39,806 research outputs found
Informality, Trade Policies And Smuggling In West Africa
In West Africa, recorded intra-regional trade is small but informal cross-border trade (ICBT) is pervasive, despite regional integration schemes intended to promote official trade. We argue that ICBT must be understood in light of two features of West African national boundaries: divergent economic policies between neighboring countries and the ease with which informal operators can ship goods across borders. We focus on two ICBT clusters: Senegal–The Gambia and Nigeria–Benin–Togo. Nigeria and Senegal have protected their domestic industries with high import barriers, whereas Benin, Togo and The Gambia have maintained lower import taxation. These differential trade policies, together with high mobility of goods and people across borders, lead to widespread smuggling, with goods imported legally in low-tax countries and re-exported unofficially to countries with higher import duties
Thermodynamic Measurements Using the Knudsen Cell Technique
The Knudsen cell technique has been used for over a century and is a valuable tool for measurement of vapor pressures and thermodynamic properties. It is based on a small enclosure (~1 cm long x 1 cm diameter) in which a condensed phase/vapor equilibria can be established. A small (<1 mm) orifice on the cell allows sampling of the vapor via a variety of techniques including weight loss, torsion effusion, target collection, and mass spectrometry. Many excellent measurements based on these methods have been reported. However in order to obtain reliable measurements, a variety of factors must be considered. They include proper cell material selection, accurate and uniform temperature control and measurement, and proper sampling of the vapor. Each of these factors are discussed in detail in this chapter. Typically these studies are conducted at high temperatures and it is a challenge to select an inert container material. Recommended materials are discussed and in some cases the container may be used as part of the system under study. Temperature control and measurement is perhaps the most important issue. In most systems, the furnace must be compact yet there can be no temperature gradient in the cell. Temperatures are measured with either a thermocouple or pyrometer and the relative advantages of each are discussed. Sampling method considerations depend on the particular technique. It is essential that all of the vapor or a representative portion of the vapor be sampled. The distribution of the effusate from a Knudsen cell is discussed and sampling positions discussed. Mass spectrometry is often used to study the effusing vapor and the relations between ion current and vapor pressure are discussed
Study of quantum current enhancement, eigenenergy spectra and magnetic moments in a multiply connected system at equilibrium
A multiply connected system in both its open and closed form variations but
in equilibrium is studied using quantum waveguide theory. The system exhibits
remarkable features, in its open form variation we see current enhancement,
hitherto seen only in non-equilibrium systems in absence of magnetic flux. In
its closed form analog parity effects are broken. Further we analyse the global
and local current densities of our system and also show that the orbital
magnetic response of the system calculated from the current densities (and
inherently linked to the topological configuration) is qualitatively not same
as that calculated from the eigenenergy spectra.Comment: 10 pages, 15 figures, 3 table
Decoherence of one-dimensional flying qubits due to their cross-talk and imperfections
We study decoherence of propagating spin-1/2 excitations in generic
(non-integrable and/or disordered) spin chains. We find the relevant
decoherence times to be shorter in both the near-critical and diffusive regimes
(if any), which fact might have important implications for the recently
proposed spin chain-based implementations of quantum information processing.Comment: Latex, 5 pages, no figure
Is the cosmic microwave background really non-Gaussian?
Two recent papers have claimed detection of non-Gaussian features in the COBE
DMR sky maps of the cosmic microwave background. We confirm these results, but
argue that Gaussianity is still not convincingly ruled out. Since a score of
non-Gaussianity tests have now been published, one might expect some mildly
significant results even by chance. Moreover, in the case of one measure which
yields a detection, a bispectrum statistic, we find that if the non-Gaussian
feature is real, it may well be due to detector noise rather than a
non-Gaussian sky signal, since a signal-to-noise analysis localizes it to
angular scales smaller than the beam. We study its spatial origin in case it is
nonetheless due to a sky signal (eg, a cosmic string wake or flat-spectrum
foreground contaminant). It appears highly localized in the direction b=39.5,
l=257, since removing a mere 5 pixels inside a single COBE beam area centered
there makes the effect statistically insignificant. We also test Guassianity
with an eigenmode analysis which allows a sky map to be treated as a random
number generator. A battery of tests of this generator all yield results
consistent with Gaussianity.Comment: Revised to match accepted ApJL version. 4 pages with 2 figs included.
Links and color fig at http://www.sns.ias.edu/~max/gaussianity_frames.html or
from [email protected]
Guaranteed Minimum-Rank Solutions of Linear Matrix Equations via Nuclear Norm Minimization
The affine rank minimization problem consists of finding a matrix of minimum
rank that satisfies a given system of linear equality constraints. Such
problems have appeared in the literature of a diverse set of fields including
system identification and control, Euclidean embedding, and collaborative
filtering. Although specific instances can often be solved with specialized
algorithms, the general affine rank minimization problem is NP-hard. In this
paper, we show that if a certain restricted isometry property holds for the
linear transformation defining the constraints, the minimum rank solution can
be recovered by solving a convex optimization problem, namely the minimization
of the nuclear norm over the given affine space. We present several random
ensembles of equations where the restricted isometry property holds with
overwhelming probability. The techniques used in our analysis have strong
parallels in the compressed sensing framework. We discuss how affine rank
minimization generalizes this pre-existing concept and outline a dictionary
relating concepts from cardinality minimization to those of rank minimization
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