Limits of the energy-momentum tensor in general relativity

A limiting diagram for the Segre classification of the energy-momentum tensor is obtained and discussed in connection with a Penrose specialization diagram for the Segre types. A generalization of the coordinate-free approach to limits of Paiva et al. to include non-vacuum space-times is made. Geroch's work on limits of space-times is also extended. The same argument also justifies part of the procedure for classification of a given spacetime using Cartan scalars.Comment: LaTeX, 21 page

The Schwarzschild-de Sitter solution in five-dimensional general relativity briefly revisited

We briefly revisit the Schwarzschild-de Sitter solution in the context of five-dimensional general relativity. We obtain a class of five-dimensional solutions of Einstein vacuum field equations into which the four-dimensional Schwarzschild-de Sitter space can be locally and isometrically embedded. We show that this class of solutions is well-behaved in the limit of lambda approaching zero. Applying the same procedure to the de Sitter cosmological model in five dimensions we obtain a class of embedding spaces which are similarly well-behaved in this limit. These examples demonstrate that the presence of a non-zero cosmological constant does not in general impose a rigid relation between the (3+1) and (4+1)-dimensional spacetimes, with degenerate limiting behaviour.Comment: 7 page

A layering model for superconductivity in the borocarbides

We propose a superlattice model to describe superconductivity in layered materials, such as the borocarbide families with the chemical formul\ae\ $RT_2$B$_2$C and $RT$BC, with $R$ being (essentially) a rare earth, and $T$ a transition metal. We assume a single band in which electrons feel a local attractive interaction (negative Hubbard-$U$) on sites representing the $T$B layers, while U=0 on sites representing the $R$C layers; the multi-band structure is taken into account minimally through a band offset $\epsilon$. The one-dimensional model is studied numerically through the calculation of the charge gap, the Drude weight, and of the pairing correlation function. A comparison with the available information on the nature of the electronic ground state (metallic or superconducting) indicates that the model provides a systematic parametrization of the whole borocarbide family.Comment: 4 figure

A Comparative Evaluation of Two Distributed Hydro-Sedimentological Models in The Semiarid Region of the State of Paraíba, Brazil

Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchiv

Virtual Meson Cloud of the Nucleon and Intrinsic Strangeness and Charm

We have applied the Meson Cloud Model (MCM) to calculate the charm and strange antiquark distribution in the nucleon. The resulting distribution, in the case of charm, is very similar to the intrinsic charm momentum distribution in the nucleon. This seems to corroborate the hypothesis that the intrinsic charm is in the cloud and, at the same time, explains why other calculations with the MCM involving strange quark distributions fail in reproducing the low x region data. From the intrinsic strange distribution in the nucleon we have extracted the strangeness radius of the nucleon, which is in agreement with other meson cloud calculations.Comment: 9 pages RevTex, 4 figure

Variabilidade para teor de tanino em sorgo (Sorghum bicolor L.) e sua associação com a resistência a pássaros.

O objetivo do presente trabalho foi verificar a variabilidade existente entre diversos materiais geneticos quanto ao teor de tanino e resistencia a passaros nas fases de grao leitoso, grao pastoso, maturacao fisiologicas e de colheita. O experimento foi conduzido no delineamento blocos casualizados em tres repeticoes e semeadura em outubro/89 no Centro Nacional de Pesquisa de Milho e Sorgo (CNPMS/EMBRAPA) em Sete Lagoas, Estado de Minas Gerais. Os resultados mostraram maior teor de tanino na materia seca na fase de grao leitoso. No entanto, quando se considerou o conteudo de tanino na materia verde do grao, os resultados mostraram maior teor nas fases de maturacao fisiologicas ou de colheita para sorgo de alto tanino (valores maiores que 1%). O dano causado por passaro iniciou-se na fase de grao pastoso, sendo que todos os materiais geneticos resistentes apresentaram alto teor de tanino no grao na fase de colheita

The case of classroom robots: teachers’ deliberations on the ethical tensions

Robots are increasingly being studied for use in education. It is expected that robots will have the potential to facilitate children’s learning and function autonomously within real classrooms in the near future. Previous research has raised the importance of designing acceptable robots for different practices. In parallel, scholars have raised ethical concerns surrounding children interacting with robots. Drawing on a Responsible Research and Innovation perspective, our goal is to move away from research concerned with designing features that will render robots more socially acceptable by end users toward a reflective dialogue whose goal is to consider the key ethical issues and long-term consequences of implementing classroom robots for teachers and children in primary education. This paper presents the results from several focus groups conducted with teachers in three European countries. Through a thematic analysis, we provide a theoretical account of teachers’ perspectives on classroom robots pertaining to privacy, robot role, effects on children and responsibility. Implications for the field of educational robotics are discussed.info:eu-repo/semantics/acceptedVersio

On the Levi-Civita solutions with cosmological constant

The main properties of the Levi-Civita solutions with the cosmological constant are studied. In particular, it is found that some of the solutions need to be extended beyond certain hypersurfaces in order to have geodesically complete spacetimes. Some extensions are considered and found to give rise to black hole structure but with plane symmetry. All the spacetimes that are not geodesically complete are Petrov type D, while in general the spacetimes are Petrov type I.Comment: Typed in Revtex, including two figures. To appear in Phys. Rev.