10,327 research outputs found
The growth of matter perturbations in f(R) models
We consider the linear growth of matter perturbations on low redshifts in
some dark energy (DE) models. We discuss the definition of dark energy
(DE) in these models and show the differences with scalar-tensor DE models. For
the model recently proposed by Starobinsky we show that the growth
parameter takes the value for
and for , allowing
for a clear distinction from CDM. Though a scale-dependence appears in
the growth of perturbations on higher redshifts, we find no dispersion for
on low redshifts up to , is also
quasi-linear in this interval. At redshift , the dispersion is still
small with . As for some scalar-tensor models, we
find here too a large value for ,
for and for
. These values are largely outside the range found for DE
models in General Relativity (GR). This clear signature provides a powerful
constraint on these models.Comment: 14 pages, 7 figures, improved presentation, references added, results
unchanged, final version to be published in JCA
Axiomatization and Models of Scientific Theories
In this paper we discuss two approaches to the axiomatization of scien- tific theories in the context of the so called semantic approach, according to which (roughly) a theory can be seen as a class of models. The two approaches are associated respectively to Suppes’ and to da Costa and Chuaqui’s works. We argue that theories can be developed both in a way more akin to the usual mathematical practice (Suppes), in an informal set theoretical environment, writing the set theoretical predicate in the language of set theory itself or, more rigorously (da Costa and Chuaqui), by employing formal languages that help us in writing the postulates to define a class of structures. Both approaches are called internal, for we work within a mathematical framework, here taken to be first-order ZFC. We contrast these approaches with an external one, here discussed briefly. We argue that each one has its strong and weak points, whose discussion is relevant for the philosophical foundations of science
Anticarsia gemmatalis Hübner, 1818 (Lepidoptera: Noctuidae) biologia, amostragem e métodos de controle.
A lagarta da soja, Anticarsia gemmatalis Hübner, 1818 (Lepidoptera: Noctuidae) é a principal desfolhadora da soja no Brasil, sendo encontrada em todos os locais de produção, representando um risco à produção e à qualidade dos cultivos brasileiros (GAZZONI e YORINIORI, 1995; MOSCARDI e SOUZA, 2002). Este inseto causa grandes danos à lavoura de soja, que vão desde o desfolhamento até a destruição completa da planta. A lagarta da soja é um inseto mastigador e se alimenta de folhas jovens. Quando a folhagem é removida, ataca outras partes da planta, como pecíolos e a haste. O desfolhamento compromete o enchimento das vagens, devido à diminuição da área foliar responsável pela fotossíntese, com conseqüente redução da produção de grãos. Quando se alimentam, além de remover os tecidos vegetais, que contem nutrientes, as lagartas injetam toxinas nas plantas (SILVA et al., 2002). Existem, também, outras culturas em que a lagarta da soja causa prejuízos como cultura da alfafa, do amendoim, do arroz, da ervilha, do feijão, do feijão-vagem e do trigo, atacando durante a fase vegetativa e, em alguns casos, até no período da floração (PRATISSOLI, 2002). A lagarta da soja costuma atacar as lavouras nas regiões setentrionais e a partir de janeiro no extremo sul do País e chega a ocasionar 100% de destruição foliar (HOFFMANN et al., 1979; GAZZONI e YORINIORI, 1995), sendo, portanto, de grande importância conhecer o comportamento desta praga e os métodos utilizados para o seu controle.bitstream/CENARGEN/27983/1/doc196.pd
Determinação do grau crítico de umidade em sementes de Cenostigma tocantinum Ducke e Dipteryx odorata Willd.
Este trabalho teve como objetivo avaliar o comportamento das sementes desta espécie quanto a sensibilidade à desidratação
Nematic liquid crystal dynamics under applied electric fields
In this paper we investigate the dynamics of liquid crystal textures in a
two-dimensional nematic under applied electric fields, using numerical
simulations performed using a publicly available LIquid CRystal Algorithm
(LICRA) developed by the authors. We consider both positive and negative
dielectric anisotropies and two different possibilities for the orientation of
the electric field (parallel and perpendicular to the two-dimensional lattice).
We determine the effect of an applied electric field pulse on the evolution of
the characteristic length scale and other properties of the liquid crystal
texture network. In particular, we show that different types of defects are
produced after the electric field is switched on, depending on the orientation
of the electric field and the sign of the dielectric anisotropy.Comment: 7 pages, 12 figure
Electrostatic self-force in (2+1)-dimensional cosmological gravity
Point sources in (2+1)-dimensional gravity are conical singularities that
modify the global curvature of the space giving rise to self-interaction
effects on classical fields. In this work we study the electrostatic
self-interaction of a point charge in the presence of point masses in
(2+1)-dimensional gravity with a cosmological constant.Comment: 9 pages, Late
Uma abordagem com cálculo fracionário ao transporte de contaminantes em solos tropicais lateríticos.
Poliploidia em seringueira: III - Estudo comparativo entre clones diplóides e novos poliplóides putativos em condições de jardim clonal.
Foram avaliadas a produção de borracha e o vigor das plantas em clones putativos de Hevea brasiliensis (Fx 3864 pl e Fx 985 pl)
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