2,731 research outputs found
Cosmological simulations using a static scalar-tensor theory
We present CDM -body cosmological simulations in the framework of
a static general scalar-tensor theory of gravity. Due to the influence of the
non-minimally coupled scalar field, the gravitational potential is modified by
a Yukawa type term, yielding a new structure formation dynamics. We present
some preliminary results and, in particular, we compute the density and
velocity profiles of the most massive group.Comment: 4 pages, 6 figures, to appear in Journal of Physics: Conference
Series: VII Mexican School on Gravitation and Mathematical Physics. 26
November to 1 December 2006, Playa del Carmen, Quintana Roo, Mexic
Expansion-Free Cavity Evolution: Some exact Analytical Models
We consider spherically symmetric distributions of anisotropic fluids with a
central vacuum cavity, evolving under the condition of vanishing expansion
scalar. Some analytical solutions are found satisfying Darmois junction
conditions on both delimiting boundary surfaces, while some others require the
presence of thin shells on either (or both) boundary surfaces. The solutions
here obtained model the evolution of the vacuum cavity and the surrounding
fluid distribution, emerging after a central explosion. This study complements
a previously published work where modeling of the evolution of such kind of
systems was achieved through a different kinematical condition.Comment: 9 pages, Revtex. Typos corrected. Published in Int. J. Mod. Phys.
Atributos microbiológicos do solo podem explicar níveis de produtividade de soja em áreas sob plantio direto no Paraná.
Edge-functionalized and substitutional doped graphene nanoribbons: electronic and spin properties
Graphene nanoribbons are the counterpart of carbon nanotubes in
graphene-based nanoelectronics. We investigate the electronic properties of
chemically modified ribbons by means of density functional theory. We observe
that chemical modifications of zigzag ribbons can break the spin degeneracy.
This promotes the onset of a semiconducting-metal transition, or of an
half-semiconducting state, with the two spin channels having a different
bandgap, or of a spin-polarized half-semiconducting state -where the spins in
the valence and conduction bands are oppositely polarized. Edge
functionalization of armchair ribbons gives electronic states a few eV away
from the Fermi level, and does not significantly affect their bandgap. N and B
produce different effects, depending on the position of the substitutional
site. In particular, edge substitutions at low density do not significantly
alter the bandgap, while bulk substitution promotes the onset of
semiconducting-metal transitions. Pyridine-like defects induce a
semiconducting-metal transition.Comment: 12 pages, 5 figure
Process model comparison based on cophenetic distance
The automated comparison of process models has received increasing attention in the last decade, due to the growing existence of process models and repositories, and the consequent need to assess similarities between the underlying processes. Current techniques for process model comparison are either structural (based on graph edit
distances), or behavioural (through activity profiles or the analysis of the execution semantics). Accordingly, there is a gap between the quality of the information provided by these two families, i.e., structural techniques may be fast but inaccurate, whilst behavioural are accurate but complex. In this paper we present a novel technique, that is based on a well-known technique to compare labeled trees through the notion of Cophenetic distance. The technique lays between
the two families of methods for comparing a process model: it has an structural nature, but can provide accurate information on the differences/similarities of two process models. The experimental evaluation on various benchmarks sets are reported, that position the proposed technique as a valuable tool for process model comparison.Peer ReviewedPostprint (author's final draft
Isotropization of Bianchi type models and a new FRW solution in Brans-Dicke theory
Using scaled variables we are able to integrate an equation valid for
isotropic and anisotropic Bianchi type I, V, IX models in Brans-Dicke (BD)
theory. We analyze known and new solutions for these models in relation with
the possibility that anisotropic models asymptotically isotropize, and/or
possess inflationary properties. In particular, a new solution of curve
() Friedmann-Robertson-Walker (FRW) cosmologies in Brans-Dicke theory
is analyzed.Comment: 15 pages, 4 postscript figures, to appear in Gen. Rel. Grav., special
issue dedicated in honour of Prof. H. Dehne
Bundle Theory of Improper Spin Transformations
{\it We first give a geometrical description of the action of the parity
operator () on non relativistic spin Pauli spinors in
terms of bundle theory. The relevant bundle, , is a
non trivial extension of the universal covering group .
is the non relativistic limit of the corresponding Dirac matrix
operator and obeys . Then, from the direct
product of O(3) by , naturally induced by the structure of the galilean
group, we identify, in its double cover, the time reversal operator ()
acting on spinors, and its product with . Both, and
, generate the group . As in the case of parity,
is the non relativistic limit of the corresponding Dirac matrix
operator , and obeys .}Comment: 8 pages, Plaintex; titled changed, minor text modifications, one
reference complete
Carbono orgânico e atributos microbiológicos em solo agrícola com diferentes níveis de produtividade de soja em sistema de plantio direto.
Atributos microbiológicos e bioquímicos do solo são geralmente negligenciados ao se relacionar a fertilidade do solo e a produtividade das culturas, em que o carbono orgânico (C) apresenta papel chave. O objetivo deste trabalho foi avaliar alguns bioindicadores de qualidade de solo e relacioná-los com o teor de C do solo em áreas de produção comercial com diferentes níveis de produtividade de soja sob sistema de plantio direto. Amostras compostas de solo foram obtidas na profundidade 0-10 cm, em 6 áreas agrícolas em Ponta Grossa, PR. Os dados foram submetidos à análise de correlação de Pearson e regressão múltipla. A regressão múltipla mostrou que a produtividade é influenciada principalmente pelo teor de C orgânico no solo, e a correlação simples de Pearson mostrou-se significativa entre o C orgânico e a biomassa microbiana de carbono (BMC), biomassa microbiana de nitrogênio (BMN), capacidade de troca de cátions (CTC) e atividades enzimáticas. Conclui-se que há uma relação entre a produtividade de grãos de soja e a atividade microbiana no solo, em que o C orgânico apresenta papel chave.Fertbio
Probing Yukawian gravitational potential by numerical simulations. I. Changing N-body codes
In the weak field limit general relativity reduces, as is well known, to the
Newtonian gravitation. Alternative theories of gravity, however, do not
necessarily reduce to Newtonian gravitation; some of them, for example, reduce
to Yukawa-like potentials instead of the Newtonian potential. Since the
Newtonian gravitation is largely used to model with success the structures of
the universe, such as for example galaxies and clusters of galaxies, a way to
probe and constrain alternative theories, in the weak field limit, is to apply
them to model the structures of the universe. In the present study, we consider
how to probe Yukawa-like potentials using N-body numerical simulations.Comment: 17 pages, 11 figures. To appear in General Relativity and Gravitatio
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