22,588 research outputs found
Scalable Robust Kidney Exchange
In barter exchanges, participants directly trade their endowed goods in a
constrained economic setting without money. Transactions in barter exchanges
are often facilitated via a central clearinghouse that must match participants
even in the face of uncertainty---over participants, existence and quality of
potential trades, and so on. Leveraging robust combinatorial optimization
techniques, we address uncertainty in kidney exchange, a real-world barter
market where patients swap (in)compatible paired donors. We provide two
scalable robust methods to handle two distinct types of uncertainty in kidney
exchange---over the quality and the existence of a potential match. The latter
case directly addresses a weakness in all stochastic-optimization-based methods
to the kidney exchange clearing problem, which all necessarily require explicit
estimates of the probability of a transaction existing---a still-unsolved
problem in this nascent market. We also propose a novel, scalable kidney
exchange formulation that eliminates the need for an exponential-time
constraint generation process in competing formulations, maintains provable
optimality, and serves as a subsolver for our robust approach. For each type of
uncertainty we demonstrate the benefits of robustness on real data from a
large, fielded kidney exchange in the United States. We conclude by drawing
parallels between robustness and notions of fairness in the kidney exchange
setting.Comment: Presented at AAAI1
Modifications of the Pisa Tube and some results from observations off Cape Point
Twelve stations were worked from 6 to 8 July 1965 aboard the R.S. SARDINOPS in a tight grid of three lines between Cape Town and the Cape of Good Hope, one line of four stations being worked each day. One subsurface current observation was made at each station with a Pisa Current Indicator (Carruthers Jelly Bottle). Results show three cyclonic eddies, with upwelling at each eddy. Surface flow, as shown by ship drift, bears no relationship to subsurface flow, possibly because of the strong thermocline throughout the area
Disadvantages of the Olson drift card, and description of a newly designed card
Disadvantages of the Olson drift card are discussed, and a newly designed polythene drift card (3 mm thick) is described. This card is compact and complete. In tests along the South African coast it has proved to be durable and convenient. The tests indicate that it compares favorably with either drift bottles or the Olson plastic envelopes
Prospective Analysis Spin- and CP-sensitive Variables in H -> ZZ -> l_1 l_1 l_2 l_2 with Atlas
A possibility to prove spin and CP-eigenvalue of a Standard Model (SM) Higgs
boson is presented. We exploit angular correlations in the subsequent decay H
-> ZZ -> 4l (muons or electrons) for Higgs masses above 200 GeV. We compare the
angular distributions of the leptons originating from the SM Higgs with those
resulting from decays of hypothetical particles with differing quantum numbers.
We restrict our analysis to the use of the Atlas-detector which is one of two
multi-purpose detectors at the upcoming 14 TeV proton-proton-collider (LHC) at
CERN. By applying a fast simulation of the Atlas detector it can be shown that
these correlations will be measured sufficiently well that consistency with the
spin-CP hypothesis 0+ of the Standard Model can be verified and the 0- and 1+-
can be ruled out with an integrated luminosity of 100 fb^-1.Comment: 25 pages, 9 figures Version 2: Minor changes made as requested by
Atlas referee and Springer editor. Added a chapter where background
subtraction is detaile
Low Dirac Eigenmodes and the Topological and Chiral Structure of the QCD Vacuum
Several lattice calculations which probe the chiral and topological structure
of QCD are discussed. The results focus attention on the low-lying eigenmodes
of the Dirac operator in typical gauge field configurations.Comment: Talk presented at the DPF2000 Conferenc
The Size and Nature of Lyman alpha Forest Clouds Probed by QSO Pairs and Groups
We describe a robust Bayesian statistical method for determining Lyman alpha
forest cloud sizes in spherical and in thin disk geometries, using absorption
in adjacent sightlines toward closely separated QSO pairs and groups, apply
this method to the available data, and discuss implications of our results for
models of Ly alpha clouds. Under the assumption of a population of uniform-
size and unclustered clouds, the data from Q1343+2640A/B give a 99% confidence
lower and upper bounds 61<R<533 kpc/h on the radius of spherical clouds at z
about 1.8, with a median value of 149 kpc/h [].
