de escolaridade, com idade em torno de 11 anos. Os resultados das qualidades psicométricas da EHC/A são considerados satisfatórios para a amostra em estudo sugerindo a sua utilização em futuras pesquisas com amostras mais alargadas e com o objetivo de sinalização de alunos mais capazes.Screening by teachers is often used in the identification of gifted children and complement assessments through psychological tests of intelligence. However this identification is not always free from bias, improving its quality when identification instruments used by teachers have good metric characteristics of validity and reliability. With this work we intend to test the empirical structure of the Scale of Cognitive Abilities and Learning (EHC/A) included in the BISAS-T (Almeida, Oliveira, & Melo, 2000) or, in other words, to assess the quality of the adjustment of its factor structure in a sample of 6th grade students. This scale was constructed based on Renzulli´s theoretical model (1976) and consisted of 16 items answered on a Likert type scale of 6 points. The sample consisted of 132 students from 6th grade, aged around 11 years. Results of the psychometric qualities of EHC/A are satisfactory for this sample, suggesting its use in future research with broader samples and aiming to identify the most able students