31,553 research outputs found
Optimization of hierarchical structures of information flow
The efficiency of a large hierarchical organisation is simulated on
Barabasi-Albert networks, when each needed link leads to a loss of information.
The optimum is found at a finite network size, corresponding to about five
hierarchical layers, provided a cost for building the network is included in
our optimization.Comment: Draft of 6 pages including all figure
Hermitian Dirac Hamiltonian in time dependent gravitational field
It is shown by a straightforward argument that the Hamiltonian generating the
time evolution of the Dirac wave function in relativistic quantum mechanics is
not hermitian with respect to the covariantly defined inner product whenever
the background metric is time dependent. An alternative, hermitian, Hamiltonian
is found and is shown to be directly related to the canonical field Hamiltonian
used in quantum field theory.Comment: 9 pages, final version, to appear in Class. Quant. Gra
Impacto do sedimento sobre o desenvolvimento do perifíton no Rio Taquari, Pantanal, MS.
As macrófitas aquáticas são muito abundantes em áreas alagadas como as do Pantanal, e fornecem um vasto substrato para a microbiota, em particular para a comunidade perifítica. Foi avaliado o efeito da deposição de sedimento sobre a comunidade perifítica ao longo de 40km do trecho final do rio Taquari, em 12 estações de amostragem, em setembro de 1998. Foram coletados pecíolos de Eichhornia azurea, raspados e filtrados para análise da biomassa perifítica (clorofila a e peso seco). Foram tomadas amostras de raízes, caules e folhas da macrófita para análise do sedimento aderido. E as características da água também foram estudadas. Do sedimento suspenso analisado, 55% foi depositado até a última estação, e 86% é de origem inorgânica. A transparência da água passou de 27cm a 50cm na última estação de coleta, próxima à foz. A biomassa ficoperifítica aumentou nas últimas estações, sugerindo que próximo da foz a fixação e desenvolvimento dos organismos, principalmente os autotróficos, é beneficiada pela diminuição acentuada do material inorgânico.bitstream/item/37415/1/BP37.pd
Spatial patterns and biodiversity in off-lattice simulations of a cyclic three-species Lotka-Volterra model
Stochastic simulations of cyclic three-species spatial predator-prey models
are usually performed in square lattices with nearest neighbor interactions
starting from random initial conditions. In this Letter we describe the results
of off-lattice Lotka-Volterra stochastic simulations, showing that the
emergence of spiral patterns does occur for sufficiently high values of the
(conserved) total density of individuals. We also investigate the dynamics in
our simulations, finding an empirical relation characterizing the dependence of
the characteristic peak frequency and amplitude on the total density. Finally,
we study the impact of the total density on the extinction probability, showing
how a low population density may jeopardize biodiversity.Comment: 5 pages, 7 figures; new version, with new title and figure
Mexilhão dourado no Pantanal - um problema ambiental e econômico.
bitstream/CPAP/56433/1/ADM032.pdfFormato eletrônico
SimPhy: Phylogenomic Simulation of Gene, Locus, and Species Trees
We present a fast and flexible software package--SimPhy--for the simulation of multiple gene families evolving under incomplete lineage sorting, gene duplication and loss, horizontal gene transfer--all three potentially leading to species tree/gene tree discordance--and gene conversion. SimPhy implements a hierarchical phylogenetic model in which the evolution of species, locus, and gene trees is governed by global and local parameters (e.g., genome-wide, species-specific, locus-specific), that can be fixed or be sampled from a priori statistical distributions. SimPhy also incorporates comprehensive models of substitution rate variation among lineages (uncorrelated relaxed clocks) and the capability of simulating partitioned nucleotide, codon, and protein multilocus sequence alignments under a plethora of substitution models using the program INDELible. We validate SimPhy's output using theoretical expectations and other programs, and show that it scales extremely well with complex models and/or large trees, being an order of magnitude faster than the most similar program (DLCoal-Sim). In addition, we demonstrate how SimPhy can be useful to understand interactions among different evolutionary processes, conducting a simulation study to characterize the systematic overestimation of the duplication time when using standard reconciliation methods. SimPhy is available at https://github.com/adamallo/SimPhy, where users can find the source code, precompiled executables, a detailed manual and example cases
The CO A-X System for Constraining Cosmological Drift of the Proton-Electron Mass Ratio
The band system of carbon monoxide,
which has been detected in six highly redshifted galaxies (), is
identified as a novel probe method to search for possible variations of the
proton-electron mass ratio () on cosmological time scales. Laboratory
wavelengths of the spectral lines of the A-X (,0) bands for have
been determined at an accuracy of
through VUV Fourier-transform absorption spectroscopy, providing a
comprehensive and accurate zero-redshift data set. For the (0,0) and (1,0)
bands, two-photon Doppler-free laser spectroscopy has been applied at the accuracy level, verifying the absorption data. Sensitivity
coefficients for a varying have been calculated for the CO A-X
bands, so that an operational method results to search for -variation.Comment: 7 pages (main article), 3 figures, includes supplementary materia
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