The baryonic mass of such large clouds is comparable to that of dwarf irregular
galaxies. Their cosmic overdensity is close to the turn-around density but
generally below the virialization density, suggesting a population of gravi-
tationally bound but unvirialized protogalactic objects at z about 2. Their
comoving volume density is similar to that of the faint blue galaxies (FBGs) at
the limiting magnitude B of 26-27. The dynamical collapsing timescale of over-
densities like these clouds is also comparable with the cosmic time difference
between z of 2 to 1. Both populations of objects show similar weak clustering
in space. All this evidence suggests a possible identification of Ly alpha
clouds as the collapsing progenitors of the FBGs at z about 1. We also
investigate the other QSO pairs: Q0307-1931/1932, Q0107-0232/0235, and the
triplet of Q1623+268. Imposing an uniform W_0 > 0.4 A threshold on all
linelists, we find a trend of larger inferred cloud radius with larger proper
separation of QSO pairs, significant at the 3.4 sigma level. This indicates
that the idealization of unclustered, uniform-sized clouds does not accurately
describe the Ly alpha cloud population.Comment: Astrophysical Journal accepted; 28 pages of uuencoded gzip compressed
postscript file (including 8 figures). Also see the uncompressed postscript
file at http://www.astro.columbia.edu/~fang
Large-area sheet task advanced dendritic web growth development
The thermal models used for analyzing dendritic web growth and calculating the thermal stress were reexamined to establish the validity limits imposed by the assumptions of the models. Also, the effects of thermal conduction through the gas phase were evaluated and found to be small. New growth designs, both static and dynamic, were generated using the modeling results. Residual stress effects in dendritic web were examined. In the laboratory, new techniques for the control of temperature distributions in three dimensions were developed. A new maximum undeformed web width of 5.8 cm was achieved. A 58% increase in growth velocity of 150 micrometers thickness was achieved with dynamic hardware. The area throughput goals for transient growth of 30 and 35 sq cm/min were exceeded
Physical Mechanisms for the Variable Spin-down of SGR 1900+14
We consider the physical implications of the rapid spindown of Soft Gamma
Repeater 1900+14, and of the apparent "braking glitch", \Delta P/P = l x 10^-4,
that was concurrent with the Aug. 27th giant flare. A radiation-hydrodynamical
outflow associated with the flare could impart the required torque, but only if
the dipole magnetic field is stronger than ~ 10^14 G and the outflow lasts
longer and/or is more energetic than the observed X-ray flare. A positive
period increment is also a natural consequence of a gradual, plastic
deformation of the neutron star crust by an intense magnetic field, which
forces the neutron superfluid to rotate more slowly than the crust. Sudden
unpinning of the neutron vortex lines during the August 27th event would then
induce a glitch opposite in sign to those observed in young pulsars, but of a
much larger magnitude as a result of the slower rotation.
The change in the persistent X-ray lightcurve following the August 27 event
is ascribed to continued particle heating in the active region of that
outburst. The enhanced X-ray output can be powered by a steady current flowing
through the magnetosphere, induced by the twisting motion of the crust. The
long term rate of spindown appears to be accelerated with respect to a simple
magnetic dipole torque. Accelerated spindown of a seismically-active magnetar
will occur when its persistent output of Alfven waves and particles exceeds its
spindown luminosity. We suggest that SGRs experience some episodes of relative
inactivity, with diminished spindown rates, and that such inactive magnetars
are observed as Anomalous X-ray Pulsars (AXPs). The rapid reappearence of
persistent X-ray emission following August 27 flare gives evidence against
accretion-powered models.Comment: 24 pages, no figure
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