On a size-structured two-phase population model with infinite states-at-birth

In this work we introduce and analyze a linear size-structured population model with infinite states-at-birth. We model the dynamics of a population in which individuals have two distinct life-stages: an “active” phase when individuals grow, reproduce and die and a second “resting” phase when individuals only grow. Transition between these two phases depends on individuals’ size. First we show that the problem is governed by a positive quasicontractive semigroup on the biologically relevant state space. Then we investigate, in the framework of the spectral theory of linear operators, the asymptotic behavior of solutions of the model. We prove that the associated semigroup has, under biologically plausible assumptions, the property of asynchronous exponential growth.

Random assignment of intervention points in two phase single-case designs: data-division-specific distributions

The present study explores the statistical properties of a randomization test based on the random assignment of the intervention point in a two-phase (AB) single-case design. The focus is on randomization distributions constructed with the values of the test statistic for all possible random assignments and used to obtain p-values. The shape of those distributions is investigated for each specific data division defined by the moment in which the intervention is introduced. Another aim of the study consisted in testing the detection of inexistent effects (i.e., production of false alarms) in autocorrelated data series, in which the assumption of exchangeability between observations may be untenable. In this way, it was possible to compare nominal and empirical Type I error rates in order to obtain evidence on the statistical validity of the randomization test for each individual data division. The results suggest that when either of the two phases has considerably less measurement times, Type I errors may be too probable and, hence, the decision making process to be carried out by applied researchers may be jeopardized.

Spreading due to heterogeneity in two-phase flow

Postprint (published version)

Contratación de la mano de obra y desempeño en los servicios intensivos en conocimiento: una visión del capital intelectual

Objeto: El desempeño de las actividades de servicios de alto valor añadido ofrecidospor las empresas manufactureras, de la misma forma que el de los servicios intensivosen conocimiento, puede verse afectado por las formas de contratación de la mano deobra utilizadas en ellas. Se estudia el impacto del uso de trabajo contingente (temporal y autónomo) sobre la productividad del trabajo en las empresas de servicios intensivos en conocimiento. Para desarrollar las hipótesis, se tiene en cuenta el impacto potencial del trabajo contingente sobre el capital intangible de la empresa, así como los resultados de la literatura empírica.Diseño/metodología: Se analizan los datos de una muestra de 279 empresas de servicios intensivos en conocimiento localizadas en Cataluña, mediante dos modelos de regresión lineal.Aportaciones y resultados: Los resultados muestran que el empleo de formas de trabajo contingentes, como el trabajo temporal y los trabajadores autónomos, tiene un impacto negativo en la productividad del trabajo. No existe, en cambio, una relación cuadrática entre estas variables. Limitaciones: La muestra utilizada procede exclusivamente de Cataluña (España), noes perfectamente extrapolable al conjunto de empresas de servicios intensivos enconocimiento y se obtuvo en la fase alcista del ciclo económico. El diseño del estudio estransversal. La clasificación de las empresas como intensivas en conocimiento esdicotómica, en función del sector al que pertenecen. Implicaciones prácticas:Las decisiones sobre la contratación de mano de obra en actividades de servicios de alto valor añadido deberían minimizar las formas de trabajocontingentes, si quieren mejorar su productividad.Implicaciones sociales: Un modelo productivo que pretenda potenciar los servicios de mayor valor añadido no puede descansar sobre la base de un mercado laboral en el que las empresas utilizan una elevada proporción de trabajo contingente en su mano de obra. Valor añadido: Este artículo ofrece nuevos datos para a la escasa literatura que relaciona el uso de trabajo contingente con la productividad en el sector de los servicios intensivos en conocimiento. La creciente importancia de los servicios de alto valor añadido, tanto en empresas de servicios como manufactureras, y el interés por conocer los determinantes de su productividad justifican la necesidad de realizar estudios como el que se presenta.

Phase behavior and rheological analysis of reverse liquid crystals and W/I2, W/H2 gel emulsions using an amphiphilic block copolymer.

This article reports the phase behavior determi- nation of a system forming reverse liquid crystals and the formation of novel disperse systems in the two-phase region. The studied system is formed by water, cyclohexane, and Pluronic L-121, an amphiphilic block copolymer considered of special interest due to its aggregation and structural proper- ties. This system forms reverse cubic (I2) and reverse hexagonal (H2) phases at high polymer concentrations. These reverse phases are of particular interest since in the two-phase region, stable high internal phase reverse emulsions can be formed. The characterization of the I2 and H2 phases and of the derived gel emulsions was performed with small-angle X-ray scattering (SAXS) and rheometry, and the influence of temperature and water content was studied. TheH2 phase experimented a thermal transition to an I2 phase when temperature was increased, which presented an Fd3m structure. All samples showed a strong shear thinning behavior from low shear rates. The elasticmodulus (G0) in the I2 phase was around 1 order of magnitude higher than in theH2 phase. G0 was predominantly higher than the viscousmodulus (G00). In the gel emulsions,G0 was nearly frequency-independent, indicating their gel type nature. Contrarily to water-in-oil (W/O) normal emulsions, in W/I2 and W/H2 gel emulsions, G0, the complex viscosity (|η*|), and the yield stress (τ0) decreased with increasing water content, since the highly viscous microstructure of the con- tinuous phase was responsible for the high viscosity and elastic behavior of the emulsions, instead of the volumefraction of dispersed phase and droplet size. A rheological analysis, in which the cooperative flow theory, the soft glass rheology model, and the slip plane model were analyzed and compared, was performed to obtain one single model that could describe the non-Maxwellian behavior of both reverse phases and highly concentrated emulsions and to characterize their microstructure with the rheological properties.

