2,105 research outputs found
Paradoxes in Fair Computer-Aided Decision Making
Computer-aided decision making--where a human decision-maker is aided by a
computational classifier in making a decision--is becoming increasingly
prevalent. For instance, judges in at least nine states make use of algorithmic
tools meant to determine "recidivism risk scores" for criminal defendants in
sentencing, parole, or bail decisions. A subject of much recent debate is
whether such algorithmic tools are "fair" in the sense that they do not
discriminate against certain groups (e.g., races) of people.
Our main result shows that for "non-trivial" computer-aided decision making,
either the classifier must be discriminatory, or a rational decision-maker
using the output of the classifier is forced to be discriminatory. We further
provide a complete characterization of situations where fair computer-aided
decision making is possible
Scale-dependent angle of alignment between velocity and magnetic field fluctuations in solar wind turbulence
Under certain conditions, freely decaying magnetohydrodynamic (MHD) turbulence evolves in such a way that velocity and magnetic field fluctuations delta v and delta B approach a state of alignment in which delta v proportional to delta B. This process is called dynamic alignment. Boldyrev has suggested that a similar kind of alignment process occurs as energy cascades from large to small scales through the inertial range in strong incompressible MHD turbulence. In this study, plasma and magnetic field data from the Wind spacecraft, data acquired in the ecliptic plane near 1 AU, are employed to investigate the angle theta(tau) between velocity and magnetic field fluctuations in the solar wind as a function of the time scale tau of the fluctuations and to look for the scaling relation similar to tau(1/4) predicted by Boldyrev. We find that the angle appears to scale like a power law at large inertial range scales, but then deviates from power law behavior at medium to small inertial range scales. We also find that small errors in the velocity vector measurements can lead to large errors in the angle measurements at small time scales. As a result, we cannot rule out the possibility that the observed deviations from power law behavior arise from errors in the velocity measurements. When we fit the data from 2 x 10(3) s to 2 x 10(4) s with a power law of the form proportional to tau(p), our best fit values for p are in the range 0.27-0.36
A new Fermi smearing approach for scattering of multi-GeV electrons by nuclei
The cross section for electron scattering by nuclei at high momentum
transfers is calculated within the Fermi smearing approximation (FSA), where
binding effects on the struck nucleon are introduced via the relativistic
Hartree approximation (RHA). The model naturally preserves current
conservation, since the response tensor for an off-shell nucleon conserves the
same form that for a free one but with an effective mass. Different
parameterizations for the inelastic nucleon structure function, are analyzed.
The smearing at the Fermi surface is introduced through a momentum distribution
obtained from a perturbative nuclear matter calculation. Recent CEBAF data on
inclusive scattering of 4.05 GeV electrons on Fe are well reproduced for
all measured geometries for the first time, as is evident from the comparison
with previous calculations.Comment: 8 pages in Revtex4 style, 6 eps figures, to appear in Physical Review
Scale dependent alignment between velocity and magnetic field fluctuations in the solar wind and comparisons to Boldyrev's phenomenological theory
(Abridged abstract) A theory of incompressible MHD turbulence recently
developed by Boldyrev predicts the existence of a scale dependent angle of
alignment between velocity and magnetic field fluctuations that is proportional
to the lengthscale of the fluctuations to the power 1/4. In this study, plasma
and magnetic field data from the Wind spacecraft are used to investigate the
angle between velocity and magnetic field fluctuations in the solar wind as a
function of the timescale of the fluctuations and to look for the power law
scaling predicted by Boldyrev.Comment: Particle Acceleration and Transport in the Heliosphere and Beyond,
7th Annual International Astrophysics Conference, Kauai, Hawaii, G. Li, Q.
