11,595 research outputs found
Intermediate Tail Dependence: A Review and Some New Results
The concept of intermediate tail dependence is useful if one wants to
quantify the degree of positive dependence in the tails when there is no strong
evidence of presence of the usual tail dependence. We first review existing
studies on intermediate tail dependence, and then we report new results to
supplement the review. Intermediate tail dependence for elliptical, extreme
value and Archimedean copulas are reviewed and further studied, respectively.
For Archimedean copulas, we not only consider the frailty model but also the
recently studied scale mixture model; for the latter, conditions leading to
upper intermediate tail dependence are presented, and it provides a useful way
to simulate copulas with desirable intermediate tail dependence structures.Comment: 25 pages, 1 figur
Deep Mid-Infrared Silicate Absorption as a Diagnostic of Obscuring Geometry Toward Galactic Nuclei
The silicate cross section peak near 10um produces emission and absorption
features in the spectra of dusty galactic nuclei observed with the Spitzer
Space Telescope. Especially in ultraluminous infrared galaxies, the observed
absorption feature can be extremely deep, as IRAS 08572+3915 illustrates. A
foreground screen of obscuration cannot reproduce this observed feature, even
at large optical depth. Instead, the deep absorption requires a nuclear source
to be deeply embedded in a smooth distribution of material that is both
geometrically and optically thick. In contrast, a clumpy medium can produce
only shallow absorption or emission, which are characteristic of
optically-identified active galactic nuclei. In general, the geometry of the
dusty region and the total optical depth, rather than the grain composition or
heating spectrum, determine the silicate feature's observable properties. The
apparent optical depth calculated from the ratio of line to continuum emission
generally fails to accurately measure the true optical depth. The obscuring
geometry, not the nature of the embedded source, also determines the far-IR
spectral shape.Comment: To appear in ApJ
Extended matrix Gelfand-Dickey hierarchies: reduction to classical Lie algebras
The Drinfeld-Sokolov reduction method has been used to associate with
extensions of the matrix r-KdV system. Reductions of these systems to the fixed
point sets of involutive Poisson maps, implementing reduction of to
classical Lie algebras of type , are here presented. Modifications
corresponding, in the first place to factorisation of the Lax operator, and
then to Wakimoto realisations of the current algebra components of the
factorisation, are also described.Comment: plain TeX, 12 page
Experimental tests of hidden variable theories from dBB to Stochastic Electrodynamics
In this paper we present some of our experimental results on testing hidden
variable theories, which range from Bell inequalities measurements to a
conclusive test of stochastic electrodynamics
Positive maps, majorization, entropic inequalities, and detection of entanglement
In this paper, we discuss some general connections between the notions of
positive map, weak majorization and entropic inequalities in the context of
detection of entanglement among bipartite quantum systems. First, basing on the
fact that any positive map can
be written as the difference between two completely positive maps
, we propose a possible way to generalize the
Nielsen--Kempe majorization criterion. Then we present two methods of
derivation of some general classes of entropic inequalities useful for the
detection of entanglement. While the first one follows from the aforementioned
generalized majorization relation and the concept of the Schur--concave
decreasing functions, the second is based on some functional inequalities. What
is important is that, contrary to the Nielsen--Kempe majorization criterion and
entropic inequalities, our criteria allow for the detection of entangled states
with positive partial transposition when using indecomposable positive maps. We
also point out that if a state with at least one maximally mixed subsystem is
detected by some necessary criterion based on the positive map , then
there exist entropic inequalities derived from (by both procedures)
that also detect this state. In this sense, they are equivalent to the
necessary criterion [I\ot\Lambda](\varrho_{AB})\geq 0. Moreover, our
inequalities provide a way of constructing multi--copy entanglement witnesses
and therefore are promising from the experimental point of view. Finally, we
discuss some of the derived inequalities in the context of recently introduced
protocol of state merging and possibility of approximating the mean value of a
linear entanglement witness.Comment: the published version, 25 pages in NJP format, 6 figure
Maximally entangled mixed states of two qubits
We consider mixed states of two qubits and show under which global unitary
operations their entanglement is maximized. This leads to a class of states
that is a generalization of the Bell states. Three measures of entanglement are
considered: entanglement of formation, negativity and relative entropy of
entanglement. Surprisingly all states that maximize one measure also maximize
the others. We will give a complete characterization of these generalized Bell
states and prove that these states for fixed eigenvalues are all equivalent
under local unitary transformations. We will furthermore characterize all