Stability and asymptotic analysis of a fluid-particle interaction model

We are interested in coupled microscopic/macroscopic models describing the evolution of particles dispersed in a fluid. The system consists in a Vlasov-Fokker-Planck equation to describe the microscopic motion of the particles coupled to the Euler equations for a compressible fluid. We investigate dissipative quantities, equilibria and their stability properties and the role of external forces. We also study some asymptotic problems, their equilibria and stability and the derivation of macroscopic two-phase models.

Application of standard and refined heat balance integral methods to one-dimensional Stefan problems

The work in this paper concerns the study of conventional and refined heat balance integral methods for a number of phase change problems. These include standard test problems, both with one and two phase changes, which have exact solutions to enable us to test the accuracy of the approximate solutions. We also consider situations where no analytical solution is available and compare these to numerical solutions. It is popular to use a quadratic profile as an approximation of the temperature, but we show that a cubic profile, seldom considered in the literature, is far more accurate in most circumstances. In addition, the refined integral method can give greater improvement still and we develop a variation on this method which turns out to be optimal in some cases. We assess which integral method is better for various problems, showing that it is largely dependent on the specified boundary conditions.

Application for fault location in electrical power distribution systems

Fault location has been studied deeply for transmission lines due to its importance in power systems. Nowadays the problem of fault location on distribution systems is receiving special attention mainly because of the power quality regulations. In this context, this paper presents an application software developed in Matlabtrade that automatically calculates the location of a fault in a distribution power system, starting from voltages and currents measured at the line terminal and the model of the distribution power system data. The application is based on a N-ary tree structure, which is suitable to be used in this application due to the highly branched and the non- homogeneity nature of the distribution systems, and has been developed for single-phase, two-phase, two-phase-to-ground, and three-phase faults. The implemented application is tested by using fault data in a real electrical distribution power system

Critical behavior of a system with orientational and positional degrees of freedom: A Monte Carlo simulation study

The critical behavior of a system constituted by molecules with a preferred symmetry axis is studied by means of a Monte Carlo simulation of a simplified two-dimensional model. The system exhibits two phase transitions, associated with the vanishing of the positional order of the center of mass of the molecules and with the orientational order of the symmetry axis. The evolution of the order parameters and the specific heat is also studied. The transition associated with the positional degrees of freedom is found to change from a second-order to a first-order behavior when the two phase transitions are close enough, due to the coupling with the orientational degrees of freedom. This fact is qualitatively compared with similar results found in pure liquid crystals and liquid-crystal mixtures.

Atomic structure of the nanocrystalline Si particles appearing in nanostructured Si thin films produced in low-temperature radiofrequency plasmas

Nanostructured Si thin films, also referred as polymorphous, were grown by plasma-enhanced chemical vapor deposition. The term "polymorphous" is used to define silicon material that consists of a two-phase mixture of amorphous and ordered Si. The plasma conditions were set to obtain Si thin films from the simultaneous deposition of radical and ordered nanoparticles. Here, a careful analysis by electron transmission microscopy and electron diffraction is reported with the aim to clarify the specific atomic structure of the nanocrystalline particles embedded in the films. Whatever the plasma conditions, the electron diffraction images always revealed the existence of a well-defined crystalline structure different from the diamondlike structure of Si. The formation of nanocrystallinelike films at low temperature is discussed. A Si face-cubic-centered structure is demonstrated here in nanocrystalline particles produced in low-pressure silane plasma at room temperature.

Influence of the dipolar interactions in the magnetization reversal asymmetry of hard-soft magnetic ribbons

Partial crystallization of the metallic glass Co66Si16B12Fe4Mo2 was performed by annealing at temperatures between 500 and 540°C for 10-20 min, resulting in crystallite volume fractions of (0.7-5)×10¿3 and sizes of 50-100 nm. This two-phase alloy presents a remarkable feature: a hysteresis loop shift that can be tailored by simply premagnetizing the sample in the adequate magnetic field. Shifts as large as five times the coercive field have been obtained which make them interesting for application as magnetic cores in dc pulse transformers. The asymetrical magnetic reversal is explained in terms of the magnetic dipolar field interaction and the observed hysteresis loops have been satisfactorily simulated by a modification of Stoner-Wohlfarth¿s model of coherent rotations.

