12,541 research outputs found
Simulations of a mortality plateau in the sexual Penna model for biological ageing
The Penna model is a strategy to simulate the genetic dynamics of
age-structured populations, in which the individuals genomes are represented by
bit-strings. It provides a simple metaphor for the evolutionary process in
terms of the mutation accumulation theory. In its original version, an
individual dies due to inherited diseases when its current number of
accumulated mutations, n, reaches a threshold value, T. Since the number of
accumulated diseases increases with age, the probability to die is zero for
very young ages (n = T). Here, instead
of using a step function to determine the genetic death age, we test several
other functions that may or may not slightly increase the death probability at
young ages (n < T), but that decreases this probability at old ones. Our
purpose is to study the oldest old effect, that is, a plateau in the mortality
curves at advanced ages. Imposing certain conditions, it has been possible to
obtain a clear plateau using the Penna model. However, a more realistic one
appears when a modified version, that keeps the population size fixed without
fluctuations, is used. We also find a relation between the birth rate, the
age-structure of the population and the death probability.Comment: submitted to Phys. Rev.
Analysis of noise-induced transitions from regular to chaotic oscillations in the Chen system
The stochastically perturbed Chen system is studied within the parameter region which permits both regular and chaotic oscillations. As noise intensity increases and passes some threshold value, noise-induced hopping between close portions of the stochastic cycle can be observed. Through these transitions, the stochastic cycle is deformed to be a stochastic attractor that looks like chaotic. In this paper for investigation of these transitions, a constructive method based on the stochastic sensitivity function technique with confidence ellipses is suggested and discussed in detail. Analyzing a mutual arrangement of these ellipses, we estimate the threshold noise intensity corresponding to chaotization of the stochastic attractor. Capabilities of this geometric method for detailed analysis of the noise-induced hopping which generates chaos are demonstrated on the stochastic Chen system. © 2012 American Institute of Physics
Quantifying morphological variability within extant mammalian species
Main articleIn this study we attempt to establish a baseline for measuring species variability in the
palaeontological record by using linear regression analysis on sets of measurements from extant
mammalian species (males and females within species). In particular, this study explores the
standard error of the m-coefficient (s.e.m) , derived from linear regression analyses associated with
equations of the form y = mx + c, to quantify the degree of morphological variability within extant
mammalian species. The slope m generally centres around 1.00. Even though the species in this
study range in size from small rodents to large ungulates, s.e'm values show a normal distribution
around a mean of 0.035 . The approach outlined here has potential application in palaeontological
contexts where there is uncertainty about any two specimens being conspecific.Foundation for Research Development
Star-forming galaxies in low-redshift clusters: Data and integrated galaxy properties
This paper is a continuation of an ongoing study of the evolutionary
processes affecting cluster galaxies. Both CCD R band and H alpha narrow-band
imaging was used to determine photometric parameters (m_(r), r_(24), H alpha
flux and equivalent width) and derive star formation rates for 227 CGCG
galaxies in 8 low-redshift clusters. The galaxy sample is a subset of CGCG
galaxies in an objective prism survey of cluster galaxies for H alpha emission.
It is found that detection of emission-line galaxies in the OPS is 85%, 70%,
and 50% complete at the mean surface brightness values of 1.25 x 10^(-19), 5.19
x 10^(-20), and 1.76 x 10^(-20) W m^(-2) arcsec^(-2), respectively, measured
within the R band isophote of 24 mag arcsec^(-2) for the galaxy. The CCD data,
together with matched data from a recent H alpha galaxy survey of UGC galaxies
within 3000 km s^(-1), will be used for a comparative study of R band and H
alpha surface photometry between cluster and field spirals.Comment: Accepted for publication in A&A. 11 pages, including 6 figure
Electronic, optical and thermal properties of the hexagonal and fcc Ge2Sb2Te5 chalcogenide from first-principle calculations
We present a comprehensive computational study on the properties of
face-centered cubic and hexagonal chalcogenide Ge2Sb2Te5. We calculate the
electronic structure using density functional theory (DFT); the obtained
density of states (DOS) compares favorably with experiments, also looking
suitable for transport analysis. Optical constants including refraction index
and absorption coefficient capture major experimental features, aside from an
energy shift owed to an underestimate of the band gap that is typical of DFT
calculations. We also compute the phonon DOS for the hexagonal phase, obtaining
a speed of sound and thermal conductivity in good agreement with the
experimental lattice contribution. The calculated heat capacity reaches ~ 1.4 x
106 J/(m3 K) at high temperature, in agreement with experimental data, and
provides insight into the low-temperature range (< 150 K), where data are
unavailable.Comment: 19 pages, 8 figure
Gravitational quasinormal modes for Anti-de Sitter black holes
Quasinormal mode spectra for gravitational perturbations of black holes in
four dimensional de Sitter and anti-de Sitter space are investigated. The
anti-de Sitter case is relevant to the ADS-CFT correspondence in superstring
theory. The ADS-CFT correspondence suggests a prefered set of boundary
conditions.Comment: 12 pages, 6 figures in ReVTe
Towards A Mean-Field Formulation Of The Babcock-Leighton Type Solar Dynamo. I. Alpha Coefficient Versus Durney's Double Ring Approach
We develop a model of the solar dynamo in which, on the one hand, we follow
the Babcock-Leighton approach to include surface processes like the production
of poloidal field from the decay of active regions, and, on the other hand, we
attempt to develop a mean field theory that can be studied in quantitative
detail. One of the main challenges in developing such models is to treat the
buoyant rise of toroidal field and the production of poloidal field from it
near the surface. We build up a dynamo model with two contrasting methods of
treating buoyancy. In one method, we incorporate the generation of the poloidal
field near the solar surface by Durney's procedure of double ring eruption. In
the second method, the poloidal field generation is treated by a positive
alpha-effect concentrated near the solar surface, coupled with an algorithm for
handling buoyancy. The two methods are found to give qualitatively similar
results.Comment: 32 pages, 27 figures, uses aastex.cls and epsfig.st
Moving \u3cem\u3eH. pylori\u3c/em\u3e Vaccine Development Forward with Bioinformatics and Immunomics
Memory Effects in Turbulent Dynamo: Generation and Propagation of Large Scale Magnetic Field
We are concerned with large scale magnetic field dynamo generation and
propagation of magnetic fronts in turbulent electrically conducting fluids. An
effective equation for the large scale magnetic field is developed here that
takes into account the finite correlation times of the turbulent flow. This
equation involves the memory integrals corresponding to the dynamo source term
describing the alpha-effect and turbulent transport of magnetic field. We find
that the memory effects can drastically change the dynamo growth rate, in
particular, non-local turbulent transport might increase the growth rate
several times compared to the conventional gradient transport expression.
Moreover, the integral turbulent transport term leads to a large decrease of
the speed of magnetic front propagation.Comment: 13 pages, 2 figure
- …