12,486 research outputs found
Spectral density method in quantum nonextensive thermostatistics and magnetic systems with long-range interactions
Motived by the necessity of explicit and reliable calculations, as a valid
contribution to clarify the effectiveness and, possibly, the limits of the
Tsallis thermostatistics, we formulate the Two-Time Green Functions Method in
nonextensive quantum statistical mechanics within the optimal Lagrange
multiplier framework, focusing on the basic ingredients of the related Spectral
Density Method. Besides, to show how the SDM works we have performed, to the
lowest order of approximation, explicit calculations of the low-temperature
properties for a quantum -dimensional spin-1/2 Heisenberg ferromagnet with
long-range interactions decaying as ( is the distance between
spins in the lattice)Comment: Contribution to Next-SigmaPhi conference in Kolymbari, Crete, Greece,
August 13-18, 2005, 9 page
The Classical Spectral Density Method at Work: The Heisenberg Ferromagnet
In this article we review a less known unperturbative and powerful many-body
method in the framework of classical statistical mechanics and then we show how
it works by means of explicit calculations for a nontrivial classical model.
The formalism of two-time Green functions in classical statistical mechanics is
presented in a form parallel to the well known quantum counterpart, focusing on
the spectral properties which involve the important concept of spectral
density. Furthermore, the general ingredients of the classical spectral density
method (CSDM) are presented with insights for systematic nonperturbative
approximations to study conveniently the macroscopic properties of a wide
variety of classical many-body systems also involving phase transitions. The
method is implemented by means of key ideas for exploring the spectrum of
elementary excitations and the damping effects within a unified formalism.
Then, the effectiveness of the CSDM is tested with explicit calculations for
the classical -dimensional spin- Heisenberg ferromagnetic model with
long-range exchange interactions decaying as () with distance
between spins and in the presence of an external magnetic field. The analysis
of the thermodynamic and critical properties, performed by means of the CSDM to
the lowest order of approximation, shows clearly that nontrivial results can be
obtained in a relatively simple manner already to this lower stage. The basic
spectral density equations for the next higher order level are also presented
and the damping of elementary spin excitations in the low temperature regime is
studied. The results appear in reasonable agreement with available exact ones
and Monte Carlo simulations and this supports the CSDM as a promising method of
investigation in classical many-body theory.Comment: Latex, 58 pages, 12 figure
Two-time Green's functions and spectral density method in nonextensive quantum statistical mechanics
We extend the formalism of the thermodynamic two-time Green's functions to
nonextensive quantum statistical mechanics. Working in the optimal Lagrangian
multipliers representation, the -spectral properties and the methods for a
direct calculation of the two-time % -Green's functions and the related
-spectral density ( measures the nonextensivity degree) for two generic
operators are presented in strict analogy with the extensive ()
counterpart. Some emphasis is devoted to the nonextensive version of the less
known spectral density method whose effectiveness in exploring equilibrium and
transport properties of a wide variety of systems has been well established in
conventional classical and quantum many-body physics. To check how both the
equations of motion and the spectral density methods work to study the
-induced nonextensivity effects in nontrivial many-body problems, we focus
on the equilibrium properties of a second-quantized model for a high-density
Bose gas with strong attraction between particles for which exact results exist
in extensive conditions. Remarkably, the contributions to several thermodynamic
quantities of the -induced nonextensivity close to the extensive regime are
explicitly calculated in the low-temperature regime by overcoming the
calculation of the grand-partition function.Comment: 48 pages, no figure
Public health training in Europe. Development of European masters degrees in public health.
BACKGROUND: Changing political and economic relations in Europe mean that there are new challenges for public health and public health training. There have been several attempts to develop training at the master's level in public health which is focused on meeting the new needs. These have failed due to being too inflexible to allow participation by schools of public health. METHODS: A project funded by the European Union involving public health trainers has developed a new approach which allows participating schools to retain their national differences and work within local rules and traditions, but which aims to introduce the European dimension into public health training. This paper reports the conclusions of this project. CONCLUSIONS: A network of schools wishing to develop European Master's degrees is being established and other schools offering good quality programmes will be able to join
Prices and Supply Disruptions during Natural Disasters
We study the daily behavior of supermarket prices and product availability following two recent natural disasters: the 2010 earthquake in Chile and the 2011 earthquake in Japan. In both cases there was an immediate and persistent effect on product availability. The number of goods available for sale fell 32 percent in Chile and 17 percent in Japan from the day of the disaster to its lowest point, which occurred 61 and 18 days after the earthquakes, respectively. Product availability recovered slowly, and a significant share of goods remained out of stock after six months. By contrast, prices were relatively stable and did not increase for months after the earthquakes, even for goods that were experiencing severe stockouts. These trends are present at all levels of aggregation, but appear strongly in non-perishable goods and emergency products. Our findings shed light into the determinants of sticky prices in conditions where traditional adjustment costs are less important. In particular, we look at the frequency and magnitudes of price changes in both countries and find that the results in Chile are consistent with pricing models where retailers have fear of “customer anger”. In Japan, by contrast, the evidence suggests a bigger role for supply disruptions that restricted the ability of retailers to re-stock goods after the earthquake
The Theory Behind TheoryMine
Abstract. We describe the technology behind the TheoryMine novelty gift company, which sells the rights to name novel mathematical theorems. A tower of four computer systems is used to generate recursive theories, then to speculate conjectures in those theories and then to prove these conjectures. All stages of the process are entirely automatic. The process guarantees large numbers of sound, novel theorems of some intrinsic merit.
Online and official price indexes: Measuring Argentina's inflation
Prices collected from online retailers can be used to construct daily price indexes that complement official statistics. This paper studies their ability to match official inflation estimates in five Latin American countries, with a focus on Argentina, where official statistics have been heavily criticized in recent years. The data were collected between October 2007 and March 2011 from the largest supermarket in each country. In Brazil, Chile, Colombia, and Venezuela, online price indexes approximate both the level and main dynamics of official inflation. By contrast, Argentina's online inflation rate is nearly three times higher than the official estimate
Sudden Stops: Determinants and Output Effects in the First Era of Globalization, 1880-1913
Using a sample of 20 emerging countries from 1880 to 1913, we study the determinants and output effects of sudden stops in capital inflows during an era of intensified globalization. We find that higher levels of original sin (hard currency debt to total debt) and large current account deficits associated with reliance on foreign capital greatly increased the likelihood of experiencing a sudden stop. Trade openness and stronger commitment to the gold standard had the opposite effect. These results are robust for many sudden stop definitions used in the literature. Finally, we use a treatment effects model to show that after controlling for endogeneity sudden stops have a strong negative association with growth in per capita output. We also show that banking, currency and debt crises that were preceded by a sudden stop have much greater negative relation with growth than in the absence of a sudden stop.
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