Precursor phenomena in frustrated systems

To understand the origin of the dynamical transition, between high-temperature exponential relaxation and low-temperature nonexponential relaxation, that occurs well above the static transition in glassy systems, a frustrated spin model, with and without disorder, is considered. The model has two phase transitions, the lower being a standard spin glass transition (in the presence of disorder) or fully frustrated Ising (in the absence of disorder), and the higher being a Potts transition. Monte Carlo results clarify that in the model with (or without) disorder the precursor phenomena are related to the Griffiths (or Potts) transition. The Griffiths transition is a vanishing transition which occurs above the Potts transition and is present only when disorder is present, while the Potts transition which signals the effect due to frustration is always present. These results suggest that precursor phenomena in frustrated systems are due either to disorder and/or to frustration, giving a consistent interpretation also for the limiting cases of Ising spin glass and of Ising fully frustrated model, where also the Potts transition is vanishing. This interpretation could play a relevant role in glassy systems beyond the spin systems case.

Prevalencia de la discapacidad en España por comunidades autónomas: el papel de los factores individuales y del entorno geográfico en su variabilidad

Fundamento: La prevalencia de discapacidad en la población general presenta una gran variabilidad geográfica, de manera que identificar aquellos factores que pudieran explicarla será importante para la planificación de políticas sociales. En este trabajo se analiza la variabilidad de la discapacidad por comunidades autónomas desde una doble vertiente, los factores individuales y del entorno. Métodos: Los datos proceden principalmente de la Encuesta de Discapacidad, Deficiencias y Estado de Salud de 1999 y del Inebase, ambas del Instituto Nacional de Estadística (INE). Se calculó la prevalencia de discapacidad simple y ajustada por edad de las CCAA. Se analizan los factores individuales asociados a la discapacidad mediante una regresión logística y los factores individuales y de la comunidad autónoma conjuntamente con una regresión logística de dos niveles. Resultados: La prevalencia de discapacidad muestra una diferencia máxima de 5,75 puntos entre las comunidades autónomas. En la regresión logística la comunidad de residencia fue estadísticamente significativa (OR: 3,35 en la de mayor prevalencia respecto a la de menor) junto con otras variables individuales: edad (OR de 40-64= 1,78 OR de 65-79= 1,87 y OR de >79= 3,34), sexo (OR mujer= 0,66), situación laboral (OR sin trabajo=2,25 OR amas casa/estudiante=1,39 y OR otros=2,03), estado de salud (OR regular= 1,69 OR malo/muy malo= 2,05) y enfermedades crónicas (OR 1-3=1,56 OR4-6=1,82 OR>6=2,59). En la regresión de dos niveles las variables individuales explican poca varianza (s=0,261) y ninguna de las variables relativas a las CCAA mejora el modelo. Conclusiones: Las características individuales no explican suficientemente la variabilidad de la discapacidad entre CCAA y no se han identificado variables del entorno que sean significativas.

Estimating 750 years of temperature variations and uncertainties in the Pyrenees by tree-ring reconstructions and climate simulations.

Past temperature variations are usually inferred from proxy data or estimated using general circulation models. Comparisons between climate estimations derived from proxy records and from model simulations help to better understand mechanisms driving climate variations, and also offer the possibility to identify deficiencies in both approaches. This paper presents regional temperature reconstructions based on tree-ring maximum density series in the Pyrenees, and compares them with the output of global simulations for this region and with regional climate model simulations conducted for the target region. An ensemble of 24 reconstructions of May-to-September regional mean temperature was derived from 22 maximum density tree-ring site chronologies distributed over the larger Pyrenees area. Four different tree-ring series standardization procedures were applied, combining two detrending methods: 300-yr spline and the regional curve standardization (RCS). Additionally, different methodological variants for the regional chronology were generated by using three different aggregation methods. Calibration verification trials were performed in split periods and using two methods: regression and a simple variance matching. The resulting set of temperature reconstructions was compared with climate simulations performed with global (ECHO-G) and regional (MM5) climate models. The 24 variants of May-to-September temperature reconstructions reveal a generally coherent pattern of inter-annual to multi-centennial temperature variations in the Pyrenees region for the last 750 yr. However, some reconstructions display a marked positive trend for the entire length of the reconstruction, pointing out that the application of the RCS method to a suboptimal set of samples may lead to unreliable results. Climate model simulations agree with the tree-ring based reconstructions at multi-decadal time scales, suggesting solar variability and volcanism as the main factors controlling preindustrial mean temperature variations in the Pyrenees. Nevertheless, the comparison also highlights differences with the reconstructions, mainly in the amplitude of past temperature variations and in the 20th century trends. Neither proxy-based reconstructions nor model simulations are able to perfectly track the temperature variations of the instrumental record, suggesting that both approximations still need further improvements.