2,936 research outputs found
Adaptive Wavelet Collocation Method for Simulation of Time Dependent Maxwell's Equations
This paper investigates an adaptive wavelet collocation time domain method
for the numerical solution of Maxwell's equations. In this method a
computational grid is dynamically adapted at each time step by using the
wavelet decomposition of the field at that time instant. In the regions where
the fields are highly localized, the method assigns more grid points; and in
the regions where the fields are sparse, there will be less grid points. On the
adapted grid, update schemes with high spatial order and explicit time stepping
are formulated. The method has high compression rate, which substantially
reduces the computational cost allowing efficient use of computational
resources. This adaptive wavelet collocation method is especially suitable for
simulation of guided-wave optical devices
Modeling Dual Pathways for the Metazoan Spindle Assembly Checkpoint
Using computational modelling, we investigate mechanisms of signal
transduction focusing on the spindle assembly checkpoint where a single
unattached kinetochore is able to signal to prevent cell cycle progression.
This inhibitory signal switches off rapidly once spindle microtubules have
attached to all kinetochores. This requirement tightly constrains the possible
mechanisms. Here we investigate two possible mechanisms for spindle checkpoint
operation in metazoan cells, both supported by recent experiments. The first
involves the free diffusion and sequestration of cell-cycle regulators. This
mechanism is severely constrained both by experimental fluorescence recovery
data and also by the large volumes involved in open mitosis in metazoan cells.
Using a simple mathematical analysis and computer simulation, we find that this
mechanism can generate the inhibition found in experiment but likely requires a
two stage signal amplification cascade. The second mechanism involves spatial
gradients of a short-lived inhibitory signal that propagates first by diffusion
but then primarily via active transport along spindle microtubules. We propose
that both mechanisms may be operative in the metazoan spindle assembly
checkpoint, with either able to trigger anaphase onset even without support
from the other pathway.Comment: 9 pages, 2 figure
Scattering of rare-gas atoms at a metal surface: evidence of anticorrugation of the helium-atom potential-energy surface and the surface electron density
Recent measurements of the scattering of He and Ne atoms at Rh(110) suggest
that these two rare-gas atoms measure a qualitatively different surface
corrugation: While Ne atom scattering seemingly reflects the electron-density
undulation of the substrate surface, the scattering potential of He atoms
appears to be anticorrugated. An understanding of this perplexing result is
lacking. In this paper we present density functional theory calculations of the
interaction potentials of He and Ne with Rh(110). We find that, and explain
why, the nature of the interaction of the two probe particles is qualitatively
different, which implies that the topographies of their scattering potentials
are indeed anticorrugated.Comment: RevTeX, 4 pages, 10 figure
Facebook polls as proto-democratic instruments in the Egyptian revolution: The ‘We Are All Khaled Said’ Facebook page
A Page and Its Politics: Situating Kullinā Khālid Saʿīd in Egypt’s Ideological Landscape at the Time of Revolution
In discussions concerning the importance of social media in the 25January revolution, a central role is given to the "Kullinā Khālid Saʿīd" [We’re all Khaled Said] Facebook page. Using an advanced data collection and extraction application called Netvizz, a research team consisting of Arabists and Media studies specialists has collected and analysed all of the posts and comments exchanged through the page. This data set allows for a systematic analysis of the page. This article offers an outline of the ideological nature of "Kullinā Khālid Saʿīd", with particular emphasis on the "revolutionary" period between 1 January - 11 February 2011. It argues that the page shows no evidence of political bias in the sense of explicit favoring of a political group. Rather, the page constituted a community of users who abstained from using politically factional language. Reflecting the mood and concerns of the revolution’s grassroots masses, it clearly illustrates the disinclination to engage with formal politics
Demographic characteristics of patients with extrapulmonary tuberculosis in Germany
