605 research outputs found
Self-sustained current oscillations in the kinetic theory of semiconductor superlattices
We present the first numerical solutions of a kinetic theory description of
self-sustained current oscillations in n-doped semiconductor superlattices. The
governing equation is a single-miniband Boltzmann-Poisson transport equation
with a BGK (Bhatnagar-Gross-Krook) collision term. Appropriate boundary
conditions for the distribution function describe electron injection in the
contact regions. These conditions seamlessly become Ohm's law at the injecting
contact and the zero charge boundary condition at the receiving contact when
integrated over the wave vector. The time-dependent model is numerically solved
for the distribution function by using the deterministic Weighted Particle
Method. Numerical simulations are used to ascertain the convergence of the
method. The numerical results confirm the validity of the Chapman-Enskog
perturbation method used previously to derive generalized drift-diffusion
equations for high electric fields because they agree very well with numerical
solutions thereof.Comment: 26 pages, 16 figures, to appear in J. Comput. Phy
Alfonso A. CUADRÓN (coord.), Manual de Doctrina Social de la Iglesia, "Biblioteca de Autores Cristianos», Madrid 1993, 814 pp., 15 x 23,5. [RECENSIÓN]
lldefonso CAMACHO, Creyentes en la vida pública (Iniciación a la doctrina social de la Iglesia), Ed. San Pablo, Madrid 1995, 199 pp., 13, 5 x 21. [RECENSIÓN]
Dynamic regression model for hourly river level forecasting under risk situations: An application to the Ebro River
This work proposes a new statistical modelling approach to forecast the hourly river level at a gauging station, under potential flood risk situations and over a medium-term prediction horizon (around three days). For that aim we introduce a new model, the switching regression model with ARMA errors, which takes into account the serial correlation structure of the hourly level series, and the changing time delay between them. A whole modelling approach is developed, including a two-step estimation, which improves the medium-term prediction performance of the model, and uncertainty measures of the predictions. The proposed model not only provides predictions for longer periods than other statistical models, but also helps to understand the physics of the river, by characterizing the relationship between the river level in a gauging station and its influential factors. This approach is applied to forecast the Ebro River level at Zaragoza (Spain), using as input the series at Tudela. The approach has shown to be useful and the resulting model provides satisfactory hourly predictions, which can be fast and easily updated, together with their confidence intervals. The fitted model outperforms the predictions from other statistical and numerical models, specially in long prediction horizons
Acerca de las nacionalidades. Pasado, presente ¿y futuro? de un término escurridizo
Este trabajo analiza el concepto nacionalidades, presente en el artÃculo 2 de la Constitución Española, a la vez que lleva a cabo un recorrido por la historia del término y su presencia en la actualidad, planteándose asimismo su viabilidad futura.
This paper analyzes the concept nationalities, included in the Article 2 of the Spanish Constitution, going through the history of the term, its present and thinking about its viability in the future
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