Hu, O. Verkhoglyadova, G. P. Zank, R. P. Lin, J. Luhmann (eds), AIP
Conference Proceedings 1039, 81-8
Response Functions to Critical Shocks in Social Sciences: An Empirical and Numerical Study
We show that, provided one focuses on properly selected episodes, one can
apply to the social sciences the same observational strategy that has proved
successful in natural sciences such as astrophysics or geodynamics. For
instance, in order to probe the cohesion of a policy, one can, in different
countries, study the reactions to some huge and sudden exogenous shocks, which
we call Dirac shocks. This approach naturally leads to the notion of structural
(as opposed or complementary to temporal) forecast. Although structural
predictions are by far the most common way to test theories in the natural
sciences, they have been much less used in the social sciences. The Dirac shock
approach opens the way to testing structural predictions in the social
sciences. The examples reported here suggest that critical events are able to
reveal pre-existing ``cracks'' because they probe the social cohesion which is
an indicator and predictor of future evolution of the system, and in some cases
foreshadows a bifurcation. We complement our empirical work with numerical
simulations of the response function (``damage spreading'') to Dirac shocks in
the Sznajd model of consensus build-up. We quantify the slow relaxation of the
difference between perturbed and unperturbed systems, the conditions under
which the consensus is modified by the shock and the large variability from one
realization to another
Effect of FK506 in experimental organ transplantation.
FK506 is the most potent immunosuppressive agent known. Its toxicity is substantial in dogs, minor in rats, and unknown in subhuman primates. In small doses that are nontoxic even in dogs, it can be used in synergistic combination with cyclosporine, steroids, and presumably in other drugs
Accurate estimation of third-order moments from turbulence measurements
Politano and Pouquet's law, a generalization of Kolmogorov's four-fifths law
to incompressible MHD, makes it possible to measure the energy cascade rate in
incompressible MHD turbulence by means of third-order moments. In
hydrodynamics, accurate measurement of third-order moments requires large
amounts of data because the probability distributions of velocity-differences
are nearly symmetric and the third-order moments are relatively small.
Measurements of the energy cascade rate in solar wind turbulence have recently
been performed for the first time, but without careful consideration of the
accuracy or statistical uncertainty of the required third-order moments. This
paper investigates the statistical convergence of third-order moments as a
function of the sample size N. It is shown that the accuracy of the
third-moment depends on the number of correlation lengths spanned by the data
set and a method of estimating the statistical uncertainty of the third-moment
is developed. The technique is illustrated using both wind tunnel data and
solar wind data.Comment: Submitted to: Nonlinear Processes in Geophysic
Using Synthetic Spacecraft Data to Interpret Compressible Fluctuations in Solar Wind Turbulence
Kinetic plasma theory is used to generate synthetic spacecraft data to
analyze and interpret the compressible fluctuations in the inertial range of
solar wind turbulence. The kinetic counterparts of the three familiar linear
MHD wave modes---the fast, Alfven, and slow waves---are identified and the
properties of the density-parallel magnetic field correlation for these kinetic
wave modes is presented. The construction of synthetic spacecraft data, based
on the quasi-linear premise---that some characteristics of magnetized plasma
turbulence can be usefully modeled as a collection of randomly phased, linear
wave modes---is described in detail. Theoretical predictions of the
density-parallel magnetic field correlation based on MHD and Vlasov-Maxwell
linear eigenfunctions are presented and compared to the observational
determination of this correlation based on 10 years of Wind spacecraft data. It
is demonstrated that MHD theory is inadequate to describe the compressible
turbulent fluctuations and that the observed density-parallel magnetic field
correlation is consistent with a statistically negligible kinetic fast wave
energy contribution for the large sample used in this study. A model of the
solar wind inertial range fluctuations is proposed comprised of a mixture of a
critically balanced distribution of incompressible Alfvenic fluctuations and a
critically balanced or more anisotropic than critical balance distribution of
compressible slow wave fluctuations. These results imply that there is little
or no transfer of large scale turbulent energy through the inertial range down
to whistler waves at small scales.Comment: Accepted to Astrophysical Journal. 28 pages, 7 figure
Comparison of 3D flux-driven scrape-off layer turbulence simulations with gas-puff imaging of Alcator C-Mod inner-wall limited discharges
We carry out a quantitative comparison between gas-puff imaging (GPI) turbulence measurements in Alcator C-Mod inner-wall limited discharges (Zweben et al 2009 Phys. Plasmas 18 082505) and 3D flux-driven drift-reduced Braginskii turbulence simulations of scrape-off layer dynamics. The comparison is carried out for a series of inner-wall limited discharges where the magnetic field and the density are varied. The comparison between GPI data and non-linear simulations yields overall good agreement for several observables, such as the D-alpha emission levels and intermittency, the radial and poloidal correlation lengths and propagation velocities, and the power and frequency spectral density
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