5,128 research outputs found
When best practice is bad medicine: a new approach to rationing tertiary health services in South Africa.
Quadri-tilings of the plane
We introduce {\em quadri-tilings} and show that they are in bijection with
dimer models on a {\em family} of graphs arising from rhombus
tilings. Using two height functions, we interpret a sub-family of all
quadri-tilings, called {\em triangular quadri-tilings}, as an interface model
in dimension 2+2. Assigning "critical" weights to edges of , we prove an
explicit expression, only depending on the local geometry of the graph ,
for the minimal free energy per fundamental domain Gibbs measure; this solves a
conjecture of \cite{Kenyon1}. We also show that when edges of are
asymptotically far apart, the probability of their occurrence only depends on
this set of edges. Finally, we give an expression for a Gibbs measure on the
set of {\em all} triangular quadri-tilings whose marginals are the above Gibbs
measures, and conjecture it to be that of minimal free energy per fundamental
domain.Comment: Revised version, minor changes. 30 pages, 13 figure
Collation of data on the form and composition of the sea bed in the south-western approaches to the British Isles and the Celtic Sea
Circumbinary Chaos: Using Pluto's Newest Moon to Constrain the Masses of Nix & Hydra
The Pluto system provides a unique local laboratory for the study of binaries
with multiple low mass companions. In this paper, we study the orbital
stability of P4, the most recently discovered moon in the Pluto system. This
newfound companion orbits near the plane of the Pluto-Charon binary, roughly
halfway between the two minor moons Nix and Hydra. We use a suite of few body
integrations to constrain the masses of Nix and Hydra, and the orbital
parameters of P4. For the system to remain stable over the age of the Solar
System, the masses of Nix and Hydra likely do not exceed 5e16 kg and 9e16 kg,
respectively. These upper limits assume a fixed mass ratio between Nix and
Hydra at the value implied by their median optical brightness. Our study finds
that stability is more sensitive to their total mass and that a downward
revision of Charon's eccentricity (from our adopted value of 0.0035) is
unlikely to significantly affect our conclusions. Our upper limits are an order
of magnitude below existing astrometric limits on the masses of Nix and Hydra.
For a density at least that of ice, the albedos of Nix and Hydra would exceed
0.3. This constraint implies they are icy, as predicted by giant impact models.
Even with these low masses, P4 only remains stable if its eccentricity e <
0.02. The 5:1 commensurability with Charon is particularly unstable, Combining
stability constraints with the observed mean motion places the preferred orbit
for P4 just exterior to the 5:1 resonance. These predictions will be tested
when the New Horizons satellite visits Pluto. Based on the results for the
Pluto-Charon system, we expect that circumbinary, multi-planet systems will be
more widely spaced than their singleton counterparts. Further, circumbinary
exoplanets close to the three-body stability boundary, such as those found by
Kepler, are less likely to have other companions nearby.Comment: 12 pages, 7 figure
Stellar Velocity Dispersion of the Leo A Dwarf Galaxy
We measure the first stellar velocity dispersion of the Leo A dwarf galaxy,
\sigma = 9.3 +- 1.3 km/s. We derive the velocity dispersion from the radial
velocities of ten young B supergiants and two HII regions in the central region
of Leo A. We estimate a projected mass of 8 +- 2.7 x10^7 solar masses within a
radius of 2 arcmin, and a mass to light ratio of at least 20 +- 6 M_sun/L_sun.
