1,724 research outputs found
Statistical mechanics of the international trade network
Analyzing real data on international trade covering the time interval
1950-2000, we show that in each year over the analyzed period the network is a
typical representative of the ensemble of maximally random weighted networks,
whose directed connections (bilateral trade volumes) are only characterized by
the product of the trading countries' GDPs. It means that time evolution of
this network may be considered as a continuous sequence of equilibrium states,
i.e. quasi-static process. This, in turn, allows one to apply the linear
response theory to make (and also verify) simple predictions about the network.
In particular, we show that bilateral trade fulfills fluctuation-response
theorem, which states that the average relative change in import (export)
between two countries is a sum of relative changes in their GDPs. Yearly
changes in trade volumes prove that the theorem is valid.Comment: 6 pages, 2 figure
Effect of Edge Roughness on resistance and switching voltage of Magnetic Tunnel Junctions
We investigate the impact of edge roughness on the electrical transport
properties of magnetic tunnel junctions using non-equilibrium Greens function
formalism. We have modeled edge roughness as a stochastic variation in the
cross-sectional profile of magnetic tunnel junction characterized by the
stretched exponential decay of the correlation function. The stochastic
variation in the shape and size changes the transverse energy mode profile and
gives rise to the variations in the resistance and switching voltage of the
magnetic tunnel junction. We find that the variations are larger as the
magnetic tunnel junction size is scaled down due to the quantum confinement
effect. A model is proposed for the efficient calculation of edge roughness
effects by approximating the cross-sectional geometry to a circle with the same
cross-sectional area. Further improvement can be obtained by approximating the
cross-sectional area to an ellipse with an aspect ratio determined by the first
transverse eigenvalue corresponding to the 2D cross section. These results
would be useful for reliable design of the spin transfer torque-magnetic random
access memory (STT-MRAM) with ultra-small magnetic tunnel junctions.Comment: 3 pages, 3 figure
Measurement of the anisotropy power spectrum of the radio synchrotron background
We present the first targeted measurement of the power spectrum of
anisotropies of the radio synchrotron background, at 140 MHz where it is the
overwhelmingly dominant photon background. This measurement is important for
understanding the background level of radio sky brightness, which is dominated
by steep-spectrum synchrotron radiation at frequencies below 0.5 GHz and has
been measured to be significantly higher than that which can be produced by
known classes of extragalactic sources and most models of Galactic halo
emission. We determine the anisotropy power spectrum on scales ranging from 2
degrees to 0.2 arcminutes with LOFAR observations of two 18 square degree
fields -- one centered on the Northern hemisphere coldest patch of radio sky
where the Galactic contribution is smallest and one offset from that location
by 15 degrees. We find that the anisotropy power is higher than that
attributable to the distribution of point sources above 100 micro-Jy in flux.
This level of radio anisotropy power indicates that if it results from point
sources, those sources are likely at low fluxes and incredibly numerous, and
likely clustered in a specific manner.Comment: 8 pages, 5 figures, published in MNRAS, updated to published versio
Diffuse Sources, Clustering and the Excess Anisotropy of the Radio Synchrotron Background
We present the largest low frequency (120~MHz) arcminute resolution image of
the radio synchrotron background (RSB) to date, and its corresponding angular
power spectrum of anisotropies (APS) with angular scales ranging from
to . We show that the RSB around the North Celestial Pole has a
significant excess anisotropy power at all scales over a model of unclustered
point sources based on source counts of known source classes. This anisotropy
excess, which does not seem attributable to the diffuse Galactic emission,
could be linked to the surface brightness excess of the RSB. To better
understand the information contained within the measured APS, we model the RSB
varying the brightness distribution, size, and angular clustering of potential
sources. We show that the observed APS could be produced by a population of
faint clustered point sources only if the clustering is extreme and the size of
the Gaussian clusters is . We also show that the observed APS
could be produced by a population of faint diffuse sources with sizes , and this is supported by features present in our image. Both of these
cases would also cause an associated surface brightness excess. These classes
of sources are in a parameter space not well probed by even the deepest radio
surveys to date.Comment: 13 pages, 14 figures. Accepted for publication in MNRA
Stochastic volatility and leverage effect
We prove that a wide class of correlated stochastic volatility models exactly
measure an empirical fact in which past returns are anticorrelated with future
volatilities: the so-called ``leverage effect''. This quantitative measure
allows us to fully estimate all parameters involved and it will entail a deeper
study on correlated stochastic volatility models with practical applications on
option pricing and risk management.Comment: 4 pages, 2 figure
Continuous Equilibrium in Affine and Information-Based Capital Asset Pricing Models
We consider a class of generalized capital asset pricing models in continuous
time with a finite number of agents and tradable securities. The securities may
not be sufficient to span all sources of uncertainty. If the agents have
exponential utility functions and the individual endowments are spanned by the
securities, an equilibrium exists and the agents' optimal trading strategies
are constant. Affine processes, and the theory of information-based asset
pricing are used to model the endogenous asset price dynamics and the terminal
payoff. The derived semi-explicit pricing formulae are applied to numerically
analyze the impact of the agents' risk aversion on the implied volatility of
simultaneously-traded European-style options.Comment: 24 pages, 4 figure
Testing stock market convergence: a non-linear factor approach
This paper applies the Phillips and Sul (Econometrica 75(6):1771–1855, 2007) method to test for convergence in stock returns to an extensive dataset including monthly stock price indices for five EU countries (Germany, France, the Netherlands, Ireland and the UK) as well as the US between 1973 and 2008. We carry out the analysis on both sectors and individual industries within sectors. As a first step, we use the Stock and Watson (J Am Stat Assoc 93(441):349–358, 1998) procedure to filter the data in order to extract the long-run component of the series; then, following Phillips and Sul (Econometrica 75(6):1771–1855, 2007), we estimate the relative transition parameters. In the case of sectoral indices we find convergence in the middle of the sample period, followed by divergence, and detect four (two large and two small) clusters. The analysis at a disaggregate, industry level again points to convergence in the middle of the sample, and subsequent divergence, but a much larger number of clusters is now found. Splitting the cross-section into two subgroups including euro area countries, the UK and the US respectively, provides evidence of a global convergence/divergence process not obviously influenced by EU policies
The Quality vs. the Quantity of Schooling: What Drives Economic Growth?
This paper challenges Hanushek and Woessmann’s [2008] contention that the quality and not the quantity of schooling determines a nation’s rate of economic growth. I first show that their statistical analysis is flawed. I then show that when a nation’s average test scores and average schooling attainment are included in a national income model, both measures explain income differences, but schooling attainment has greater statistical significance. The high correlation between a nation’s average schooling attainment, cumulative investment in schooling, and average tests scores indicates that average schooling attainment implicitly measures the quality as well as the quantity of schooling
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