1,057 research outputs found
A command for fitting mixture regression models for bounded dependent variables using the beta distribution
In this article, we describe the betamix command, which fits mixture regression models for dependent variables bounded in an interval. The model is a generalization of the truncated inflated beta regression model introduced in Pereira, Botter, and Sandoval (2012, Communications in Statistics—Theory and Methods 41: 907-919) and the mixture beta regression model in Verkuilen and Smithson (2012, Journal of Educational and Behavioral Statistics 37: 82-113) for variables with truncated supports at either the top or the bottom of the distribution. betamix accepts dependent variables defined in any range that are then transformed to the interval (0, 1) before estimation
Estimated vitamin D synthesis and dietary vitamin D intake among Asians in two distinct geographical locations (Kuala Lumpur, 3°N versus Aberdeen, 57°N) and climates
Objective:
To compare the contributions of UVB exposure and diet to total vitamin D among Asians living in Kuala Lumpur (KL) and Aberdeen (AB).
Design:
Longitudinal study.
Setting:
UVB exposure (using polysulfone film badges) and skin colour and dietary vitamin D intake (by web-based questionnaire) were measured at each season in AB and during south-west (SWM) and north-east monsoons (NEM) in KL.
Subjects:
One hundred and fifteen Asians in KL and eighty-five Asians in AB aged 20–50 years.
Results:
Median summer UVB exposure of Asians in AB (0·25 SED/d) was higher than UVB exposure for the KL participants (SWM=0·20 SED/d, P=0·02; NEM= 0·14 SED/d, P<0·01). UVB exposure was the major source of vitamin D in KL year-round (60%) but only during summer in AB (59%). Median dietary vitamin D intake was higher in AB (3·50 µg/d (140 IU/d)), year-round, than in KL (SWM=2·05 µg/d (82 IU/d); NEM=1·83 µg/d (73 IU/d), P<0·01). Median total vitamin D (UVB plus diet) was higher in AB only during summer (8·45 µg/d (338 IU/d)) compared with KL (SWM=6·03 µg/d (241 IU/d), P=0·04; NEM=5·35 µg/d (214 IU/d), P<0·01), with a comparable intake across the full year (AB=5·75 µg/d (230 IU/d); KL=6·15 µg/d (246 IU/d), P=0·78).
Conclusions:
UVB exposure among Asians in their home country is low. For Asians residing at the northerly latitude of Scotland, acquiring vitamin D needs from UVB exposure alone (except in summer) may be challenging due to low ambient UVB in AB (available only from April to October)
Detailed Examination of Transport Coefficients in Cubic-Plus-Quartic Oscillator Chains
We examine the thermal conductivity and bulk viscosity of a one-dimensional
(1D) chain of particles with cubic-plus-quartic interparticle potentials and no
on-site potentials. This system is equivalent to the FPU-alpha beta system in a
subset of its parameter space. We identify three distinct frequency regimes
which we call the hydrodynamic regime, the perturbative regime and the
collisionless regime. In the lowest frequency regime (the hydrodynamic regime)
heat is transported ballistically by long wavelength sound modes. The model
that we use to describe this behaviour predicts that as the frequency goes to
zero the frequency dependent bulk viscosity and the frequency dependent thermal
conductivity should diverge with the same power law dependence on frequency.
Thus, we can define the bulk Prandtl number as the ratio of the bulk viscosity
to the thermal conductivity (with suitable prefactors to render it
dimensionless). This dimensionless ratio should approach a constant value as
frequency goes to zero. We use mode-coupling theory to predict the zero
frequency limit. Values of the bulk Prandtl number from simulations are in
agreement with these predictions over a wide range of system parameters. In the
middle frequency regime, which we call the perturbative regime, heat is
transported by sound modes which are damped by four-phonon processes. We call
the highest frequency regime the collisionless regime since at these
frequencies the observing times are much shorter than the characteristic
relaxation times of phonons. The perturbative and collisionless regimes are
discussed in detail in the appendices.Comment: Latex with references in .bib file. 36 pages, 8 figures. Submitted to
J. Stat. Phys. on Sept. 2
Structure of isobaric analog states in 91Nb populated by the 90Zr(a,t) reaction
Decay via proton emission of isobaric analog states (IAS's) in
was studied using the reaction at =180 MeV.
This study provides information about the damping mechanism of these states.
Decay to the ground state and low-lying phonon states in was
observed. The experimental data are compared with theoretical predictions
wherein the IAS `single-particle' proton escape widths are calculated in a
continuum RPA approach. The branching ratios for decay to the phonon states are
explained using a simple model.Comment: 3 figures. submitted to Phys. Lett.
