4,042 research outputs found
Performance comparison of clustered and replicated information retrieval systems
The amount of information available over the Internet is increasing daily as well as the importance and magnitude of Web search engines. Systems based on a single centralised index present several problems (such as lack of scalability), which lead to the use of distributed information retrieval systems to effectively search for and locate the required information. A distributed retrieval system can be clustered and/or replicated. In this paper, using simulations, we present a detailed performance analysis, both in terms of throughput and response time, of a clustered system compared to a replicated system. In addition, we consider the effect of changes in the query topics over time. We show that the performance obtained for a clustered system does not improve the performance obtained by the best replicated system. Indeed, the main advantage of a clustered system is the reduction of network traffic. However, the use of a switched network eliminates the bottleneck in the network, markedly improving the performance of the replicated systems. Moreover, we illustrate the negative performance effect of the changes over time in the query topics when a distributed clustered system is used. On the contrary, the performance of a distributed replicated system is query independent
From de Sitter to de Sitter: A non-singular inflationary universe driven by vacuum
A semi-classical analysis of vacuum energy in the expanding spacetime
suggests that the cosmological term decays with time, with a concomitant matter
production. For early times we find, in Planck units, ,
where H is the Hubble parameter. The corresponding cosmological solution has no
initial singularity, existing since an infinite past. During an infinitely long
period we have a quasi-de Sitter, inflationary universe, with .
However, at a given time, the expansion undertakes a phase transition, with H
and decreasing to nearly zero in a few Planck times, producing a huge
amount of radiation. On the other hand, the late-time scenario is similar to
the standard model, with the radiation phase followed by a dust era, which
tends asymptotically to a de Sitter universe, with vacuum dominating again.Comment: This essay received an "honorable mention" in the 2006 Essay
Competition of the Gravity Research Foundatio
On Useful Conformal Tranformations In General Relativity
Local conformal transformations are known as a useful tool in various
applications of the gravitational theory, especially in cosmology. We describe
some new aspects of these transformations, in particular using them for
derivation of Einstein equations for the cosmological and Schwarzschild
metrics. Furthermore, the conformal transformation is applied for the
dimensional reduction of the Gauss-Bonnet topological invariant in to the
spaces of lower dimensions.Comment: 17 pages, LaTeX. The paper is intended mainly for pedagogical
purposes and represents a collection of exercises concerning local conformal
transformations and dimensional reduction. To be published in "Gravitation
and Cosmology
Obtaining pressure versus concentration phase diagrams in spin systems from Monte Carlo simulations
We propose an efficient procedure for determining phase diagrams of systems
that are described by spin models. It consists of combining cluster algorithms
with the method proposed by Sauerwein and de Oliveira where the grand canonical
potential is obtained directly from the Monte Carlo simulation, without the
necessity of performing numerical integrations. The cluster algorithm presented
in this paper eliminates metastability in first order phase transitions
allowing us to locate precisely the first-order transitions lines. We also
produce a different technique for calculating the thermodynamic limit of
quantities such as the magnetization whose infinite volume limit is not
straightforward in first order phase transitions. As an application, we study
the Andelman model for Langmuir monolayers made of chiral molecules that is
equivalent to the Blume-Emery-Griffiths spin-1 model. We have obtained the
phase diagrams in the case where the intermolecular forces favor interactions
between enantiomers of the same type (homochiral interactions). In particular,
we have determined diagrams in the surface pressure versus concentration plane
which are more relevant from the experimental point of view and less usual in
numerical studies
Integrated metabolic flux and omics analysis of leishmania major metabolism
Leishmaniasis is a virulent parasitic infection that causes a significant threat to human health worldwide. The existing drugs are becoming less effective due to the ability of Leishmania spp. to alter its metabolism to adapt to harsh environments. Understanding how this parasite manipulates its metabolism inside the host (e.g. sandfly and human) might underpin new ways to prevent the disease and develop effective treatment strategies. Despite significant advances in omics technologies, biochemistry of parasites still lacks the understanding of molecular components that determine the metabolic behavior under varying conditions. Metabolic network modeling might be of interest to identify physiologically relevant nodes in a metabolic network. The present work proposes a metabolic model iSK570 (an extension of the iAC560 model) with additional reactions for the metabolism of lipids, long chain fatty acids and carbohydrates to study the metabolic behavior of this parasite. Gene Inactivity Moderated by Metabolism and Expression (GIMME) algorithm was used to verify the consistency between model flux predictions and gene expression data. Improved flux distributions were obtained, allowing a more accurate understanding of stage-specific metabolism in of promastigotes and amastigotes.This work was supported by the Initial Training Network, GlycoPar, funded by the FP7 Marie Curie Actions of the European Commission (FP7-PEOPLE-2013-ITN-608295). The authors gratefully express appreciation to SilicoLife Lda for providing required infrastructural facilities related to this work. We also thank Bruno Pereira (systems biologist at SilicoLife) and Hugo Giesteira (programmer at SilicoLife) for scientific and technical assistance during various phases of the project.info:eu-repo/semantics/publishedVersio
Non-singular inflation with vacuum decay
On the basis of a semi-classical analysis of vacuum energy in an expanding
spacetime, we describe a non-singular cosmological model in which the vacuum
density decays with time, with a concomitant production of matter. During an
infinitely long period we have an empty, inflationary universe, with H \approx
1. This primordial era ends in a fast phase transition, during which H and
\Lambda decrease to nearly zero in a few Planck times, with release of a huge
amount of radiation. The late-time scenario is similar to the standard model,
with the radiation phase followed by a long dust era, which tends
asymptotically to a de Sitter universe, with vacuum dominating again. An
analysis of the redshift-distance relation for supernovas Ia leads to
cosmological parameters in agreement with other current estimations.Comment: Work presented at IRGAC 2006, Barcelona, July 11-15 2006. To appear
in a special issue of Journal of Physics
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