246 research outputs found
Numerical Solutions for Linear Fredholm Integro-Differential Difference Equations with Variable Coefficients by Collocation Methods
We employed an efficient numerical collocation approximation methods to obtain an approximate solution of linear Fredholm integro-differential difference equation with variable coefficients. An assumed approximate solutions for both collocation approximation methods are substituted into the problem considered. After simplifications and collocations, resulted into system of linear algebraic equations which are then solved using MAPLE 18 modules to obtain the unknown constants involved in the assumed solution. The known constants are then substituted back into the assumed approximate solution. Numerical examples were solved to illustrate the reliability, accuracy and efficiency of these methods on problems considered by comparing the numerical solutions obtained with the exact solution and also with some other existing methods. We observed from the results obtained that the methods are reliable, accurate, fast, simple to apply and less computational which makes the valid for the classes of problems considered. Â Keywords: Approximate solution, Collocation, Fredholm, Integro-differential difference and linear algebraic equation
Rotated Rectangular Slots And Mirrored Inversed Cantor-sets On Ultrawideband Antipodal Vivaldi Antenna
Variants of antipodal Vivaldi antenna (AVA) design suitable for access point working on 0.5 â 6.0 GHz are proposed in this paper. The novel designs were produced by employing three novel techniques to conventional AVA: (i) rotated-slot pattern to shift down the frequency cutoff and enhancing bandwidth, (ii) curve design to miniaturize rotated-slot-inserted antipodal Vivaldi, and (iii) fractal-director (Cantor set) to increase the gain of antipodal Vivaldi. Using FR4 (relative permittivity of 4.4) with an overall dimension of 300 mm x 143 mm x 1.6 mm the antenna designs are able to work at a frequency of 0.472 GHz to higher than 6 GHz with a maximum gain of 11.9 dBi
Alcoholic Beverages and the Health of Young Persons: How Do Undergraduate Students in Remo, Southwest Nigeria See It?
Background: Young people were hitherto assumed to be very healthy until various lifestyle-related illnesses were reported among them. Excessive alcohol intake, often driven by a wrong perception of its consequences, is associated with several diseases and nutritional disorders. This study, therefore, assessed the perception of alcoholic beverages and their associated factors among undergraduate students in the Remo area of Ogun State.
Materials and Methods: A cross-sectional study was conducted among 420 undergraduate students in the Remo Area of Ogun State, southwest, Nigeria, selected via multi-stage sampling. Data was collected using a validated, semi-structured, self-administered questionnaire and analyzed using SPSS 23.0. Relevant descriptive and inferential statistics were calculated, with p<0.05.
Results: The mean age of respondents was 20.12±2.2 years. About 98% of respondents were full-time students. Alcoholic beverages were perceived to be stress relievers (55.1%); social drinks (57.1%); aphrodisiacs and performance enhancers (71.5%); promoters of male-female bonding (43.9%). Less than 5% were willing to recommend them to friends.
Conclusion : Perception of alcohol consumption was good (83%) among respondents. It was associated with a history of alcohol consumption and religion. Adequate and targeted nutrition education should be made available to undergraduate students of tertiary institutions on the ills of excessive alcohol consumption
Non equilibrium thermodynamics and cosmological pancakes formation
We investigate the influence of non equilibrium thermodynamics on
cosmological structure formation. In this paper, we consider the collapse of
planar perturbations usually called "Zel'dovich pancakes". We have developed
for that purpose a new two fluids (gas and dark matter) hydrodynamical code,
with three different thermodynamical species: electrons, ions and neutral
particles (T_e\ne T_i \ne T_n). We describe in details the complex structure of
accretion shock waves. We include several relevant processes for a low density,
high temperature, collisional plasma such as non-equilibrium chemical
reactions, cooling, shock heating, thermal energy equipartition between
electrons, ions and neutral particles and electronic conduction. We find two
different regions in the pancake structure: a thermal precursor ahead of the
compression front and an equipartition wave after the compression front where
electrons and ions temperatures differ significantly. This complex structure
may have two interesting consequences: pre-heating of unshocked regions in the
vicinity of massive X-ray clusters and ions and electrons temperatures
differences in the outer regions of X-rays clusters.Comment: 30 pages, including 8 figures, accepted for publication in The
Astrophysical Journa
Cosmological Backreaction from Perturbations
We reformulate the averaged Einstein equations in a form suitable for use
with Newtonian gauge linear perturbation theory and track the size of the
modifications to standard Robertson-Walker evolution on the largest scales as a
function of redshift for both Einstein de-Sitter and Lambda CDM cosmologies. In
