21,160 research outputs found
An accurate formula for the period of a simple pendulum oscillating beyond the small-angle regime
A simple approximation formula is derived here for the dependence of the
period of a simple pendulum on amplitude that only requires a pocket calculator
and furnishes an error of less than 0.25% with respect to the exact period. It
is shown that this formula describes the increase of the pendulum period with
amplitude better than other simple formulas found in literature. A good
agreement with experimental data for a low air-resistance pendulum is also
verified and it suggests, together with the current availability/precision of
timers and detectors, that the proposed formula is useful for extending the
pendulum experiment beyond the usual small-angle oscillations.Comment: 15 pages and 4 figures. to appear in American Journal of Physic
Comparison of Ising magnet on directed versus undirected Erdos-Renyi and scale-free network
Scale-free networks are a recently developed approach to model the
interactions found in complex natural and man-made systems. Such networks
exhibit a power-law distribution of node link (degree) frequencies n(k) in
which a small number of highly connected nodes predominate over a much greater
number of sparsely connected ones. In contrast, in an Erdos-Renyi network each
of N sites is connected to every site with a low probability p (of the orde r
of 1/N). Then the number k of neighbors will fluctuate according to a Poisson
distribution. One can instead assume that each site selects exactly k neighbors
among the other sites. Here we compare in both cases the usual network with the
directed network, when site A selects site B as a neighbor, and then B
influences A but A does not influence B. As we change from undirected to
directed scale-free networks, the spontaneous magnetization vanishes after an
equilibration time following an Arrhenius law, while the directed ER networks
have a positive Curie temperature.Comment: 10 pages including all figures, for Int. J, Mod. Phys. C 1
Electronic excitation of H_2O by low-energy electron impact
Cross sections for electronic excitation of H_2O by low-energy electrons have been studied using the Schwinger multichannel method. Differential and integral cross sections for excitation of the (3a_1→3sa_1)^3A_1 dissociative state are reported for impact energies of 12, 15, and 20 eV. A comparison of the calculated integral cross sections with emission measurements for OH (A^2Σ^+) produced via dissociative electron-impact excitation of H_2O in this energy range is encouraging
Critical wave-packet dynamics in the power-law bond disordered Anderson Model
We investigate the wave-packet dynamics of the power-law bond disordered
one-dimensional Anderson model with hopping amplitudes decreasing as
. We consider the critical case ().
Using an exact diagonalization scheme on finite chains, we compute the
participation moments of all stationary energy eigenstates as well as the
spreading of an initially localized wave-packet. The eigenstates
multifractality is characterized by the set of fractal dimensions of the
participation moments. The wave-packet shows a diffusive-like spread developing
a power-law tail and achieves a stationary non-uniform profile after reflecting
at the chain boundaries. As a consequence, the time-dependent participation
moments exhibit two distinct scaling regimes. We formulate a finite-size
scaling hypothesis for the participation moments relating their scaling
exponents to the ones governing the return probability and wave-function
power-law decays
Density-functionals not based on the electron gas: Local-density approximation for a Luttinger liquid
By shifting the reference system for the local-density approximation (LDA)
from the electron gas to other model systems one obtains a new class of density
functionals, which by design account for the correlations present in the chosen
reference system. This strategy is illustrated by constructing an explicit LDA
for the one-dimensional Hubbard model. While the traditional {\it ab initio}
LDA is based on a Fermi liquid (the electron gas), this one is based on a
Luttinger liquid. First applications to inhomogeneous Hubbard models, including
one containing a localized impurity, are reported.Comment: 4 pages, 4 figures (final version, contains additional applications
and discussion; accepted by Phys. Rev. Lett.
Clustering, Angular Size and Dark Energy
The influence of dark matter inhomogeneities on the angular size-redshift
test is investigated for a large class of flat cosmological models driven by
dark energy plus a cold dark matter component (XCDM model). The results are
presented in two steps. First, the mass inhomogeneities are modeled by a
generalized Zeldovich-Kantowski-Dyer-Roeder (ZKDR) distance which is
characterized by a smoothness parameter and a power index ,
and, second, we provide a statistical analysis to angular size data for a large
sample of milliarcsecond compact radio sources. As a general result, we have
found that the parameter is totally unconstrained by this sample of
angular diameter data.Comment: 9 pages, 7 figures, accepted in Physical Review
On the Temperley-Lieb reflection matrices
This work concerns the boundary integrability of the spin-s
Temperley-Lieb model. A systematic computation method is
used to constructed the solutions of the boundary Yang-Baxter equations. For
half-integer, a general free parameter solution is presented.
It turns that for integer, the general solution has free
parameters. Moreover, some particular solutions are discussed.Comment: LaTex 17 page
Uma nova abordagem para diagnóstico de doenças via web.
RESUMO: Neste trabalho é apresentada e discutida a nova plataforma do módulo de diagnóstico de doenças (módulo produtor) do sistema Diagnose Virtual que foi desenvolvida para integrar ao portal WebAgritec. O WebAgritec é uma plataforma, concretizada na forma de um website com uma interface mais interativa para transferência das tecnologias da Embrapa para os extensionistas e produtores rurais. Esta plataforma, baseada nos conceitos de Web 2.0, visa o planejamento, previsão e monitoramento da produção agrícola e reúne informações sobre as culturas de arroz, feijão, milho, soja e trigo.SBIAgro 2011
Sistema Diagnose Virtual: módulo produtor: manual do usuário
O sistema Diagnose Virtual é um ambiente virtual na WEB para diagnóstico de doenças de plantas e enfermidades de animais que se utiliza de mecanismos de inferência (investigação) aplicados sobre o conhecimento de especialistas previamente categorizado. Este documento tem por objetivo orientar o usuário do sistema Diagnose Virtual no procedimento para sua utilização visando obter resultados corretos com menor esforço. O sistema é também dotado de ajuda online, na qual cada funcionalidade do sistema é descrita de forma sucinta mostrada desde que o ponteiro do mouse fique parado por um instante em cima da funcionalidade. Outra forma de ajuda pode ser obtida a cada tela, clicando o símbolo de interrogação no canto inferior direito. O documento aborda o módulo do usuário/produtor, no qual são exploradas as características de um problema (um caso) de uma determinada cultura até obter-se o diagnóstico. Como resultados são fornecidas as possíveis desordens com seus respectivos graus de certeza.bitstream/item/11858/1/doc85.pd
Exact Lyapunov Exponent for Infinite Products of Random Matrices
In this work, we give a rigorous explicit formula for the Lyapunov exponent
for some binary infinite products of random real matrices. All
these products are constructed using only two types of matrices, and ,
which are chosen according to a stochastic process. The matrix is singular,
namely its determinant is zero. This formula is derived by using a particular
decomposition for the matrix , which allows us to write the Lyapunov
exponent as a sum of convergent series. Finally, we show with an example that
the Lyapunov exponent is a discontinuous function of the given parameter.Comment: 1 pages, CPT-93/P.2974,late
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