1,121 research outputs found
Testing some f(R,T) gravity models from energy conditions
We consider theory of gravity, where is the curvature scalar
and the trace of the energy momentum tensor. Attention is attached to the
special case, and two expressions are assumed for the
function , and ,
where , , , , , , , and are input
parameters. We observe that by adjusting suitably these input parameters,
energy conditions can be satisfied. Moreover, an analyse of the perturbations
and stabilities of de Sitter solutions and power-law solutions is performed
with the use of the two models. The results show that for some values of the
input parameters, for which energy conditions are satisfied, de Sitter
solutions and power-law solutions may be stables.Comment: 25 pages, 6 figures. Accepted for publication in Journal of Modern
Physcis (JMP
Avaliação do Pronaf, grupo "B", em Minas Gerais.
Este estudo trata da análise de uma das linhas de crédito do Programa Nacional de Apoio à Agricultura Familiar (o Pronaf "B"), no Município de Porteirinha, MG, como instrumento de desenvolvimento socioeconômico do público beneficiário. Nele são analisadas tanto a evolução dos indicadores econômicos e sociais como a dos efeitos indiretos do programa, além da capacidade de pagamento do crédito obtido. Os resultados mostram pequenos acréscimos de produtividade na pecuária leiteira, na avicultura e na suinocultura; atividades essas diretamente beneficiadas pelo microcrédito do Pronaf ?B?. Constatou-se que, entre 2000 e 2004, em todas as três atividades que receberam crédito ? pecuária leiteira, suinocultura e avicultura ?, houve elevação da relação Margem Líquida Média Auferida e Crédito Médio Obtido: entre 2000 e 2004, tendência importante para as perspectivas de manutenção e de ampliação do referido programa
Parasitismo de larvas da mosca-do-mediterrâneo por Diachasmimorpha longicaudata (Ashmead) (Hymenoptera: Braconidae) em diferentes cultivares de goiaba.
As moscas-das-frutas são as pragas que causam os maiores prejuízos à fruticultura mundial. Esses prejuízos podem ser diretos, com perdas na produção e indiretos. Por serem pragas quarentenárias, existem barreiras comerciais impostas pelos países importadores, limitando a exportação de frutos in natura. Apesar de o Brasil ser o terceiro maior produtor mundial de frutas, exporta-se apenas 2% dessa produção, enquanto que Diachasmimorpha longicaudata infestadas com larvas de Ceratitis capitata (Wiedemann) (Diptera: Tephritidae). Cinco lotes de oito frutos de goiaba, sendo dois frutos por cultivar, foram acondicionados em gaiolas contendo adultos de C. capitata, por 2h para oviposição. Após uma semana, quando as larvas já haviam se desenvolvido dentro dos frutos, estes foram expostos aos parasitóides durante 24h. Foram avaliados o peso médio dos frutos, a mortalidade das larvas, o número de pupários e as porcentagens de moscas e parasitóides emergidos
Troubles with quantum anistropic cosmological models: Loss of unitarity
The anisotropic Bianchi I cosmological model coupled with perfect fluid is
quantized in the minisuperspace. The perfect fluid is described by using the
Schutz formalism which allows to attribute dynamical degrees of freedom to
matter. A Schr\"odinger-type equation is obtained where the matter variables
play the role of time. However, the signature of the kinetic term is
hyperbolic. This Schr\"odinger-like equation is solved and a wave packet is
constructed. The norm of the resulting wave function comes out to be time
dependent, indicating the loss of unitarity in this model. The loss of
unitarity is due to the fact that the effective Hamiltonian is hermitian but
not self-adjoint. The expectation value and the bohmian trajectories are
evaluated leading to different cosmological scenarios, what is a consequence of
the absence of a unitary quantum structure. The consistency of this quantum
model is discussed as well as the generality of the absence of unitarity in
anisotropic quantum models.Comment: Latex file, 18 pages. To appear in General Relativity and Gravitatio
Quantum Perfect-Fluid Kaluza-Klein Cosmology
The perfect fluid cosmology in the 1+d+D dimensional Kaluza-Klein spacetimes
for an arbitrary barotropic equation of state is quantized by using
the Schutz's variational formalism. We make efforts in the mathematics to solve
the problems in two cases. For the first case of the stiff fluid we
exactly solve the Wheeler-DeWitt equation when the space is flat. After the
superposition of the solutions we analyze the Bohmian trajectories of the
final-stage wave-packet functions and show that the flat spaces and the
compact spaces will eventually evolve into finite scale functions. For the
second case of , we use the approximated wavefunction in the
Wheeler-DeWitt equation to find the analytic forms of the final-stage
wave-packet functions. After analyzing the Bohmian trajectories we show that
the flat spaces will be expanding forever while the scale function of the
contracting spaces would not become zero within finite time. Our
investigations indicate that the quantum effect in the quantum perfect-fluid
cosmology could prevent the extra compact spaces in the Kaluza-Klein theory
from collapsing into a singularity or that the "crack-of-doom" singularity of
the extra compact dimensions is made to occur at .Comment: Latex 18 pages, add section 2 to introduce the quantization of
perfect flui
Determinação experimental da retenção do parafuso ortopédico em cortex ósseo
Cylindrical specimens of bone measuring 15 mm in diameter were obtained from the lateral cortical layer of 10 pairs of femurs and tibias. A central hole 3.2 mm in diameter was drilled in each specimen. The hole was tapped, and a 4.5 mm cortical bone screw was inserted from the outer surface. The montage was submitted to push-out testing up to a complete strip of the bone threads. The cortical thickness and rupture load were measured, and the shear stress was calculated. The results were grouped according to the bone segment from which the specimen was obtained. The results showed that bone cortex screw holding power is dependent on the bone site. Additionally, the diaphyseal cortical bone tissue is both quantitatively and qualitatively more resistant to screw extraction than the metaphyseal tissue.Foram retirados corpos de prova cilíndricos de 15 mm de diâmetro da camada cortical da face lateral de 10 pares de fêmures e tíbias oriundos de peças anatômicas. No centro de cada um destes foi feito um orifício de 3,2 mm e nele inserido, a partir da superfície externa, um parafuso cortical de 4,5 mm de diâmetro após rosqueamento com macho. Este conjunto foi submetido a um ensaio mecânico no qual o parafuso foi extraído. A espessura da camada cortical e a carga de ruptura foram medidas e a tensão de cisalhamento foi calculada. Os resultados foram agrupados de acordo com o segmento do osso de onde provinham. Os ensaios mostraram que a retenção cortical do parafuso varia ao longo do osso e que o tecido ósseo da córtex diafisária é tanto quantitativamente quanto qualitativamente mais resistente à extração do parafuso que o tecido metafisário
Desenvolvimento de larvas de Hippodamia convergens (Coleoptera: coccinellidae) alimentadas com Schizaphis graminum.
Does Quantum Cosmology Predict a Constant Dilatonic Field?
Quantum cosmology may permit to determine the initial conditions of the
Universe. In particular, it may select a specific model between many possible
classical models. In this work, we study a quantum cosmological model based on
the string effective action coupled to matter. The Schutz's formalism is
employed in the description of the fluid. A radiation fluid is considered. In
this way, a time coordinate may be identified and the Wheeler-DeWitt equation
reduces in the minisuperspace to a Schr\"odinger-like equation. It is shown
that, under some quite natural assumptions, the expectation values indicate a
null axionic field and a constant dilatonic field. At the same time the scale
factor exhibits a bounce revealing a singularity-free cosmological model. In
some cases, the mininum value of the scale factor can be related to the value
of gravitational coupling.Comment: Latex file, 14 page
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