Arabidopsis RecQl4A suppresses homologous recombination and modulates DNA damage responses
The DNA damage response and DNA recombination are two interrelated mechanisms involved in maintaining the integrity of the genome, but in plants they are poorly understood. RecQ is a family of genes with conserved roles in the regulation of DNA recombination in eukaryotes; there are seven members in Arabidopsis. Here we report on the functional analysis of the Arabidopsis RecQl4A gene. Ectopic expression of Arabidopsis RecQl4A in yeast RecQ-deficient cells suppressed their hypersensitivity to the DNA-damaging drug methyl methanesulfonate (MMS) and enhanced their rate of homologous recombination (HR). Analysis of three recQl4A mutant alleles revealed no obvious developmental defects or telomere deregulation in plants grown under standard growth conditions. Compared with wild-type Arabidopsis, the recQl4A mutant seedlings were found to be hypersensitive to UV light and MMS, and more resistant to mitomycin C. The average frequency of intrachromosomal HR in recQl4A mutant plants was increased 7.5-fold over that observed in wild-type plants. The data reveal roles for Arabidopsis RecQl4A in maintenance of genome stability by modulation of the DNA damage response and suppression of HR.
A framework for the local information dynamics of distributed computation in complex systems
The nature of distributed computation has often been described in terms of
the component operations of universal computation: information storage,
transfer and modification. We review the first complete framework that
quantifies each of these individual information dynamics on a local scale
within a system, and describes the manner in which they interact to create
non-trivial computation where "the whole is greater than the sum of the parts".
We describe the application of the framework to cellular automata, a simple yet
powerful model of distributed computation. This is an important application,
because the framework is the first to provide quantitative evidence for several
important conjectures about distributed computation in cellular automata: that
blinkers embody information storage, particles are information transfer agents,
and particle collisions are information modification events. The framework is
also shown to contrast the computations conducted by several well-known
cellular automata, highlighting the importance of information coherence in
complex computation. The results reviewed here provide important quantitative
insights into the fundamental nature of distributed computation and the
dynamics of complex systems, as well as impetus for the framework to be applied
to the analysis and design of other systems.Comment: 44 pages, 8 figure
Synchronization and resonance in a driven system of coupled oscillators
We study the noise effects in a driven system of globally coupled
oscillators, with particular attention to the interplay between driving and
noise. The self-consistency equation for the order parameter, which measures
the collective synchronization of the system, is derived; it is found that the
total order parameter decreases monotonically with noise, indicating overall
suppression of synchronization. Still, for large coupling strengths, there
exists an optimal noise level at which the periodic (ac) component of the order
parameter reaches its maximum. The response of the phase velocity is also
examined and found to display resonance behavior.Comment: 17 pages, 3 figure
Phase synchronization and noise-induced resonance in systems of coupled oscillators
We study synchronization and noise-induced resonance phenomena in systems of
globally coupled oscillators, each possessing finite inertia. The behavior of
the order parameter, which measures collective synchronization of the system,
is investigated as the noise level and the coupling strength are varied, and
hysteretic behavior is manifested. The power spectrum of the phase velocity is
also examined and the quality factor as well as the response function is
obtained to reveal noise-induced resonance behavior.Comment: to be published in Phys. Rev.
Towards optical polarization control of laser-driven proton acceleration in foils undergoing relativistic transparency
Control of the collective response of plasma particles to intense laser light is intrinsic to relativistic optics, the development of compact laser-driven particle and radiation sources, as well as investigations of some laboratory astrophysics phenomena. We recently demonstrated that a relativistic plasma aperture produced in an ultra-thin foil at the focus of intense laser radiation can induce diffraction, enabling polarization-based control of the collective motion of plasma electrons. Here we show that under these conditions the electron dynamics are mapped into the beam of protons accelerated via strong charge-separation-induced electrostatic fields. It is demonstrated experimentally and numerically via 3D particle-in-cell simulations that the degree of ellipticity of the laser polarization strongly influences the spatial-intensity distribution of the beam of multi-MeV protons. The influence on both sheath accelerated and radiation pressure accelerated protons is investigated. This approach opens up new routes to control laser-driven ion sources
Effect of air movement on the thermal insulation of avian nests
Capsule: Air movement over a nest increases the rate of cooling within the nest cup but the walls
provide good thermal insulation.
Aims: This study compared nests of six bird species of the families Fringillidae and Motacillidae to
investigate the insulative properties in still and moving air treatments. It was hypothesized that
differences in nest size and moving air would differ between species and would have a
significant effect on insulatory values of the nests.
Methods: Nest dimensions were measured for a total of 35 nests from six species. Thermal
properties of the nests were recorded using temperature loggers within nests placed in a wind
tunnel under still and moving air conditions.
Results: Insulatory values and internal nest cooling rates were significantly increased by moving air.
There was no significant difference between species for the thermal properties of nests but nest
mass correlated with greater insulatory values and a lower rate of cooling within the nest cup.
Nest wall thickness had no significant effect on the thermal characteristics of the nests.
Conclusion: The use of a constructed nest mitigated the effects of air movement but the
differences between species reflected difference in nest mass rather than wall thickness
- …