1,415 research outputs found
The number of independent sets in a graph with small maximum degree
Let be the number of independent sets in a graph . We show
that if has maximum degree at most then
(where is vertex degree, is the number of isolated
vertices in and is the complete bipartite graph with vertices
in one partition class and in the other), with equality if and only if each
connected component of is either a complete bipartite graph or a single
vertex. This bound (for all ) was conjectured by Kahn.
A corollary of our result is that if is -regular with then with
equality if and only if is a disjoint union of copies of
. This bound (for all ) was conjectured by Alon and Kahn and
recently proved for all by the second author, without the characterization
of the extreme cases.
Our proof involves a reduction to a finite search. For graphs with maximum
degree at most the search could be done by hand, but for the case of
maximum degree or , a computer is needed.Comment: Article will appear in {\em Graphs and Combinatorics
Failed intentions? Meeting the water needs of people living with HIV in South Africa
Researchers, activists, practitioners and policy-makers have grappled with the challenge of providing people living with HIV (PLHIV) with an adequate amount of safe water. Comprising 13% of the overall population of South Africa in 2018, 7.52 million PLHIV need water for drinking and taking medication; preparing food; and personal hygiene and cleaning to minimise infections. This article examines the responses of the different stakeholders to this challenge and their impact on the water and health policy process. It finds that activists were able to emphasise the dimensions of the challenge; practitioners worked to implement provision more effectively within existing policy frameworks; and a range of stakeholders made a thoughtful and promising policy proposal for direct action, which the Department of Water and Sanitation ultimately failed to embrace. This article is based on an extensive review of academic research and publications by development agencies on HIV and water as well as engagement with policies and documents in the South African water sector related to water services delivery for PLHIV. While the widespread provision of antiretrovirals from 2004 has changed the context, the above findings are significant in understanding and reviewing the impact of various stakeholders on the water and health policy process. They raise questions regarding the effectiveness of NGO advocacy, the means of delivering improved services to specific populations, and the ability of a range of stakeholders to inform the policy approaches of government departments.
Keywords: activist HIV multi-stakeholder targeting wate
Wildlife Conservation and the Role of the Indigenous Communities Living around Conservation Areas
The Indigenous Kenyan Maasai community has coexisted with the wildlife surrounding it for decades from Nairobi National Park, Maasai Mara and Amboseli. These parks border Maasai lands. Although the northern, eastern, and western perimeters of the Nairobi National Park are fenced, the southern part is not. It is at this point that the Maasai community’s land meets the park. This area also acts as a wildlife dispersal area where wildlife can freely migrate to other parks, including Maasai Mara and Amboseli. The park is only 117sq kms and its vitality depends on the plains to the south where the Maasai live so that the animals can migrate in and out. Without that open space, the park would be little more than a zoo. The fact is that approximately 60 to 80 percent of wildlife in Kenya is outside formally protected areas.
For the Maasai community, wildlife poses an enormous threat. As herbivores migrate during the wet season, they are followed by predators such as lions. Livestock are an easy target for them. A lion attack can be devastating, ruining family lives and livelihoods. Lions have traditionally been the Maasai tribe’s greatest adversaries; they are a deadly threat to the cattle and other livestock that are both an integral part of the Maasai culture and the tribe’s greatest source of wealth. Despite the ongoing livestock predation, lions may be the tribe’s strongest hope of preserving their way of life.
With these in mind, it becomes imperative that we think of systems for better correlation between all aspects of conservation and understand that the wildlife, livestock, and the surrounding pastoral communities play a key role in each other’s survival
20 cm VLA Radio-Continuum Study of M31 - Images and Point Source Catalogues
We present a series of new high-sensitivity and high-resolution
radio-continuum images of M31 at \lambda=20 cm (\nu=1.4 GHz). These new images
were produced by merging archived 20 cm radio-continuum observations from the
Very Large Array (VLA) telescope. Images presented here are sensitive to rms=60
\mu Jy and feature high angular resolution (<10"). A complete sample of
discrete radio sources have been catalogued and analysed across 17 individual
VLA projects. We identified a total of 864 unique discrete radio sources across
the field of M31. One of the most prominent regions in M31 is the ring feature
for which we estimated total integrated flux of 706 mJy at \lambda=20 cm. We
compare here, detected sources to those listed in Gelfand et al. (2004) at
\lambda=92 cm and find 118 sources in common to both surveys. The majority
(61%) of these sources exhibit a spectral index of \alpha <-0.6 indicating that
their emission is predominantly non-thermal in nature. That is more typical for
background objects.Comment: 28 pages, 25 figures, accepted for publication in the Serbian
Astronomical Journa
The ATLAS 9.0 GHz Survey of the Extended Chandra Deep Field South: The Faint 9.0 GHz Radio Population
We present a new image of the 9.0 GHz radio emission from the extended
Chandra Deep Field South. A total of 181 hours of integration with the
Australia Telescope Compact Array has resulted in a 0.276 square degree image
with a median sensitivity of 20 Jy/beam rms, for a synthesised beam
of 4.0 1.3 arcsec. We present a catalogue of the 9.0 GHz radio
sources, identifying 70 source components and 55 individual radio galaxies.
