El absentismo escolar : constantes , problemática y proyecto

Se parte del concepto de absentismo escolar, de la legislación existente respecto al mismo y de las responsabilidades que atañen a la familia y a los poderes públicos. Se describen las circunstancias personales y sociales de los niños que faltan a la escuela y se propone un programa de actuación para combatir el absentismo, y que comprende aspectos tanto individuales como sociales.On part du concept de l'absentéisme scolaire, de la legislation existante dans ce cas précis et des responsabilités qui incombent a la famille et aux pouvoirs publics. On décrit les circonstances personnelles et sociales des enfants qui manquent l'ecole et on propose un programme actif pour combattre l'absentéisme, et qui comprend des aspects aussi individuels que sociaux.The author begins with the concept of school absenteeism, existing legislation on the matter, and the responsabilities of family and public powers. The personal and social circumstances of truant children are described, and a programme of action to combat absenteeism in proposed talking both individual and social aspects into account

Hot electron transport in suspended multilayer graphene

We study hot electron transport in short-channel suspended multilayer graphene devices created by a distinct experimental approach. For devices with semi-transparent contact barriers, a dip of differential conductance (dI/dV) has been observed at source drain bias Vd = 0, along with anomalies at higher Vd likely induced by optical phonon scattering. For devices with low contact barriers, only the dI/dV dip at Vd = 0 is observed, and we find a well-fit logarithmic dependence of dI/dV on both the bias Vd and the temperature T. The logarithmic Vd dependence is explained with the hot electron effect and the logarithmic T dependence could be attributed to the weak-localization in two-dimensions

The ages of very cool hydrogen-rich white dwarfs

The evolution of white dwarfs is essentially a cooling process that depends primarily on the energy stored in their degenerate cores and on the transparency of their envelopes. In this paper we compute accurate cooling sequences for carbon-oxygen white dwarfs with hydrogen dominated atmospheres for the full range of masses of interest. For this purpose we use the most accurate available physical inputs for both the equation of state and opacities of the envelope and for the thermodynamic quantities of the degenerate core. We also investigate the role of the latent heat in the computed cooling sequences. We present separately cooling sequences in which the effects of phase separation of the carbon-oxygen binary mixture upon crystallization have been neglected, and the delay introduced in the cooling times when this mechanism is properly taken into account, in order to compare our results with other published cooling sequences which do not include a treatment of this phenomenon. We find that the cooling ages of very cool white dwarfs with pure hydrogen atmospheres have been systematically underestimated by roughly 1.5 Gyr at log(L/Lo)=-4.5 for an otherwise typical 0.6 Mo white dwarf, when phase separation is neglected. If phase separation of the binary mixture is included then the cooling ages are further increased by roughly 10%. Cooling tracks and cooling isochrones in several color-magnitude diagrams are presented as well.Comment: 8 Pages; ApJ, accepted for publicatio

Authentication in remote controls

Este proyecto consiste en el desarrollo de sistemas de autenticación, usando lenguaje C, en un canal ideal (sin errores). Estos sistemas de autenticación usan LFSR y un algoritmo de multiplicación rápida en curvas elípticas sobre campos de Galois binarios, sin pre cálculos. Sobre estos sistemas de autenticación fue implementado un algoritmo de verificación de identidad digital usando curvas elípticas. Estos sistemas de autenticación son diferentes a anteriores desarrollados porque su aplicación es ideal para aplicaciones de bajo consumo de energía y bajo tiempo de computo.This project consists of developing, using C language, different authentication schemes in an ideal Cryptographic channel, using Clock Controlled LFSRs and Fast Multiplication in Elliptic Curves over GF(2^m) without precomputation working in an inversionfree coordinate system. Over these schemes are implemented an Elliptic Curve Digital Signature Algorithm different from the classic signature scheme. They are different because of their applications on restricted resources like energy consumption, processing capacity, not facilities of dual communication channel and calculation time.Ingeniero (a) ElectrónicoPregrad

Exploring Halo Substructure with Giant Stars: Spectroscopy of Stars in the Galactic Anticenter Stellar Structure

To determine the nature of the recently discovered, ring-like stellar structure at the Galactic anticenter, we have collected spectra of a set of presumed constituent M giants selected from the 2MASS point source catalog. Radial velocities have been obtained for stars spanning ~100 degrees, exhibiting a trend in velocity with Galactic longitude and an estimated dispersion of 20 +/- 4 km/sec. A mean metallicity [Fe/H] = -0.4 +/- 0.3 measured for these stars combines with previous evidence from the literature to suggest a population with a significant metallicity spread. In addition, a curious alignment of at least four globular clusters of lower mean metallicity is noted to be spatially and kinematically consistent with this stellar distribution. We interpret the M giant sample position and velocity variation with Galactic longitude as suggestive of a satellite galaxy currently undergoing tidal disruption in a non-circular, prograde orbit about the Milky Way.Comment: (1) University of Virginia, 4 pages, 3 figures, accepted for publication in The Astrophysical Journal Letter