587 research outputs found
Wage Dynamics: Reconciling Theory and Evidence
U.S. macroeconomic evidence shows a negative relation between the rate of change of wages and unemployment. In contrast, most theories of wage determination imply a negative relation between the level of wages and unemployment. In this paper, we ask whether one can reconcile the empirical evidence with theoretical wage relations. We reach three main conclusions. First, we derive the condition under which the two can indeed be reconciled. We show the constraints that such a condition imposes on the determinants of workers' reservation wages as well as the relative importance of workers' outside options as opposed to match specific productivity in wage determination. Second, in the light of this condition, we reinterpret the presence of an "error correction" term in macroeconomic wage relations for most European economies but not in the United States. Third, we show that whether this condition holds or not has important implications for the effects of a number of variables -- from real interest rates to oil prices to payroll taxes -- on the natural rate of unemployment.
Inference and Evaluation of the Multinomial Mixture Model for Text Clustering
In this article, we investigate the use of a probabilistic model for
unsupervised clustering in text collections. Unsupervised clustering has become
a basic module for many intelligent text processing applications, such as
information retrieval, text classification or information extraction. The model
considered in this contribution consists of a mixture of multinomial
distributions over the word counts, each component corresponding to a different
theme. We present and contrast various estimation procedures, which apply both
in supervised and unsupervised contexts. In supervised learning, this work
suggests a criterion for evaluating the posterior odds of new documents which
is more statistically sound than the "naive Bayes" approach. In an unsupervised
context, we propose measures to set up a systematic evaluation framework and
start with examining the Expectation-Maximization (EM) algorithm as the basic
tool for inference. We discuss the importance of initialization and the
influence of other features such as the smoothing strategy or the size of the
vocabulary, thereby illustrating the difficulties incurred by the high
dimensionality of the parameter space. We also propose a heuristic algorithm
based on iterative EM with vocabulary reduction to solve this problem. Using
the fact that the latent variables can be analytically integrated out, we
finally show that Gibbs sampling algorithm is tractable and compares favorably
to the basic expectation maximization approach
What We Know and Do Not Know About the Natural Rate of Unemployment
Over the past three decades, a large amount of research has attempted to identify the determinants of the natural rate of unemployment. It is this body of work we assess in this paper. We reach two main conclusions. First, there has been considerable theoretical progress over the past 30 years. A framework has emerged. We present it, and show how it can be used to think for example about the relation between technological progrss and unemployment. Second, empirical knowledge lags behind. Economists do not have a good quantitative understanding of the determinants of the natural rate, either across time or across countries. We look at two issues, the relation of wages to unemployment, and the risk of European unemployment.
Mineral Contamination from Cemetery Soils: Case Study of Zandfontein Cemetery, South Africa
The burial of coffins may pose an environmental and health hazard since the metals that are used in coffin-making may corrode or degrade into harmful toxins. These may leach into the surrounding soils and groundwater. Very little research has been conducted world-wide on the mineral contamination potential of cemeteries, and virtually none in South Africa. The aim of the study is to determine whether burial practices affect the mineral content of soils in cemeteries. This was done by comparing the mineral concentrations of soils within the Zandfontein Cemetery in Tshwane (Gauteng, South Africa) to those off-site as well as those in zones with high burial loads with those zones with fewer burials. Twenty three soil samples were collected from various sites on- and off-site and analyzed for 31 minerals using ICP-AES. It was found that mineral concentrations of soils within the Zandfontein Cemetery were considerably higher than those off-site. Soil samples in multiple burials blocks also have elevated metal concentrations. These excess metals are probably of anthropogenic origin associated with burial practices and could pose an environmental and human health hazard. Strict monitoring of water quality in boreholes in the vicinity of the cemetery is recommended
A matrix model for the topological string II: The spectral curve and mirror geometry
In a previous paper, we presented a matrix model reproducing the topological
string partition function on an arbitrary given toric Calabi-Yau manifold.
Here, we study the spectral curve of our matrix model and thus derive, upon
imposing certain minimality assumptions on the spectral curve, the large volume
limit of the BKMP "remodeling the B-model" conjecture, the claim that
Gromov-Witten invariants of any toric Calabi-Yau 3-fold coincide with the
spectral invariants of its mirror curve.Comment: 1+37 page
Electric-Field-Induced Mott Insulating States in Organic Field-Effect Transistors
We consider the possibility that the electrons injected into organic
field-effect transistors are strongly correlated. A single layer of acenes can
be modelled by a Hubbard Hamiltonian similar to that used for the
kappa-(BEDT-TTF)(2)X family of organic superconductors. The injected electrons
do not necessarily undergo a transition to a Mott insulator state as they would
in bulk crystals when the system is half-filled. We calculate the fillings
needed for obtaining insulating states in the framework of the slave-boson
theory and in the limit of large Hubbard repulsion, U. We also suggest that
these Mott states are unstable above some critical interlayer coupling or
long-range Coulomb interaction.Comment: 9 pages, 7 figure
Perspectives in Fundamental Physics in Space
We discuss the fundamental principles underlying the current physical
theories and the prospects of further improving their knowledge through
experiments in space.Comment: Gravitational waves, gravitomagnetism, Equivalence Principle,
Antimatter, Pioneer Anomaly, Lorentz invariance. To appear in IAA - Acta
Astronautica Journal (2006
Advances in Human-Protein Interaction - Interactive and Immersive Molecular Simulations
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