Source counts derived from this sample are consistent with those reported in
the literature. The observed source counts are also generally consistent with
the source counts from simulations of the faint radio population. Using the
wealth of multiwavelength data available for this region, we classify the faint
9 GHz population and find that 91% are radio loud AGN, 7% are radio quiet AGN
and 2% are star forming galaxies. The 9.0 GHz radio sources were matched to 5.5
and 1.4 GHz sources in the literature and we find a significant fraction of
flat or inverted spectrum sources, with 36% of the 9 GHz sources having
-0.3 (for ). This flat or
inverted population is not well reproduced by current simulations of radio
source populations.Comment: 17 pages, 12 figures. Accepted for publication in MNRA
Orbital operations study. Appendix B: Operational procedures
Operational procedures for each alternate approach for each interfacing activity of the orbital operations study are presented. The applicability of the procedures to interfacing element pairs is identified
Forecasting Periods of Strong Southward Magnetic Field Following Interplanetary Shocks
Long periods of strong southward magnetic fields are known to be the primary
cause of intense geomagnetic storms. The majority of such events are caused by
the passage over Earth of a magnetic ejecta. Irrespective of the interplanetary
cause, fast-forward shocks often precede such strong southward B periods.
Here, we first look at all long periods of strong southward magnetic fields as
well as fast-forward shocks measured by the \textit{Wind} spacecraft in a
22.4-year span. We find that 76{\%} of strong southward B periods are
preceded within 48 hours by at least a fast-forward shock but only about 23{\%}
of all shocks are followed within 48 hours by strong southward B periods.
Then, we devise a threshold-based probabilistic forecasting method based on the
shock properties and the pre-shock near-Earth solar wind plasma and
interplanetary magnetic field characteristics adopting a `superposed epoch
analysis'-like approach. Our analysis shows that the solar wind conditions in
the 30 minutes interval around the arrival of fast-forward shocks have a
significant contribution to the prediction of long-duration southward B
periods. This probabilistic model may provide on average a 14-hour warning time
for an intense and long-duration southward B period. Evaluating the
forecast capability of the model through a statistical and skill score-based
approach reveals that it outperforms a coin-flipping forecast. By using the
information provided by the arrival of a fast-forward shock at L1, this model
represents a marked improvement over similar forecasting methods. We outline a
number of future potential improvements.Comment: published in Space Weather, 22 Nov 201
Melting behavior of (Th,U)O2 and (Th,Pu)O2 mixed oxides
© 2016 Elsevier B.V.The melting behaviors of pure ThO2, UO2 and PuO2 as well as (Th,U)O2 and (Th,Pu)O2 mixed oxides (MOX) have been studied using molecular dynamics (MD) simulations. The MD calculated melting temperatures (MT) of ThO2, UO2 and PuO2 using two-phase simulations, lie between 3650-3675 K, 3050–3075 K and 2800–2825 K, respectively, which match well with experiments. Variation of enthalpy increments and density with temperature, for solid and liquid phases of ThO2, PuO2 as well as the ThO2 rich part of (Th,U)O2 and (Th,Pu)O2 MOX are also reported. The MD calculated MT of (Th,U)O2 and (Th,Pu)O2 MOX show good agreement with the ideal solidus line in the high thoria section of the phase diagram, and evidence for a minima is identified around 5 atom% of ThO2 in the phase diagram of (Th,Pu)O2 MOX
Discovery of magnetic fields along stacked cosmic filaments as revealed by radio and X-ray emission
Diffuse filaments connect galaxy clusters to form the cosmic web. Detecting these filaments could yield information on the magnetic field strength, cosmic ray population and temperature of intercluster gas, yet, the faint and large-scale nature of these bridges makes direct detections very challenging. Using multiple independent all-sky radio and X-ray maps we stack pairs of luminous red galaxies as tracers for cluster pairs. For the first time, we detect an average surface brightness between the clusters from synchrotron (radio) and thermal (X-ray) emission with 73 5\u3c3 significance, on physical scales larger than observed to date ( 653 Mpc). We obtain a synchrotron spectral index of \u3b1 43 -1.0 and estimates of the average magnetic field strength of 30 64 B 64 60 nG, derived from both equipartition and Inverse Compton arguments, implying a 5 to 15 per cent degree of field regularity when compared with Faraday rotation measure estimates. While the X-ray detection is inline with predictions, the average radio signal comes out higher than predicted by cosmological simulations and dark matter annihilation and decay models. This discovery demonstrates that there are connective structures between mass concentrations that are significantly magnetised, and the presence of sufficient cosmic rays to produce detectable synchrotron radiation
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