402 research outputs found
Blurred Lines Between Competition and Parasitism
Accurately describing the ecological relationships between species is more than mere semantics-doing so has profound practical and applied implications, not the least of which is that inaccurate descriptions can lead to fundamentally incorrect predicted outcomes of community composition and functioning. Accurate ecological classifications are particularly important in the context of global change, where species interactions can change rapidly following shifts in species composition. Here, we argue that many common ecological interactions-particularly competition and parasitism-can be easily confused and that we often lack empirical evidence for the full reciprocal interaction among species. To make our case and to propose a theoretical framework for addressing this problem, we use the interactions between lianas and trees, whose outcomes have myriad implications for the ecology and conservation of tropical forests (e.g., Schnitzer et al. 2015)
Stellar models with Schwarzschild and non-Schwarzschild vacuum exteriors
A striking characteristic of non-Schwarzschild vacuum exteriors is that they
contain not only the total gravitational mass of the source, but also an {\it
arbitrary} constant. In this work, we show that the constants appearing in the
"temporal Schwarzschild", "spatial Schwarzschild" and
"Reissner-Nordstr{\"o}m-like" exteriors are not arbitrary but are completely
determined by star's parameters, like the equation of state and the
gravitational potential. Consequently, in the braneworld scenario the
gravitational field outside of a star is no longer determined by the total mass
alone, but also depends on the details of the internal structure of the source.
We show that the general relativistic upper bound on the gravitational
potential , for perfect fluid stars, is significantly increased in
these exteriors. Namely, , and for the
temporal Schwarzschild, spatial Schwarzschild and Reissner-Nordstr{\"o}m-like
exteriors, respectively. Regarding the surface gravitational redshift, we find
that the general relativistic Schwarzschild exterior as well as the braneworld
spatial Schwarzschild exterior lead to the same upper bound, viz., .
However, when the external spacetime is the temporal Schwarzschild metric or
the Reissner-Nordstr{\"o}m-like exterior there is no such constraint: . This infinite difference in the limiting value of is because for
these exteriors the effective pressure at the surface is negative. The results
of our work are potentially observable and can be used to test the theory.Comment: 19 pages, 3 figures and caption
Lifetime Assessment of Load-Bearing Polymer Glasses: The Influence of Physical Ageing
The timescale at which ductile failure occurs in loaded glassy polymers can be successfully predicted using the engineering approach presented in a previous publication. In this paper the influence of progressive physical ageing on the plastic deformation behaviour of unplasticised poly(vinyl chloride) (uPVC) is characterised and incorporated in the existing approach. With the modification it is possible to quantitatively predict long-term failures which show a so-called endurance limit. The predictions are compared with failure data of uPVC specimens which were subjected to constant or dynamic loads. In dynamic loading conditions a second type of failure mode was observed: fatigue crack growth. A brief study on the influence of the frequency and stress ratio of the applied stress signal shows that crack growth failure is not expected to occur within experimentally reasonable timescales for constant loading conditions
Wormhole solutions in the Randall-Sundrum scenario
In the simplest form of the Randall-Sundrum model, we consider the metric
generated by a static, spherically symmetric distribution of matter on the
physical brane. The solution to the five-dimensional Einstein equations,
obtained numerically, describes a wormhole geometry.Comment: 10 pages, 2 figures, revtex
Higher Dimensional Wormhole Geometries with Compact Dimensions
This paper studies wormhole solutions to Einstein gravity with an arbitrary
number of time dependent compact dimensions and a matter-vacuum boundary. A new
gauge is utilized which is particularly suited for studies of the wormhole
throat. The solutions possess arbitrary functions which allow for the
description of infinitely many wormhole systems of this type and, at the
stellar boundary, the matter field is smoothly joined to vacuum. It turns out
that the classical vacuum structure differs considerably from the four
dimensional theory and is therefore studied in detail. The presence of the
vacuum-matter boundary and extra dimensions places interesting restrictions on
the wormhole. For example, in the static case, the radial size of a weak energy
condition (WEC) respecting throat is restricted by the extra dimensions. There
is a critical dimension, D=5, where this restriction is eliminated. In the time
dependent case, one \emph{cannot} respect the WEC at the throat as the time
dependence actually tends the solution towards WEC violation. This differs
considerably from the static case and the four dimensional case.Comment: 28 pages, 5 figures (quality reduced to reduce file size). Update
includes added references and more detail regarding role extra dimensions
play in wormhole analysis. Accepted for publication in Nuclear Phys.
Wesson's IMT with a Weylian bulk
The foundations of Wesson's induced matter theory are analyzed. It is shown
that the 5D empty bulk must be regarded rather as a Weylian space than as a
Riemannian one.The framework of a Weyl-Dirac version of Wesson's theory is
elaborated and discussed. The bulk possesses in addition to the metric tensor a
Weylian connection vector as well Dirac's gauge function; there are no sources
(mass, current) in the bulk. On the 4D brane one obtains a geometrically based
unified theory of gravitation and electromagnetism with mass, currents and
equations induced by the 5D bulkComment: 29 page
The Vaccine and Cervical Cancer Screen project 2 (VACCS 2) : linking cervical cancer screening to a two-dose HPV vaccination schedule in the south-west district of Tshwane, Gauteng, South Africa
BACKGROUND : Cervical cancer is a preventable disease with a high prevalence in South Africa (SA), where screening is opportunistic.
