15,126 research outputs found
The Apparent Fractal Conjecture: Scaling Features in Standard Cosmologies
This paper presents an analysis of the smoothness problem in cosmology by
focussing on the ambiguities originated in the simplifying hypotheses aimed at
observationally verifying if the large-scale distribution of galaxies is
homogeneous, and conjecturing that this distribution should follow a fractal
pattern in perturbed standard cosmologies. This is due to a geometrical effect,
appearing when certain types of average densities are calculated along the past
light cone. The paper starts reviewing the argument concerning the possibility
that the galaxy distribution follows such a scaling pattern, and the premises
behind the assumption that the spatial homogeneity of standard cosmology can be
observable. Next, it is argued that to discuss observable homogeneity one needs
to make a clear distinction between local and average relativistic densities,
and showing how the different distance definitions strongly affect them,
leading the various average densities to display asymptotically opposite
behaviours. Then the paper revisits Ribeiro's (1995: astro-ph/9910145) results,
showing that in a fully relativistic treatment some observational average
densities of the flat Friedmann model are not well defined at z ~ 0.1, implying
that at this range average densities behave in a fundamentally different manner
as compared to the linearity of the Hubble law, well valid for z < 1. This
conclusion brings into question the widespread assumption that relativistic
corrections can always be neglected at low z. It is also shown how some key
features of fractal cosmologies can be found in the Friedmann models. In view
of those findings, it is suggested that the so-called contradiction between the
cosmological principle, and the galaxy distribution forming an unlimited
fractal structure, may not exist.Comment: 30 pages, 2 figures, LaTeX. This paper is a follow-up to
gr-qc/9909093. Accepted for publication in "General Relativity and
Gravitation
Confining potential in a color dielectric medium with parallel domain walls
We study quark confinement in a system of two parallel domain walls
interpolating different color dielectric media. We use the phenomenological
approach in which the confinement of quarks appears considering the QCD vacuum
as a color dielectric medium. We explore this phenomenon in QCD_2, where the
confinement of the color flux between the domain walls manifests, in a scenario
where two 0-branes (representing external quark and antiquark) are connected by
a QCD string. We obtain solutions of the equations of motion via first-order
differential equations. We find a new color confining potential that increases
monotonically with the distance between the domain walls.Comment: RevTex4, 5 pages, 1 figure; version to appear in Int. J. Mod. Phys.
Mutual information in random Boolean models of regulatory networks
The amount of mutual information contained in time series of two elements
gives a measure of how well their activities are coordinated. In a large,
complex network of interacting elements, such as a genetic regulatory network
within a cell, the average of the mutual information over all pairs is a
global measure of how well the system can coordinate its internal dynamics. We
study this average pairwise mutual information in random Boolean networks
(RBNs) as a function of the distribution of Boolean rules implemented at each
element, assuming that the links in the network are randomly placed. Efficient
numerical methods for calculating show that as the number of network nodes
N approaches infinity, the quantity N exhibits a discontinuity at parameter
values corresponding to critical RBNs. For finite systems it peaks near the
critical value, but slightly in the disordered regime for typical parameter
variations. The source of high values of N is the indirect correlations
between pairs of elements from different long chains with a common starting
point. The contribution from pairs that are directly linked approaches zero for
critical networks and peaks deep in the disordered regime.Comment: 11 pages, 6 figures; Minor revisions for clarity and figure format,
one reference adde
Análise da atividade anti-apoptótica do gene iap-3 de Anticarsia gemmatalis MNPV.
bitstream/CENARGEN/24245/1/bp042.pd
Mathematical model of welding parameters for rapid prototyping using robot welding
Rapid Prototyping is a relatively new technology that allows the creation of prototypes in a very short period of time compared with traditional manufacturing techniques. First, a model of the prototype is drawn, using a computer aided design program, which is then mathematically ‘sliced’ and used to build the prototype layer by layer, using material such as paper, resins, or thermoplastics, depending on the process. The main disadvantage of these processes is that they do not allow metal as a raw material. Rapid Prototyping using Robot welding is another approach that overcomes this problem by using a welding robot that deposits metal. As the success of the final component quality depends very much on the welding parameters, it is important to automate their calculation. To automate the task of determining the welding parameters and to generate welded components with consistent quality, a very simple mathematical algorithm was created. The tests carried out to gather the necessary information to generate this model, the mathematical model itself, the limitations of the equations, and the tests to check their feasibility are described.
At the time the work was carried out, the authors were in the welding Engineering Groups, SIMS, Cranfield University, Cranfield, Beds. MK43 0SY, UK. Dr Ribeiro is now in the Department of Industrial Electronics, University of Minho, 4800 Guimarães, Portugal and Professor Norrish is in the Faculty of Engineering, University of Wollongong, Wollongong, NSW 2522, Australia. Manuscript received 12 May 1997; in final form 20 June 1997
Comportamento alimentar de Scaptotrigona bipunctata (Hymenoptera, Apidae, Meliponini) em três Municípios do Estado do Ceará.
As abelhas sem ferrão Scaptotrigona bipunctata tem uma ampla distribuição no território brasileiro, inclusive no Estado do Ceará, e apresentam um comportamento generalista de coleta de alimento floral em uma grande variedade de espécies vegetais. Com o objetivo de conhecer as plantas visitadas por S. bipunctata foi realizado um trabalho de identificação da flora fornecedora de recursos tróficos. As amostras de pólen foram coletadas diretamente dos potes de alimento de S. bipunctata em meliponários nos municípios de Fortaleza, Horizonte e Maracanaú. As coletas contaram com nove, doze e três repetições respectivamente. As análises polínicas foram conduzidas ao Laboratório de Abelha s da UFC. Para a identificação dos grãos de pólen foram montadas triplicatas de lâminas seguindo a metodologia de Miranda e Andrade (1990). Após a identificação dos tipos polínicos, foi realizada uma contagem de 300 grãos de pólen por repetição e foram calculadas as médias e os desvios padrões. Os resultados mostraram que as abelhas coletaram pólen de cinco espécies em Horizonte e Maracanaú: Cássia (Cássia fistula ), Guaxuma (Waltheria brachypetala ), Juazeiro (Zizyphus joazeiro), Sabiá (Mimosa caesalpinifolia) e Vassourinha de botão (Spermacoce verticillata). Vassourinha de botão foi a espécie dominante em Horizonte e Guaxuma em Maracanaú. Em Fortaleza, os tipos polínicos encontrados foram: Acácia (Acacia glomerosa), Cássia (C. fistula), Coqueiro (Cocus nucifera), Guaxuma (W. brachypetala), Juazeiro (Z. joazeiro ), Mamona (Ricinus communis), Mutre (Aloysia virgata), Quebra-panela (Alternanthera tenella), Sabiá(M. caesalpinifolia) e Vassourinha de botão (S. verticillata). O pólen dominante nas amostras de Fortaleza foram de Mamona ( R. communis)
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