nearly entangled states closest to the maximally mixed state and derive a new
lower bound on the volume of separable mixed states
The use and calibration of read-out streaks to increase the dynamic range of the Swift Ultraviolet/Optical Telescope
The dynamic range of photon counting micro-channel-plate (MCP) intensified
charged-coupled device (CCD) instruments such as the Swift Ultraviolet/Optical
Telescope (UVOT) and the XMM-Newton Optical Monitor (XMM-OM) is limited at the
bright end by coincidence loss, the superposition of multiple photons in the
individual frames recorded by the CCD. Photons which arrive during the brief
period in which the image frame is transferred for read out of the CCD are
displaced in the transfer direction in the recorded images. For sufficiently
bright sources, these displaced counts form read-out streaks. Using UVOT
observations of Tycho-2 stars, we investigate the use of these read-out streaks
to obtain photometry for sources which are too bright (and hence have too much
coincidence loss) for normal aperture photometry to be reliable. For
read-out-streak photometry, the bright-source limiting factor is coincidence
loss within the MCPs rather than the CCD. We find that photometric measurements
can be obtained for stars up to 2.4 magnitudes brighter than the usual
full-frame coincidence-loss limit by using the read-out streaks. The resulting
bright-limit Vega magnitudes in the UVOT passbands are UVW2=8.80, UVM2=8.27,
UVW1=8.86, u=9.76, b=10.53, v=9.31 and White=11.71; these limits are
independent of the windowing mode of the camera. We find that a photometric
precision of 0.1 mag can be achieved through read-out streak measurements. A
suitable method for the measurement of read-out streaks is described and all
necessary calibration factors are given.Comment: 11 pages, accepted for publication in MNRAS. Code available from the
calibration link at http://www.mssl.ucl.ac.uk/www_astro/uvo
The Swift-UVOT ultraviolet and visible grism calibration
We present the calibration of the Swift UVOT grisms, of which there are two,
providing low-resolution field spectroscopy in the ultraviolet and optical
bands respectively. The UV grism covers the range 1700-5000 Angstrom with a
spectral resolution of 75 at 2600 Angstrom for source magnitudes of u=10-16
mag, while the visible grism covers the range 2850-6600 Angstrom with a
spectral resolution of 100 at 4000 Angstrom for source magnitudes of b=12-17
mag. This calibration extends over all detector positions, for all modes used
during operations. The wavelength accuracy (1-sigma) is 9 Angstrom in the UV
grism clocked mode, 17 Angstrom in the UV grism nominal mode and 22 Angstrom in
the visible grism. The range below 2740 Angstrom in the UV grism and 5200
Angstrom in the visible grism never suffers from overlapping by higher spectral
orders. The flux calibration of the grisms includes a correction we developed
for coincidence loss in the detector. The error in the coincidence loss
correction is less than 20%. The position of the spectrum on the detector only
affects the effective area (sensitivity) by a few percent in the nominal modes,
but varies substantially in the clocked modes. The error in the effective area
is from 9% in the UV grism clocked mode to 15% in the visible grism clocked
mode .Comment: 27 pages, 31 figures; MNRAS accepted 23 February 201
Le Chatelier principle in replicator dynamics
The Le Chatelier principle states that physical equilibria are not only
stable, but they also resist external perturbations via short-time
negative-feedback mechanisms: a perturbation induces processes tending to
diminish its results. The principle has deep roots, e.g., in thermodynamics it
is closely related to the second law and the positivity of the entropy
production. Here we study the applicability of the Le Chatelier principle to
evolutionary game theory, i.e., to perturbations of a Nash equilibrium within
the replicator dynamics. We show that the principle can be reformulated as a
majorization relation. This defines a stability notion that generalizes the
concept of evolutionary stability. We determine criteria for a Nash equilibrium
to satisfy the Le Chatelier principle and relate them to mutualistic
interactions (game-theoretical anticoordination) showing in which sense
mutualistic replicators can be more stable than (say) competing ones. There are
globally stable Nash equilibria, where the Le Chatelier principle is violated
even locally: in contrast to the thermodynamic equilibrium a Nash equilibrium
can amplify small perturbations, though both this type of equilibria satisfy
the detailed balance condition.Comment: 12 pages, 3 figure
Towards a framework for critical citizenship education
Increasingly countries around the world are promoting forms of "critical" citizenship in the planned curricula of schools. However, the intended meaning behind this term varies markedly and can range from a set of creative and technical skills under the label "critical thinking" to a desire to encourage engagement, action and political emancipation, often labelled "critical pedagogy". This paper distinguishes these manifestations of the "critical" and, based on an analysis of the prevailing models of critical pedagogy and citizenship education, develops a conceptual framework for analysing and comparing the nature of critical citizenship
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