Primary prevention is now possible through HPV vaccination. In VACCS 1 the feasibility of linking cervical cancer with HPV vaccination
was demonstrated.
OBJECTIVES : To investigate the feasibility of linking HPV self-testing with a two-dose HPV vaccination schedule and to compare results with
VACCS 1.
METHODS : The project was conducted in five schools in the South-West District of Tshwane, Gauteng, SA. Leaflet information on cervical
cancer and screening was provided, with requests for consent and assent for a two-dose HPV vaccination of schoolgirls. Female caregivers
were invited to take part in HPV self-screening.
RESULTS : Of 965 girls invited for vaccination, 519 (53.7%) had full consent and 518 (99.8%) received at least one vaccine dose. The invited
uptake rate was 53.7% and 495 girls received both doses, giving a completion rate of 95.4% v. 82.6% in VACCS 1. Of 1 135 self-screen kits
handed out, 560 (49.3%) were not returned. The mean age (standard deviation) of the 160 women who participated in self-screening was
38.7 (7.7) years. HPV testing was negative in 116 women (72.5%), 15 women (9.4%) tested positive for HPV 16 and/or 18, and 27 (16.9%)
were positive for non-16/18 oncogenic HPV.
CONCLUSION : Data from the VACCS projects suggest that school-based vaccine programmes can be successfully implemented. A two-dose
schedule allowed for higher completion rates. Linking self-collected HPV screening to HPV vaccination is feasible, is a promising and viable
screening strategy, and reached the appropriate age group for screening.Cancer Research Initiative of South Africa,South African Medical Research Council and
the Cancer Association of South Africa.First for Women Insurance.GlaxoSmithKline Biologicals SA (117280).http://www.samj.org.zahb201
Nematic Structure of Space-Time and its Topological Defects in 5D Kaluza-Klein Theory
We show, that classical Kaluza-Klein theory possesses hidden nematic
dynamics. It appears as a consequence of 1+4-decomposition procedure, involving
4D observers 1-form \lambda. After extracting of boundary terms the, so called,
"effective matter" part of 5D geometrical action becomes proportional to square
of anholonomicity 3-form \lambda\wedge d\lambda. It can be interpreted as twist
nematic elastic energy, responsible for elastic reaction of 5D space-time on
presence of anholonomic 4D submanifold, defined by \lambda. We derive both 5D
covariant and 1+4 forms of 5D nematic equilibrium equations, consider simple
examples and discuss some 4D physical aspects of generic 5D nematic topological
defects.Comment: Latex-2e, 14 pages, 1 Fig., submitted to GR
The Vaccine and Cervical Cancer Screen project 2 (VACCS 2): Linking cervical cancer screening to a two-dose HPV vaccination schedule in the South-West District of Tshwane, Gauteng, South Africa
Background. Cervical cancer is a preventable disease with a high prevalence in South Africa (SA), where screening is opportunistic. Primary prevention is now possible through HPV vaccination. In VACCS 1 the feasibility of linking cervical cancer with HPV vaccination was demonstrated.Objectives. To investigate the feasibility of linking HPV self-testing with a two-dose HPV vaccination schedule and to compare results with VACCS 1.Methods. The project was conducted in five schools in the South-West District of Tshwane, Gauteng, SA. Leaflet information on cervical cancer and screening was provided, with requests for consent and assent for a two-dose HPV vaccination of schoolgirls. Female caregivers were invited to take part in HPV self-screening.Results. Of 965 girls invited for vaccination, 519 (53.7%) had full consent and 518 (99.8%) received at least one vaccine dose. The invited uptake rate was 53.7% and 495 girls received both doses, giving a completion rate of 95.4% v. 82.6% in VACCS 1. Of 1 135 self-screen kits handed out, 560 (49.3%) were not returned. The mean age (standard deviation) of the 160 women who participated in self-screening was 38.7 (7.7) years. HPV testing was negative in 116 women (72.5%), 15 women (9.4%) tested positive for HPV 16 and/or 18, and 27 (16.9%) were positive for non-16/18 oncogenic HPV.Conclusion. Data from the VACCS projects suggest that school-based vaccine programmes can be successfully implemented. A two-dose schedule allowed for higher completion rates. Linking self-collected HPV screening to HPV vaccination is feasible, is a promising and viable screening strategy, and reached the appropriate age group for screening
The (2+1)-dimensional charged gravastars
This is a continuation and generalization of our earlier work on {\it
gravastar} in (2+1) anti-de Sitter space-time to 2+1 dimensional solution of
charged gravastar. Morphologically this gravastar contains three regions
namely: (i) charged interior, (ii) charged shell and (iii) electrovacuum
exterior. We have studied different characteristics in terms of Length and
Energy, Entropy, and Junction conditions of the spherical charged distribution.
It is shown that the present model of charged gravastar is non-singular and
represents itself an alternative of Black Hole.Comment: 14 pages, Accepted in Phys. Lett.
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