These results imply Leo A is at least ~80% dark matter by mass.Comment: 6 pages, accepted to Ap
Formation of Kuiper Belt Binaries by Gravitational Collapse
A large fraction of 100-km-class low-inclination objects in the classical
Kuiper Belt (KB) are binaries with comparable mass and wide separation of
components. A favored model for their formation was capture during the
coagulation growth of bodies in the early KB. Instead, recent studies suggested
that large objects can rapidly form in the protoplanetary disks when swarms of
locally concentrated solids collapse under their own gravity. Here we examine
the possibility that KB binaries formed during gravitational collapse when the
excess of angular momentum prevented the agglomeration of available mass into a
solitary object. We find that this new mechanism provides a robust path toward
the formation of KB binaries with observed properties, and can explain wide
systems such as 2001 QW322 and multiples such as (47171) 1999 TC36. Notably,
the gravitational collapse is capable of producing 100% binary fraction for a
wide range of the swarm's initial angular momentum values. The binary
components have similar masses (80% have the secondary-over-primary radius
ratio >0.7) and their separation ranges from ~1,000 to ~100,000 km. The binary
orbits have eccentricities from e=0 to ~1, with the majority having e<0.6. The
binary orbit inclinations with respect to the initial angular momentum of the
swarm range from i=0 to ~90 deg, with most cases having i<50 deg. Our binary
formation mechanism implies that the primary and secondary components in each
binary pair should have identical bulk composition, which is consistent with
the current photometric data. We discuss the applicability of our results to
the Pluto-Charon, Orcus-Vanth, (617) Patroclus-Menoetius and (90) Antiope
binary systems.Comment: Astronomical Journal, in pres
Tighter Relations Between Sensitivity and Other Complexity Measures
Sensitivity conjecture is a longstanding and fundamental open problem in the
area of complexity measures of Boolean functions and decision tree complexity.
The conjecture postulates that the maximum sensitivity of a Boolean function is
polynomially related to other major complexity measures. Despite much attention
to the problem and major advances in analysis of Boolean functions in the past
decade, the problem remains wide open with no positive result toward the
conjecture since the work of Kenyon and Kutin from 2004.
In this work, we present new upper bounds for various complexity measures in
terms of sensitivity improving the bounds provided by Kenyon and Kutin.
Specifically, we show that deg(f)^{1-o(1)}=O(2^{s(f)}) and C(f) < 2^{s(f)-1}
s(f); these in turn imply various corollaries regarding the relation between
sensitivity and other complexity measures, such as block sensitivity, via known
results. The gap between sensitivity and other complexity measures remains
exponential but these results are the first improvement for this difficult
problem that has been achieved in a decade.Comment: This is the merged form of arXiv submission 1306.4466 with another
work. Appeared in ICALP 2014, 14 page
The risks of concurrent treatment with tenofovir and aminoglycosides in patients with HIV-associated tuberculosis
The South African public sector antiretroviral treatment (ART) guidelines have recently been changed to include tenofovir in the first-line regimen.1 Injectable drugs from the aminoglycoside class are part of the intensive phase of regimen 2 tuberculosis (TB) treatment and the multidrug-resistant (MDR) TB treatment regimen in the South African TB programme. We wish to draw the attention of clinicians managing patients with HIVassociated TB to the potential dangers of concurrent administration of these drugs. We present two illustrative cases
The TAOS Project: Upper Bounds on the Population of Small KBOs and Tests of Models of Formation and Evolution of the Outer Solar System
We have analyzed the first 3.75 years of data from TAOS, the Taiwanese
American Occultation Survey. TAOS monitors bright stars to search for
occultations by Kuiper Belt Objects (KBOs). This dataset comprises 5e5
star-hours of multi-telescope photometric data taken at 4 or 5 Hz. No events
consistent with KBO occultations were found in this dataset. We compute the
number of events expected for the Kuiper Belt formation and evolution models of
Pan & Sari (2005), Kenyon & Bromley (2004), Benavidez & Campo Bagatin (2009),
and Fraser (2009). A comparison with the upper limits we derive from our data
constrains the parameter space of these models. This is the first detailed
comparison of models of the KBO size distribution with data from an occultation
survey. Our results suggest that the KBO population is comprised of objects
with low internal strength and that planetary migration played a role in the
shaping of the size distribution.Comment: 18 pages, 16 figures, Aj submitte
Geological Processes on the Northeast Atlantic Margin - Preliminary results of geological and geophysical investigations during the lTR-8 cruise of R/V Professor Logachev
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