7,289 research outputs found
Modelling aggregation on the large scale and regularity on the small scale in spatial point pattern datasets
We consider a dependent thinning of a regular point process with the aim of
obtaining aggregation on the large scale and regularity on the small scale in
the resulting target point process of retained points. Various parametric
models for the underlying processes are suggested and the properties of the
target point process are studied. Simulation and inference procedures are
discussed when a realization of the target point process is observed, depending
on whether the thinned points are observed or not. The paper extends previous
work by Dietrich Stoyan on interrupted point processes
Relativity accommodates superluminal mean velocities
Contrary to a widespread belief, measures of velocity can yield a value
larger than , the instantaneous light speed in vacuum, without contradicting
Einstein's relativity. Nevertheless, the effect turns out to be too small to
explain the recently claimed superluminal velocity by the OPERA collaboration.
Several other general relativistic effects acting on the OPERA neutrinos are
also analyzed. All of them are unable to explain the OPERA result.Comment: 5 pages; Latex source, 2 eps figures (expanded discussion, a few
typos corrected, some refs. added
Energy and angular momentum of general 4-dimensional stationary axi-symmetric spacetime in teleparallel geometry
We derive an exact general axi-symmetric solution of the coupled
gravitational and electromagnetic fields in the tetrad theory of gravitation.
The solution is characterized by four parameters (mass), (charge),
(rotation) and (NUT). We then, calculate the total exterior energy using
the energy-momentum complex given by M{\o}ller in the framework of
Weitzenbck geometry. We show that the energy contained in a sphere is
shared by its interior as well as exterior. We also calculate the components of
the spatial momentum to evaluate the angular momentum distribution. We show
that the only non-vanishing components of the angular momentum is in the Z
direction.Comment: Latex. Will appear in IJMP
Foreign Firms, Domestic Wages
Many papers have documented a wage premium in foreign-owned and large firms. However, there is very little formal theory in the literature and empirical analyses are typically not based on hypotheses which are rigorously derived from theory. This paper contributes to the theory-empirics gap by developing a model that allows for two âpureâ explanations for the wage premium. The first is a heterogenous-worker explanation along the lines of Yeaple (2005), where firms that select more scaleintensive technologies select ex-ante more productive workers. In this case, the wage premium is a pure selection phenomenon. The second explanation builds on the heterogeneous-firm model of Melitz (2003) combined with on-the-job learning as in Markusen (2001). Productivity differences between firms are internalized by ex-ante homogeneous workers, so the wage premium is a pure learning phenomenon due to ex-post higher productivity in foreign firms. Our model yields a number of precise empirical hypotheses. When these predictions are tested on Danish matched employer-employee data, we find that both explanations play a role in explaining the observed wage premium. Specifically, the foreign- and large-firm premiums explained by selection are in the neighborhood of 30-65% of the total premium, with the remainder consistent with learning. There is also considerable support for a number of other predictions specific to the worker-learning explanation.
Foreign Firms, Domestic Wages
Foreign-owned firms are often hypothesized to generate productivity "spillovers" to the host country, but both theoretical micro-foundations and empirical evidence for this are limited. We develop a heterogeneous-firm model in which ex-ante identical workers learn from their employers in proportion to the firm?s productivity. Foreign-owned firms have, on average, higher productivity in equilibrium due to entry costs, which means that low-productivity foreign firms cannot enter. Foreign firms have higher wage growth and, with some exceptions, pay higher average wages, but not when compared to similarly large domestic firms. The empirical implications of the model are tested on matched employer-employee data from Denmark. Consistent with the theory, we find considerable evidence of higher wages and wage growth in large and/or foreign-owned firms. These effects survive controlling for individual characteristics, but, as expected, are reduced significantly when controlling for unobservable firm heterogeneity. Furthermore, acquired skills in foreign-owned and large firms appear to be transferable to both subsequent wage work and self-employment.
Scaling Laws in High-Energy Inverse Compton Scattering. II. Effect of Bulk Motions
We study the inverse Compton scattering of the CMB photons off high-energy
nonthermal electrons. We extend the formalism obtained by the previous paper to
the case where the electrons have non-zero bulk motions with respect to the CMB
frame. Assuming the power-law electron distribution, we find the same scaling
law for the probability distribution function P_{1,K}(s) as P_{1}(s) which
corresponds to the zero bulk motions, where the peak height and peak position
depend only on the power-index parameter. We solved the rate equation
analytically. It is found that the spectral intensity function also has the
same scaling law. The effect of the bulk motions to the spectral intensity
function is found to be small. The present study will be applicable to the
analysis of the X-ray and gamma-ray emission models from various astrophysical
objects with non-zero bulk motions such as radio galaxies and astrophysical
jets.Comment: 10 pages, 3 figures, accepted version by Physical Review
Kerr-Newman Solution and Energy in Teleparallel Equivalent of Einstein Theory
An exact charged axially symmetric solution of the coupled gravitational and
electromagnetic fields in the teleparallel equivalent of Einstein theory is
derived. It is characterized by three parameters ``the gravitational mass
, the charge parameter and the rotation parameter " and its
associated metric gives Kerr-Newman spacetime. The parallel vector field and
the electromagnetic vector potential are axially symmetric. We then, calculate
the total energy using the gravitational energy-momentum. The energy is found
to be shared by its interior as well as exterior. Switching off the charge
parameter we find that no energy is shared by the exterior of the Kerr-Newman
black hole.Comment: 11 pages, Latex. Will appear in Mod. Phys. Lett.
Do Foreign Experts Increase the Productivity of Domestic Firms?
While most countries welcome (and some even subsidise) high-skilled immigrants, there is very limited evidence of their importance for domestic firms. To guide our empirical analysis, we first set up a simple theoretical model to show how foreign experts may impact on the productivity and wages of domestic firms. Using matched worker-firm data from Denmark and a difference-indifferences matching approach, we then find that firms that hire foreign experts â defined as employees eligible for reduced taxation under the Danish "Tax scheme for foreign researchers and key employees" â both become more productive (pay higher wages) and increase their exports of goods and services.foreign experts, export, immigrants, productivity, difference-in-differences matching
On the Clock Paradox in the case of circular motion of the moving clock
In this paper we deal analytically with a version of the so called clock
paradox in which the moving clock performs a circular motion of constant
radius. The rest clock is denoted as (1), the rotating clock is (2), the
inertial frame in which (1) is at rest and (2) moves is I and, finally, the
accelerated frame in which (2) is at rest and (1) rotates is A. By using the
General Theory of Relativity in order to describe the motion of (1) as seen in
A we will show the following features. I) A differential aging between (1) and
(2) occurs at their reunion and it has an absolute character, i.e. the proper
time interval measured by a given clock is the same both in I and in A. II)
From a quantitative point of view, the magnitude of the differential aging
between (1) and (2) does depend on the kind of rotational motion performed by
A. Indeed, if it is uniform there is no any tangential force in the direction
of motion of (2) but only normal to it. In this case, the proper time interval
reckoned by (2) does depend only on its constant velocity v=romega. On the
contrary, if the rotational motion is uniformly accelerated, i.e. a constant
force acts tangentially along the direction of motion, the proper time
intervals on the angular acceleration alpha. III) Finally, in
regard to the sign of the aging, the moving clock (2) measures always a
interval of proper time with respect to (1).Comment: LaTex2e, 9 pages, no figures, no tables. It is the follow-on of the
paper physics/040503
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