17,753 research outputs found
On Ptolemaic metric simplicial complexes
We show that under certain mild conditions, a metric simplicial complex which
satisfies the Ptolemy inequality is a CAT(0) space. Ptolemy's inequality is
closely related to inversions of metric spaces. For a large class of metric
simplicial complexes, we characterize those which are isometric to Euclidean
space in terms of metric inversions.Comment: 13 page
Is Accra a superstar city ?
A recent study of house price behavior in U.S. cities by Gyourko, Mayer, and Sinai (2006) raises questions about so-called superstar cities in which housing is so inelastically supplied that it becomes unaffordable, as higher-income families outbid residents. We consider the case of Accra, Ghana, in this light, estimating the elasticity of housing supply and discussing the implications for growth and income distribution. There is not a great deal of data available to examine trends in Accra, so our method is indirect. First, we use a variant of the traditional monocentric city model to calculate the elasticity of Accra's housing supply relative to those of other similarly-sized African cities. This suggests that housing supply responsiveness is much higher elsewhere. This muted supply responsiveness is consistent with the observed higher housing prices. Second, we estimate a number of traditional housing demand equations and reduced form equations. Placing a number of restrictions on the equations allows us to infer Accra's housing supply elasticity. Taken together, our approaches suggest that lower-income families in Accra have such poor housing conditions because the market is extremely unresponsive to demand.Although the outcomes we have traced-high housing prices and low quality-are not unusual relative to the other developed country superstar cities, they are extreme. The welfare costs are considerable, so much so that in addition to direct housing market effects, these policies also appear to have potentially significant implications for the achievement of more equitable growth.Economic Theory&Research,Housing&Human Habitats,Banks&Banking Reform,,Public Sector Management and Reform
Case study : a patient-clinician collaboration that identified and prioritized evidence gaps and stimulated research development
Copyright © 2013 Elsevier Inc. All rights reserved.Peer reviewedPostprin
Core-periphery organization of complex networks
Networks may, or may not, be wired to have a core that is both itself densely
connected and central in terms of graph distance. In this study we propose a
coefficient to measure if the network has such a clear-cut core-periphery
dichotomy. We measure this coefficient for a number of real-world and model
networks and find that different classes of networks have their characteristic
values. For example do geographical networks have a strong core-periphery
structure, while the core-periphery structure of social networks (despite their
positive degree-degree correlations) is rather weak. We proceed to study radial
statistics of the core, i.e. properties of the n-neighborhoods of the core
vertices for increasing n. We find that almost all networks have unexpectedly
many edges within n-neighborhoods at a certain distance from the core
suggesting an effective radius for non-trivial network processes
A Resonant soft x-ray powder diffraction study to determine the orbital ordering in A-site ordered SmBaMn2O6
Soft X-ray resonant powder diffraction has been performed at the Mn L2,3
edges of A-site ordered SmBaMn2O6. The energy and polarization dependence of
the (1/2 1/2 0) reflection provide direct evidence for a (x2-z2)/(y2-z2) type
orbital ordering in contrast to the single layer manganite. The temperature
dependence of the reflection indicates an orbital reorientation transition at
210 K, below which the charge and orbital ordered MnO2 sheets show AAAA type of
stacking. The concurring reduction of the ferromagnetic super exchange
correlations leads to further charge localization
The Sensitivity of the IceCube Neutrino Detector to Dark Matter Annihilating in Dwarf Galaxies
In this paper, we compare the relative sensitivities of gamma-ray and
neutrino observations to the dark matter annihilation cross section in
leptophilic models such as have been designed to explain PAMELA data. We
investigate whether the high energy neutrino telescope IceCube will be
competitive with current and upcoming searches by gamma-ray telescopes, such as
the Atmospheric Cerenkov Telescopes (ACTs) (HESS, VERITAS and MAGIC), or the
Fermi Gamma Ray Space Telescope, in detecting or constraining dark matter
particles annihilating in dwarf spheroidal galaxies. We find that after ten
years of observation of the most promising nearby dwarfs, IceCube will have
sensitivity comparable to the current sensitivity of gamma-ray telescopes only
for very heavy (m_X > 7 TeV) or relatively light (m_X < 200 GeV) dark matter
particles which annihilate primarily to mu+mu-. If dark matter particles
annihilate primarily to tau+tau-, IceCube will have superior sensitivity only
for dark matter particle masses below the 200 GeV threshold of current ACTs. If
dark matter annihilations proceed directly to neutrino-antineutrino pairs a
substantial fraction of the time, IceCube will be competitive with gamma-ray
telescopes for a much wider range of dark matter masses.Comment: 7 pages, 3 figures. v2: references added and minor revisions. v3: as
published in PRD
Optical studies of two LMC X-ray transients : RX J0544.1-7100 and RX J0520.5-6932
We report observations which confirm the identities of the optical
counterpart to the transient sources RX J0544.1-7100 and RX J0520.5-6932. The
counterparts are suggested to be a B-type stars. Optical data from the
observations carried out at ESO and SAAO, together with results from the OGLE
data base, are presented. In addition, X-ray data from the RXTE all-sky monitor
are investigated for long term periodicities. A strong suggestion for a binary
period of 24.4d is seen in RX J0520.5-6932 from the OGLE data.Comment: 6 pages, 7 figure
Recommended from our members
Replication enhancer elements within the open reading frame of tick-borne encephalitis virus and their evolution within the Flavivirus genus
We provide experimental evidence of a replication enhancer element (REE) within the capsid gene of tick-borne encephalitis virus (TBEV, genus Flavivirus). Thermodynamic and phylogenetic analyses predicted that the REE folds as a long stable stem–loop (designated SL6), conserved among all tick-borne flaviviruses (TBFV). Homologous sequences and potential base pairing were found in the corresponding regions of mosquito-borne flaviviruses, but not in more genetically distant flaviviruses. To investigate the role
of SL6, nucleotide substitutions were introduced which changed a conserved hexanucleotide motif, the conformation of the terminal loop and the base-paired dsRNA stacking. Substitutions were made within a TBEV reverse genetic system and recovered mutants were compared for plaque
morphology, single-step replication kinetics and cytopathic effect. The greatest phenotypic changes were observed in mutants with a destabilized stem. Point mutations in the conserved hexanucleotide motif of the terminal loop caused
moderate virus attenuation. However, all mutants
eventually reached the titre of wild-type virus late post-infection. Thus, although not essential for growth in tissue culture, the SL6 REE acts to up-regulate virus replication. We hypothesize that this modulatory role may be important for TBEV survival in nature, where the virus circulates by non-viraemic transmission between infected and
non-infected ticks, during co-feeding on local rodents
High-Energy Neutrino Signatures of Dark Matter Decaying into Leptons
Decaying dark matter has previously been proposed as a possible explanation
for the excess high energy cosmic ray electrons and positrons seen by PAMELA
and the Fermi Gamma-Ray Space Telescope (FGST). To accommodate these signals
however, the decays must be predominantly leptonic, to muons or taus, and
therefore produce neutrinos, potentially detectable with the IceCube neutrino
observatory. We find that, with five years of data, IceCube (supplemented by
DeepCore) will be able to significantly constrain the relevant parameter space
of decaying dark matter, and may even be capable of discovering dark matter
decaying in the halo of the Milky Way.Comment: 4 pages, 1 figur
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