11,283 research outputs found
Double-impulse magnetic focusing of launched cold atoms.
We have theoretically investigated three-dimensional focusing of a launched cloud of cold atoms using a pair of magnetic lens pulses (the alternate-gradient method). Individual lenses focus radially and defocus axially or vice versa. The performance of the two possible pulse sequences are compared and found to be ideal for loading both 'pancake' and 'sausage' shaped magnetic/optical microtraps. It is shown that focusing aberrations are considerably smaller for double-impulse magnetic lenses compared to single-impulse magnetic lenses. An analysis of clouds focused by the double-impulse technique is presented
Directional genetic differentiation and asymmetric migration
Understanding the population structure and patterns of gene flow within
species is of fundamental importance to the study of evolution. In the fields
of population and evolutionary genetics, measures of genetic differentiation
are commonly used to gather this information. One potential caveat is that
these measures assume gene flow to be symmetric. However, asymmetric gene flow
is common in nature, especially in systems driven by physical processes such as
wind or water currents. Since information about levels of asymmetric gene flow
among populations is essential for the correct interpretation of the
distribution of contemporary genetic diversity within species, this should not
be overlooked. To obtain information on asymmetric migration patterns from
genetic data, complex models based on maximum likelihood or Bayesian approaches
generally need to be employed, often at great computational cost. Here, a new
simpler and more efficient approach for understanding gene flow patterns is
presented. This approach allows the estimation of directional components of
genetic divergence between pairs of populations at low computational effort,
using any of the classical or modern measures of genetic differentiation. These
directional measures of genetic differentiation can further be used to
calculate directional relative migration and to detect asymmetries in gene flow
patterns. This can be done in a user-friendly web application called
divMigrate-online introduced in this paper. Using simulated data sets with
known gene flow regimes, we demonstrate that the method is capable of resolving
complex migration patterns under a range of study designs.Comment: 25 pages, 8 (+3) figures, 1 tabl
On the complexity of strongly connected components in directed hypergraphs
We study the complexity of some algorithmic problems on directed hypergraphs
and their strongly connected components (SCCs). The main contribution is an
almost linear time algorithm computing the terminal strongly connected
components (i.e. SCCs which do not reach any components but themselves).
"Almost linear" here means that the complexity of the algorithm is linear in
the size of the hypergraph up to a factor alpha(n), where alpha is the inverse
of Ackermann function, and n is the number of vertices. Our motivation to study
this problem arises from a recent application of directed hypergraphs to
computational tropical geometry.
We also discuss the problem of computing all SCCs. We establish a superlinear
lower bound on the size of the transitive reduction of the reachability
relation in directed hypergraphs, showing that it is combinatorially more
complex than in directed graphs. Besides, we prove a linear time reduction from
the well-studied problem of finding all minimal sets among a given family to
the problem of computing the SCCs. Only subquadratic time algorithms are known
for the former problem. These results strongly suggest that the problem of
computing the SCCs is harder in directed hypergraphs than in directed graphs.Comment: v1: 32 pages, 7 figures; v2: revised version, 34 pages, 7 figure
Should I Stay or Should I Go? Gender Gaps in the Lateral Market for SEC Lawyers
This article examines the gender gap in the lateral market for government lawyers. Using data on lawyers from the Enforcement Division of the Securities and Exchange Commission (SEC), from 2004 to 2016, we find the following: First, gender gaps in pay and promotion appear to be minimal. Second, and confounding the first finding, we find significant gaps in assignments, with men receiving the more challenging and career-enhancing projects. Third, men are more likely than women to move laterally; and when they do, are more likely than the women to move to lucrative private sector jobs
Probing the first galaxies with the SKA
Observations of anisotropies in the brightness temperature of the 21 cm line
of neutral hydrogen from the period before reionization would shed light on the
dawn of the first stars and galaxies. In this paper, we use large-scale
semi-numerical simulations to analyse the imprint on the 21 cm signal of
spatial fluctuations in the Lyman-alpha flux arising from the clustering of the
first galaxies. We show that an experiment such as the Square Kilometer Array
(SKA) can probe this signal at the onset of reionization, giving us important
information about the UV emission spectra of the first stars and characterizing
their host galaxies. SKA-pathfinders with ~ 10% of the full collecting area
should be capable of making a statistical detection of the 21 cm power spectrum
at redshifts z 67 MHz). We then show
that the SKA should be able to measure the three dimensional power spectrum as
a function of the angle with the line of sight and discuss the use of the
redshift space distortions as a way to separate out the different components of
the 21 cm power spectrum. We demonstrate that, at least on large scales where
the Lyman-alpha fluctuations are linear, they can be used as a model
independent way to extract the power spectra due to these Lyman-alpha
fluctuations.Comment: 13 pages, 17 figures. New version to match version accepted by A&A.
Improved discussions on the Lyman-alpha simulation, adiabatic cooling
fluctuations, the Fisher matrix approach and the Poisson term calculation.
New version of the code available at: http://www.SimFast21.or
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