12,833 research outputs found
Pair Production in a Time Dependent Magnetic Field
The production of electron-positron pairs in a time-dependent magnetic field
is estimated in the hypotheses that the magnetic field is uniform over large
distances with respect to the pair localization and it is so strong that the
spacing of the Landau levels is larger than the rest mass of the particles.
This calculation is presented since it has been suggested that extremely
intense and varying magnetic fields may be found around some astrophysical
objects.Comment: 11 pages, Plain TeX, no figures. Submitted to Modern Physics Letter
A distributional approach to fractional Sobolev spaces and fractional variation: existence of blow-up
We introduce the new space of functions with
bounded fractional variation in of order via
a new distributional approach exploiting suitable notions of fractional
gradient and fractional divergence already existing in the literature. In
analogy with the classical theory, we give a new notion of set of
(locally) finite fractional Caccioppoli -perimeter and we define its
fractional reduced boundary . We are able to show that
continuously and,
similarly, that sets with (locally) finite standard fractional
-perimeter have (locally) finite fractional Caccioppoli
-perimeter, so that our theory provides a natural extension of the
known fractional framework. Our main result partially extends De Giorgi's
Blow-up Theorem to sets of locally finite fractional Caccioppoli
-perimeter, proving existence of blow-ups and giving a first
characterisation of these (possibly non-unique) limit sets.Comment: 46 page
Endogenous space in the Net era
Libre Software communities are among the most interesting and advanced socio-economic laboratories on the Net. In terms of directions of Regional Science research, this paper addresses a simple question: âIs the socio-economics of digital nets out of scope for Regional Science, or might the latter expand to a cybergeography of digitally enhanced territories ?â As for most simple questions, answers are neither so obvious nor easy. The authors start drafting one in a positive sense, focussing upon a file rouge running across the paper: endogenous spaces woven by socio-economic processes. The drafted answer declines on an Evolutionary Location Theory formulation, together with two computational modelling views. Keywords: Complex networks, Computational modelling, Economics of Internet, Endogenous spaces, Evolutionary location theory, Free or Libre Software, Path dependence, Positionality.
Measuring the equilibrium real interest rate
The equilibrium real interest rate represents the real rate of return required to keep the economyâs output equal to potential output. This article discusses how to measure the equilibrium real interest rate, using an empirical structural model of the economy.Interest rates ; Monetary policy
Latent Markov model for longitudinal binary data: An application to the performance evaluation of nursing homes
Performance evaluation of nursing homes is usually accomplished by the
repeated administration of questionnaires aimed at measuring the health status
of the patients during their period of residence in the nursing home. We
illustrate how a latent Markov model with covariates may effectively be used
for the analysis of data collected in this way. This model relies on a not
directly observable Markov process, whose states represent different levels of
the health status. For the maximum likelihood estimation of the model we apply
an EM algorithm implemented by means of certain recursions taken from the
literature on hidden Markov chains. Of particular interest is the estimation of
the effect of each nursing home on the probability of transition between the
latent states. We show how the estimates of these effects may be used to
construct a set of scores which allows us to rank these facilities in terms of
their efficacy in taking care of the health conditions of their patients. The
method is used within an application based on data concerning a set of nursing
homes located in the Region of Umbria, Italy, which were followed for the
period 2003--2005.Comment: Published in at http://dx.doi.org/10.1214/08-AOAS230 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
The Time Varying Volatility of Macroeconomic Fluctuations
In this paper we investigate the sources of the important shifts in the volatility of U.S. macroeconomic variables in the postwar period. To this end, we propose the estimation of DSGE models allowing for time variation in the volatility of the structural innovations. We apply our estimation strategy to a large-scale model of the business cycle and find that investment specific technology shocks account for most of the sharp decline in volatility of the last two decades.
Item selection by Latent Class-based methods
The evaluation of nursing homes is usually based on the administration of
questionnaires made of a large number of polytomous items. In such a context,
the Latent Class (LC) model represents a useful tool for clustering subjects in
homogenous groups corresponding to different degrees of impairment of the
health conditions. It is known that the performance of model-based clustering
and the accuracy of the choice of the number of latent classes may be affected
by the presence of irrelevant or noise variables. In this paper, we show the
application of an item selection algorithm to real data collected within a
project, named ULISSE, on the quality-of-life of elderly patients hosted in
italian nursing homes. This algorithm, which is closely related to that
proposed by Dean and Raftery in 2010, is aimed at finding the subset of items
which provides the best clustering according to the Bayesian Information
Criterion. At the same time, it allows us to select the optimal number of
latent classes. Given the complexity of the ULISSE study, we perform a
validation of the results by means of a sensitivity analysis to different
specifications of the initial subset of items and of a resampling procedure
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