4,385 research outputs found
Regulation mechanisms in spatial stochastic development models
The aim of this paper is to analyze different regulation mechanisms in
spatial continuous stochastic development models. We describe the density
behavior for models with global mortality and local establishment rates. We
prove that the local self-regulation via a competition mechanism (density
dependent mortality) may suppress a unbounded growth of the averaged density if
the competition kernel is superstable.Comment: 19 page
A jackknife variance estimator for self-weighted two-stage samples
Self-weighted two-stage sampling designs are popular in practice as they simplify field-work. It is common in practice to compute variance estimates only from the first sampling stage, neglecting the second stage. This omission may induce a bias in variance estimation; especially in situations where there is low variability between clusters or when sampling fractions are non-negligible. We propose a design-consistent jackknife variance estimator that takes account of all stages via deletion of clusters and observations within clusters. The proposed jackknife can be used for a wide class of point estimators. It does not need joint-inclusion probabilities and naturally includes finite population corrections. A simulation study shows that the proposed estimator can be more accurate than standard jackknifes (Rao, Wu, and Yue (1992)) for self-weighted two-stage sampling designs
Numerical simulation of liquid sloshing in LNG tanks using a compressible two-fluid flow model
In this investigation the Reynolds-Averaged Navier-Stokes (RANS)
equations are modified to account for variable density and viscosity of
the two-fluids flow (i.e. water-air), assuming both fluids compressible.
By introducing a preconditioner, the governing equations in terms of
primitive variables are solved for both fluids in a unified manner. The
non-conservative implicit Split Coefficient Matrix Method (SCMM) is
modified to approximate convective flux vectors in the dual time
formulation. The free surface waves inside the tank, due to sloshing, are
implicitly captured by using a level set approach.
The method is illustrated through applications to rectangular and
chamfered tanks subject to sway or roll motions at different filling
levels and excitation conditions (i.e. amplitude and frequency of
oscillation). Comparisons are made between calculated and
experimental pressures, where available
Empirical likelihood confidence intervals for complex sampling designs
We define an empirical likelihood approach which gives consistent design-based confidence intervals which can be calculated without the need of variance estimates, design effects, resampling, joint inclusion probabilities and linearization, even when the point estimator is not linear. It can be used to construct confidence intervals for a large class of sampling designs and estimators which are solutions of estimating equations. It can be used for means, regressions coefficients, quantiles, totals or counts even when the population size is unknown. It can be used with large sampling fractions and naturally includes calibration constraints. It can be viewed as an extension of the empirical likelihood approach to complex survey data. This approach is computationally simpler than the pseudoempirical likelihood and the bootstrap approaches. The simulation study shows that the confidence interval proposed may give better coverages than the confidence intervals based on linearization, bootstrap and pseudoempirical likelihood. Our simulation study shows that, under complex sampling designs, standard confidence intervals based on normality may have poor coverages, because point estimators may not follow a normal sampling distribution and their variance estimators may be biased.<br/
Empirical prediction of traffic noise transmission loss across plenum windows
A parametric study on the traffic noise transmission loss across plenum windows was carried out experimentally in this investigation in an attempt to establish a simple empirical model for predicting this transmission loss. A total of fourteen full scale plenum windows were included in this study. The results of a site mockup measurement were used for model validation. The present model was developed based on the existing plenum chamber theory in which the sound fields inside the plenum window cavities were assumed to make up of a diffracted wave and a reverberant field. Results suggest that both the diffracted and reverberant field inside the plenum window cavities are weaker than those assumed in existing plenum chamber theory. It is found that a model, which assumes frequency-independent diffraction directivity and percentage reverberant field attenuation, gives the best prediction of traffic noise transmission loss. This prediction model is also able to predict site test results with good accurac
The impact of leverage on stock returns in the hospitality sector: evidence from the UK
This paper examines the relation between capital structure and abnormal returns for the UK hospitality sector by using an investment strategy based on hospitality firmsā capital structure. We find that abnormal returns are higher, 0.53 percent per annum, for medium leverage hospitality firms, and it can be increased up to 0.91 percent by investing in medium leverage and low price-to-book value firms. The findings raise an important issue for the hospitality sector as the firms in this sector are continually aiming to raise external finance to fund expansion. This is a unique situation when compared to other sectors in the economy whereby investors earn higher abnormal returns when investing in low levered firms (Muradoglu and Sivaprasad, 2012a)
Correlation functions evolution for the Glauber dynamics in continuum
We construct a correlation functions evolution corresponding to the Glauber
dynamics in continuum. Existence of the corresponding strongly continuous
contraction semigroup in a proper Banach space is shown. Additionally we prove
the existence of the evolution of states and study their ergodic properties
Coisotropic Branes, Noncommutativity, and the Mirror Correspondence
We study coisotropic A-branes in the sigma model on a four-torus by
explicitly constructing examples. We find that morphisms between coisotropic
branes can be equated with a fundamental representation of the noncommutatively
deformed algebra of functions on the intersection. The noncommutativity
parameter is expressed in terms of the bundles on the branes. We conjecture
these findings hold in general. To check mirror symmetry, we verify that the
dimensions of morphism spaces are equal to the corresponding dimensions of
morphisms between mirror objects.Comment: 13 page
Integral field spectroscopy of protoplanetary disks in Orion with VLT FLAMES
We discuss integral field spectroscopy of proplyds in M42 using the FLAMES
Argus unit and report the first detection of recombination lines of C II and O
II from the archetypical Laques-Vidal-2 object. These lines can provide
important new diagnostics of the physical conditions in proplyds. We also draw
attention to the future capabilities of the MUSE spectrograph in relation to
similar studies.Comment: 5 pages; 3 figs; To appear in proceedings of "Science with the VLT in
the ELT era" (ESO Garching, Oct. 2007
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