2,233 research outputs found
Separability in the Ambient Logic
The \it{Ambient Logic} (AL) has been proposed for expressing properties of
process mobility in the calculus of Mobile Ambients (MA), and as a basis for
query languages on semistructured data. We study some basic questions
concerning the discriminating power of AL, focusing on the equivalence on
processes induced by the logic . As underlying calculi besides MA we
consider a subcalculus in which an image-finiteness condition holds and that we
prove to be Turing complete. Synchronous variants of these calculi are studied
as well. In these calculi, we provide two operational characterisations of
: a coinductive one (as a form of bisimilarity) and an inductive one
(based on structual properties of processes). After showing to be stricly
finer than barbed congruence, we establish axiomatisations of on the
subcalculus of MA (both the asynchronous and the synchronous version), enabling
us to relate to structural congruence. We also present some
(un)decidability results that are related to the above separation properties
for AL: the undecidability of on MA and its decidability on the
subcalculus.Comment: logical methods in computer science, 44 page
A new algorithm for point spread function subtraction in high-contrast imaging: a demonstration with angular differential imaging
Direct imaging of exoplanets is limited by bright quasi-static speckles in
the point spread function (PSF) of the central star. This limitation can be
reduced by subtraction of reference PSF images. We have developed an algorithm
to construct an optimized reference PSF image from a set of reference images.
This image is built as a linear combination of the reference images available
and the coefficients of the combination are optimized inside multiple
subsections of the image independently to minimize the residual noise within
each subsection. The algorithm developed can be used with many high-contrast
imaging observing strategies relying on PSF subtraction, such as angular
differential imaging (ADI), roll subtraction, spectral differential imaging,
reference star observations, etc. The performance of the algorithm is
demonstrated for ADI data. It is shown that for this type of data the new
algorithm provides a gain in sensitivity by up to a factor 3 at small
separation over the algorithm used in Marois et al. (2006).Comment: 7 pages, 11 figures, to appear in May 10, 2007 issue of Ap
Employment Adjustment and Part-time Jobs: The U.S. and the U.K. in the Great Recession
We document that fluctuations in part-time employment play a major role in movements in hours per worker, especially during cyclical swings in the labor market. Building on this result, we propose a novel representation of the intensive margin based on a stock-flow framework. The evolution of part-time employment is predominantly explained by cyclical changes in transitions between full-time and part-time employment, which occur overwhelmingly at the same employer and entail large changes in individuals' working hours. We discuss implications for a large class of macroeconomic models that map individual decisions along the extensive/intensive margins onto aggregate labor market outcomes
The Rise of Part-time Employment
We construct new monthly time series of U.S. labor market stocks and flows from 1976 onwards. These data reveal an upward secular trend in turnover between full-time and part-time employment, and a large cyclical component chiefly explained by fluctuations in involuntary part-time work. Both short-run and long-run reallocations occur mostly without an intervening spell of non-employment, and therefore cannot be uncovered without splitting employment into finer categories. We emphasize the importance of our findings for several active debates, such as the slowdown in U.S. labor-market dynamism, changes in job stability and security, and the assessment of labor-market slack
Jordan symmetry reduction for conic optimization over the doubly nonnegative cone: theory and software
A common computational approach for polynomial optimization problems (POPs)
is to use (hierarchies of) semidefinite programming (SDP) relaxations. When the
variables in the POP are required to be nonnegative, these SDP problems
typically involve nonnegative matrices, i.e. they are conic optimization
problems over the doubly nonnegative cone. The Jordan reduction, a symmetry
reduction method for conic optimization, was recently introduced for symmetric
cones by Parrilo and Permenter [Mathematical Programming 181(1), 2020]. We
extend this method to the doubly nonnegative cone, and investigate its
application to known relaxations of the quadratic assignment and maximum stable
set problems. We also introduce new Julia software where the symmetry reduction
is implemented.Comment: 19 pages, titled change from earlier version. arXiv admin note: text
overlap with arXiv:1908.0087
A flexible HCF modeling framework leading to a probabilistic multiaxial Kitagawa-Takahashi diagram
This article describes a flexible modeling framework which leads to the construction of a probabilistic, multiaxial Kitagawa-Takahashi diagram. This framework has been developed following experimental observations that clearly indicate that two independent fatigue damage mechanisms can be activated, at the same time, in metallic materials. Specifically, one damage mechanism is associated with crack initiation and the other with crack arrest. It is postulated that these damage mechanisms are more appropriately modeled using two different fatigue criteria or, more specifically, two completely different approaches to fatigue (i.e. a classical multiaxial fatigue criterion and a LEFM type criterion). Hence, the proposed modeling framework provides the possibility of combining any two suitable criteria, in a probabilistic framework based on the weakest link hypothesis and results in the continuous description of the Kitagawa diagram for any multiaxial stress state. It is shown that under certain conditions this approach is equivalent to the classical El Haddad approach to the short crack problem encountered in LEFM. However, the proposed framework is easily extended to multiaxial loading conditions. This modeling framework is demonstrated in detail via its application to multiaxial fatigue data for data taken from the literature
Interstate Vibronic Coupling Constants Between Electronic Excited States for Complex Molecules
In the construction of diabatic vibronic Hamiltonians for quantum dynamics in
the excited-state manifold of molecules, the coupling constants are often
extracted solely from information on the excited-state energies. Here, a new
protocol is applied to get access to the interstate vibronic coupling constants
at the time-dependent density functional theory level through the overlap
integrals between excited-state adiabatic auxiliary wavefunctions. We discuss
the advantages of such method and its potential for future applications to
address complex systems, in particular those where multiple electronic states
are energetically closely lying and interact. As examples, we apply the
protocol to the study of prototype rhenium carbonyl complexes
[Re(CO)(N,N)(L)] for which non-adiabatic quantum dynamics within the
linear vibronic coupling model and including spin-orbit coupling have been
reported recently.Comment: 36 pages, 7 figures, 4 table
The Welfare Effects of Involuntary Part-Time Work
Employed individuals in the U.S. are increasingly more likely to work part-time involuntarily than to be unemployed. Spells of involuntary part-time work are different from unemployment spells: a full-time worker who takes on a part-time job suffers an earnings loss while remaining employed, and is unlikely to receive income compensation from publicly-provided insurance programs.We analyze these differences through the lens of an incomplete-market, job-search model featuring unemployment risk alongside an additional risk of involuntary part-time employment.A calibration of the model consistent with U.S. institutions and labor-market dynamics shows that involuntary part-time work generates lower welfare losses relative to unemployment. This finding relies critically on the much higher probability to return to full-time employment from part-time work. We interpret it as a premium in access to full-time work faced by involuntary part-time
workers, and use our model to tabulate its value in consumption-equivalent units
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