both cases the effective energy density arising from linear perturbations is of
the order of 10^-5 the matter density, as would be expected, with an effective
equation of state w ~ -1/19. Employing a modified Halofit code to extend our
results to quasilinear scales, we find that, while larger, the deviations from
Robertson-Walker behaviour remain of the order of 10^-5.Comment: 15 pages, 8 figures; replaced by version accepted by JCA
LTB solutions in Newtonian gauge: from strong to weak fields
Lemaitre-Tolman-Bondi (LTB) solutions are used frequently to describe the
collapse or expansion of spherically symmetric inhomogeneous mass distributions
in the Universe. These exact solutions are obtained in the synchronous gauge
where nonlinear dynamics (with respect to the FLRW background) induce large
deviations from the FLRW metric. In this paper we show explicitly that this is
a gauge artefact (for realistic sub-horizon inhomogeneities). We write down the
nonlinear gauge transformation from synchronous to Newtonian gauge for a
general LTB solution using the fact that the peculiar velocities are small. In
the latter gauge we recover the solution in the form of a weakly perturbed FLRW
metric that is assumed in standard cosmology. Furthermore we show how to obtain
the LTB solutions directly in Newtonian gauge and illustrate how the Newtonian
approximation remains valid in the nonlinear regime where cosmological
perturbation theory breaks down. Finally we discuss the implications of our
results for the backreaction scenario.Comment: 17 page
Correspondence between kinematical backreaction and scalar field cosmologies - the `morphon field'
Spatially averaged inhomogeneous cosmologies in classical general relativity
can be written in the form of effective Friedmann equations with sources that
include backreaction terms. In this paper we propose to describe these
backreaction terms with the help of a homogeneous scalar field evolving in a
potential; we call it the `morphon field'. This new field links classical
inhomogeneous cosmologies to scalar field cosmologies, allowing to reinterpret,
e.g., quintessence scenarios by routing the physical origin of the scalar field
source to inhomogeneities in the Universe. We investigate a one-parameter
family of scaling solutions to the backreaction problem. Subcases of these
solutions (all without an assumed cosmological constant) include
scale-dependent models with Friedmannian kinematics that can mimic the presence
of a cosmological constant or a time-dependent cosmological term. We explicitly
reconstruct the scalar field potential for the scaling solutions, and discuss
those cases that provide a solution to the Dark Energy and coincidence
problems. In this approach, Dark Energy emerges from morphon fields, a
mechanism that can be understood through the proposed correspondence: the
averaged cosmology is characterized by a weak decay (quintessence) or growth
(phantom quintessence) of kinematical fluctuations, fed by `curvature energy'
that is stored in the averaged 3-Ricci curvature. We find that the late-time
trajectories of those models approach attractors that lie in the future of a
state that is predicted by observational constraints.Comment: 36 pages and 6 Figures, matches published version in Class.Quant.Gra
Scalar-Tensor Cosmological Models
We analyze the qualitative behaviors of scalar-tensor cosmologies with an
arbitrary monotonic function. In particular, we are interested
on scalar-tensor theories distinguishable at early epochs from General
Relativity (GR) but leading to predictions compatible with solar-system
experiments. After extending the method developed by Lorentz-Petzold and
Barrow, we establish the conditions required for convergence towards GR at
. Then, we obtain all the asymptotic analytical solutions
at early times which are possible in the framework of these theories. The
subsequent qualitative evolution, from these asymptotic solutions until their
later convergence towards GR, has been then analyzed by means of numerical
computations. From this analysis, we have been able to establish a
classification of the different qualitative behaviors of scalar-tensor
cosmological models with an arbitrary monotonic function.Comment: uuencoded compressed postscript file containing 41 pages, with 9
figures, accepted for publication in Physical Review
Accelerated expansion from structure formation
We discuss the physics of backreaction-driven accelerated expansion. Using
the exact equations for the behaviour of averages in dust universes, we explain
how large-scale smoothness does not imply that the effect of inhomogeneity and
anisotropy on the expansion rate is small. We demonstrate with an analytical
toy model how gravitational collapse can lead to acceleration. We find that the
conjecture of the accelerated expansion being due to structure formation is in
agreement with the general observational picture of structures in the universe,
and more quantitative work is needed to make a detailed comparison.Comment: 44 pages, 1 figure. Expanded treatment of topics from the Gravity
Research Foundation contest essay astro-ph/0605632. v2: Added references,
clarified wordings. v3: Published version. Minor changes and corrections,
added a referenc
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