7,475 research outputs found
Network Topology of an Experimental Futures Exchange
Many systems of different nature exhibit scale free behaviors. Economic
systems with power law distribution in the wealth is one of the examples. To
better understand the working behind the complexity, we undertook an empirical
study measuring the interactions between market participants. A Web server was
setup to administer the exchange of futures contracts whose liquidation prices
were coupled to event outcomes. After free registration, participants started
trading to compete for the money prizes upon maturity of the futures contracts
at the end of the experiment. The evolving `cash' flow network was
reconstructed from the transactions between players. We show that the network
topology is hierarchical, disassortative and scale-free with a power law
exponent of 1.02+-0.09 in the degree distribution. The small-world property
emerged early in the experiment while the number of participants was still
small. We also show power law distributions of the net incomes and
inter-transaction time intervals. Big winners and losers are associated with
high degree, high betweenness centrality, low clustering coefficient and low
degree-correlation. We identify communities in the network as groups of the
like-minded. The distribution of the community sizes is shown to be power-law
distributed with an exponent of 1.19+-0.16.Comment: 6 pages, 12 figure
Asymptotic stability, concentration, and oscillation in harmonic map heat-flow, Landau-Lifshitz, and Schroedinger maps on R^2
We consider the Landau-Lifshitz equations of ferromagnetism (including the
harmonic map heat-flow and Schroedinger flow as special cases) for degree m
equivariant maps from R^2 to S^2. If m \geq 3, we prove that near-minimal
energy solutions converge to a harmonic map as t goes to infinity (asymptotic
stability), extending previous work down to degree m = 3. Due to slow spatial
decay of the harmonic map components, a new approach is needed for m=3,
involving (among other tools) a "normal form" for the parameter dynamics, and
the 2D radial double-endpoint Strichartz estimate for Schroedinger operators
with sufficiently repulsive potentials (which may be of some independent
interest). When m=2 this asymptotic stability may fail: in the case of
heat-flow with a further symmetry restriction, we show that more exotic
asymptotics are possible, including infinite-time concentration (blow-up), and
even "eternal oscillation".Comment: 34 page
Coupling between surface ozone and leaf area index in a chemical transport model: strength of feedback and implications for ozone air quality and vegetation health
Tropospheric ozone is an air pollutant that substantially harms vegetation
and is also strongly dependent on various vegetation-mediated processes. The
interdependence between ozone and vegetation may constitute feedback
mechanisms that can alter ozone concentration itself but have not been
considered in most studies to date. In this study we examine the importance
of dynamic coupling between surface ozone and leaf area index (LAI) in
shaping ozone air quality and vegetation. We first implement an empirical
scheme for ozone damage on vegetation in the Community Land Model (CLM) and
simulate the steady-state responses of LAI to long-term exposure to a range
of prescribed ozone levels (from 0 to 100 ppb). We find that most plant
functional types suffer a substantial decline in LAI as ozone level
increases. Based on the CLM-simulated results, we develop and implement in
the GEOS-Chem chemical transport model a parameterization that computes
fractional changes in monthly LAI as a function of local mean ozone levels.
By forcing LAI to respond to ozone concentrations on a monthly timescale, the
model simulates ozone–LAI coupling dynamically via biogeochemical processes
including biogenic volatile organic compound (VOC) emissions and dry
deposition, without the complication from meteorological changes. We find
that ozone-induced damage on LAI can lead to changes in ozone concentrations
by −1.8 to +3 ppb in boreal summer, with a corresponding ozone feedback
factor of −0.1 to +0.6 that represents an overall self-amplifying effect
from ozone–LAI coupling. Substantially higher simulated ozone due to strong
positive feedbacks is found in most tropical forests, mainly due to the
ozone-induced reductions in LAI and dry deposition velocity, whereas reduced
isoprene emission plays a lesser role in these low-NOx
environments. In high-NOx regions such as the eastern US,
Europe,
and China, however, the feedback effect is much weaker and even negative in
some regions, reflecting the compensating effects of reduced dry deposition
and reduced isoprene emission (which reduces ozone in
high-NOx environments). In remote, low-LAI regions, including
most of the Southern Hemisphere, the ozone feedback is generally slightly
negative due to the reduced transport of NOx–VOC reaction
products that serve as NOx reservoirs. This study represents
the first step to accounting for dynamic ozone–vegetation coupling in a chemical
transport model with ramifications for a more realistic joint assessment of
ozone air quality and ecosystem health.</p
Pattern formation and selection in quasi-static fracture
Fracture in quasi-statically driven systems is studied by means of a discrete
spring-block model. Developed from close comparison with desiccation
experiments, it describes crack formation induced by friction on a substrate.
The model produces cellular, hierarchical patterns of cracks, characterized by
a mean fragment size linear in the layer thickness, in agreement with
experiments. The selection of a stationary fragment size is explained by
exploiting the correlations prior to cracking. A scaling behavior associated
with the thickness and substrate coupling, derived and confirmed by
simulations, suggests why patterns have similar morphology despite their
disparity in scales.Comment: 4 pages, RevTeX, two-column, 5 PS figures include
Correlator of Fundamental and Anti-symmetric Wilson Loops in AdS/CFT Correspondence
We study the two circular Wilson loop correlator in which one is of
anti-symmetric representation, while the other is of fundamental representation
in 4-dimensional super Yang-Mills theory. This correlator has a
good AdS dual, which is a system of a D5-brane and a fundamental string. We
calculated the on-shell action of the string, and clarified the Gross-Ooguri
transition in this correlator. Some limiting cases are also examined.Comment: 22 pages, 5 figures, v2: typos corrected, v3: final version in JHE
Enhancement of singly and multiply strangeness in p-Pb and Pb-Pb collisions at 158A GeV/c
The idea that the reduction of the strange quark suppression in string
fragmentation leads to the enhancement of strange particle yield in
nucleus-nucleus collisions is applied to study the singly and multiply strange
particle production in p-Pb and Pb-Pb collisions at 158A GeV/c. In this
mechanism the strange quark suppression factor is related to the effective
string tension, which increases in turn with the increase of the energy, of the
centrality and of the mass of colliding system. The WA97 observation that the
strange particle enhancement increases with the increasing of centrality and of
strange quark content in multiply strange particles in Pb-Pb collisions with
respect to p-Pb collisions was accounted reasonably.Comment: 8 pages, 3 PostScript figures, in Latex form. submitted to PR
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