The aim of the present study was to determine the demographics of patients with extrapulmonary tuberculosis in Germany. Data on 26,302 tuberculosis cases from a national survey carried out during the period 1996-2000 were analysed. The crude proportion of tuberculosis patients with extrapulmonary manifestations was 21.6%. Extrapulmonary tuberculosis was most likely among females, children aged <15 yrs and persons originating from Africa and Asia. Females tended to be more likely to have any form of extrapulmonary tuberculosis than males, except pleural tuberculosis. The strength of this association was strongest in the age range 25-64 yrs and less pronounced amongst the oldest patients. Children were particularly prone to the development of lymphatic and meningeal tuberculosis, whereas the likelihood of genitourinary tuberculosis increased with increasing age. Asian and African patients were generally more likely than persons from other areas to have lymphatic, osteoarticular, meningeal and miliary tuberculosis. The analysis shows important differences, by age, sex and origin, in the likelihood of a tuberculosis patient presenting with extrapulmonary tuberculosis. Since the relative contribution of the foreign-born to tuberculosis in low-prevalence countries is rising, extrapulmonary tuberculosis must be taken into account more often in the differential diagnostic work-up of these patients, particularly among those originating from Asia and Africa
In vivo expression and purification of aptamer-tagged small RNA regulators
Small non-coding RNAs (sRNAs) are an emerging class of post-transcriptional regulators of bacterial gene expression. To study sRNAs and their potential protein interaction partners, it is desirable to purify sRNAs from cells in their native form. Here, we used RNA-based affinity chromatography to purify sRNAs following their expression as aptamer-tagged variants in vivo. To this end, we developed a family of plasmids to express sRNAs with any of three widely used aptamer sequences (MS2, boxB, eIF4A), and systematically tested how the aptamer tagging impacted on intracellular accumulation and target regulation of the Salmonella GcvB, InvR or RybB sRNAs. In addition, we successfully tagged the chromosomal rybB gene with MS2 to observe that RybB-MS2 is fully functional as an envelope stress-induced repressor of ompN mRNA following induction of sigmaE. We further demonstrate that the common sRNA-binding protein, Hfq, co-purifies with MS2-tagged sRNAs of Salmonella. The presented affinity purification strategy may facilitate the isolation of in vivo assembled sRNA-protein complexes in a wide range of bacteria
Chemical and physical influences on aerosol activation in liquid clouds: a study based on observations from the Jungfraujoch, Switzerland
A simple statistical model to predict the number of aerosols which activate to form cloud droplets in warm clouds has been established, based on regression analysis of data from four summertime Cloud and Aerosol Characterisation Experiments (CLACE) at the high-altitude site Jungfraujoch (JFJ). It is shown that 79 % of the observed variance in droplet numbers can be represented by a model accounting only for the number of potential cloud condensation nuclei (defined as number of particles larger than 80 nm in diameter), while the mean errors in the model representation may be reduced by the addition of further explanatory variables, such as the mixing ratios of O3, CO, and the height of the measurements above cloud base. The statistical model has a similar ability to represent the observed droplet numbers in each of the individual years, as well as for the two predominant local wind directions at the JFJ (northwest and southeast). Given the central European location of the JFJ, with air masses in summer being representative of the free troposphere with regular boundary layer in-mixing via convection, we expect that this statistical model is generally applicable to warm clouds under conditions where droplet formation is aerosol limited (i.e. at relatively high updraught velocities and/or relatively low aerosol number concentrations). A comparison between the statistical model and an established microphysical parametrization shows good agreement between the two and supports the conclusion that cloud droplet formation at the JFJ is predominantly controlled by the number concentration of aerosol particles
Anomalous thermal conductivity and local temperature distribution on harmonic Fibonacci chains
The harmonic Fibonacci chain, which is one of a quasiperiodic chain
constructed with a recursion relation, has a singular continuous
frequency-spectrum and critical eigenstates. The validity of the Fourier law is
examined for the harmonic Fibonacci chain with stochastic heat baths at both
ends by investigating the system size N dependence of the heat current J and
the local temperature distribution. It is shown that J asymptotically behaves
as (ln N)^{-1} and the local temperature strongly oscillates along the chain.
These results indicate that the Fourier law does not hold on the harmonic
Fibonacci chain. Furthermore the local temperature exhibits two different
distribution according to the generation of the Fibonacci chain, i.e., the
local temperature distribution does not have a definite form in the
thermodynamic limit. The relations between N-dependence of J and the
frequency-spectrum, and between the local temperature and critical eigenstates
are discussed.Comment: 10 pages, 4 figures, submitted to J. Phys.: Cond. Ma
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