Strict calculation of the light statistics at the output of a travelling wave optical amplifier

A new method for calculating the probability density function of the photon number propagating through a travelling wave optical amplifier with no restriction on its working regime (linear and nonlinear) is reported. The authors show that the widely used Gaussian approximation of the probability density function does not match the real statistics if the incident optical power is small.Peer ReviewedPostprint (published version

Practical method for modelling the nonlinear behaviour of a travelling wave semiconductor optical amplifier

The authors present a simple model of a travelling wave semiconductor optical amplifier with an appreciable degree of saturation. The model uses a particular way of linearising the total carrier recombination R(N) to find useful expressions for the saturation parameter and/or the carrier lifetime. By combining these expressions and the gain measurements realised, it is possible to establish the dependence between the mentioned parameters and the input optical power.Peer ReviewedPostprint (published version

SPANISH COMPANY FOREIGN MARKET ENTRY

The aim of this paper is to analyse the determinants of diversification mode (acquisition versus greenfield) through foreign direct investment considering various theories, such as those of mergers and acquisitions, transaction costs, the learning organisation, the institutional context of the company and the cultural environment of the host country, and to analyse the determinants of entry mode combining diversification mode with ownership structure decision (greenfield wholly-owned subsidiaries, greenfield joint ventures, full acquisition and partial acquisition) and proposing various research hypotheses. The methodology used estimates various binomial and multinomial logit models, over a sample of 141 Spanish manufacturing companies between 1998 and 2000, finding that entry mode into a foreign country through direct investment is mainly explained by related diversification, international and local experience, advertising intensity, cultural distance, company size and ownership structure. El propósito de este trabajo consiste en analizar, por un lado, los determinantesdel modo de diversificación (adquisición versus desarrollo interno) de la inversióndirecta en el exterior considerando diversas aproximaciones teóricas, como las defusiones y adquisiciones, costes de transacción, del aprendizaje organizativo, así comodel contexto institucional de la empresa y del entorno cultural del país receptor. Por otrolado, se examinan los determinantes de la inversión directa exterior combinando elmodo de diversificación con la estructura de la propiedad empresarial (adquisición total,adquisición parcial, filial de plena propiedad y filial de propiedad compartida)proponiendo diversas hipótesis de investigación. La metodología aplicada estimadiversos modelos logit, binomial y multinomial, sobre 141 empresas españolasmanufactureras entre 1998 y 2000, detectando que el modo de entrada medianteinversión directa exterior viene explicada por la diversificación relacionada, laexperiencia internacional y local de la empresa, la intensidad publicitaria, la distanciacultural, así como por el tamaño y estructura de la propiedad de la empresa.marketing internacional, modo de entrada, inversión directa en el exterior, joint-venture, adquisición. international marketing, entry mode, foreign direct investment, jointventure, acquisition

Signcryption schemes with threshold unsigncryption, and applications

The final publication is available at link.springer.comThe goal of a signcryption scheme is to achieve the same functionalities as encryption and signature together, but in a more efficient way than encrypting and signing separately. To increase security and reliability in some applications, the unsigncryption phase can be distributed among a group of users, through a (t, n)-threshold process. In this work we consider this task of threshold unsigncryption, which has received very few attention from the cryptographic literature up to now (maybe surprisingly, due to its potential applications). First we describe in detail the security requirements that a scheme for such a task should satisfy: existential unforgeability and indistinguishability, under insider chosen message/ciphertext attacks, in a multi-user setting. Then we show that generic constructions of signcryption schemes (by combining encryption and signature schemes) do not offer this level of security in the scenario of threshold unsigncryption. For this reason, we propose two new protocols for threshold unsigncryption, which we prove to be secure, one in the random oracle model and one in the standard model. The two proposed schemes enjoy an additional property that can be very useful. Namely, the unsigncryption protocol can be divided in two phases: a first one where the authenticity of the ciphertext is verified, maybe by a single party; and a second one where the ciphertext is decrypted by a subset of t receivers, without using the identity of the sender. As a consequence, the schemes can be used in applications requiring some level of anonymity, such as electronic auctions.Peer ReviewedPostprint (author's final draft

Energy partition and segregation for an intruder in a vibrated granular system under gravity

The difference of temperatures between an impurity and the surrounding gas in an open vibrated granular system is studied. It is shown that, in spite of the high inhomogeneity of the state, the temperature ratio remains constant in the bulk of the system. The lack of energy equipartition is associated to the change of sign of the pressure diffusion coefficient for the impurity at certain values of the parameters of the system, leading to a segregation criterium. The theoretical predictions are consistent with previous experimental results, and also in agreement with molecular dynamics simulation results reported in this paper.Comment: To appear in Phys. Rev. Let

OPTIMAL REGULATION AND GROWTH IN A NATURAL-RESOURCE-BASED ECONOMY

This paper develops a two-sector model for a renewable natural resource based economy. Pareto efficient results show the optimal harvesting rate that allows for sustained long-run optimal growth, which is upper-bounded by the biological rate of reproduction. Regulation prevents from resource over-exploitation and exhaustion which arise under open access. The Ramsey policy allowing the competitive economy to reach the first-best solution, leads the government to tax harvesting activity from firms and distribute the receipts among households. In the short-run the tax is variable. In the long-run, the lower the intrinsic rate of reproduction the higher the constant unit tax on the resource use.Natural Resources, Efficiency, Open Access, Ramsey Regulation, Growth.

Soluciones analíticas del problema de Cauchy. teoremas de Cauchy y Cauchy-Kowalewski

In first place, the goal of this work is to get a result which guarantees that the Cauchy problem, with certain conditions of analyticity, has a unique analytical solution, and its power series can be obtained. We will also provide a result, the Cauchy-Kowalewski theorem, to guarantee that, in a Cauchy problem defined by a partial differential equation, and with certain hypotheses of analyticity, the analytical solution exists and it is unique, too. We will prove these results by the majorant method. Finally, in last chapter, we will see how to apply the previous result on a Cauchy problem defined by a higher-order PDE. Then, we will study certain surfaces where the initial conditions can’t be given. These surfaces are called characteristic surfaces, and the problem isn’t well defined on them. In order to achieve these objetives, we will need to introduce some notions and results of infinitesimal calculus, as well as the concepts of analytical and majorant functions. We will also provide other results which we will use throughout the project.El objetivo de este trabajo es obtener, en primer lugar, un resultado que garantice la existencia y unicidad de solución analítica del problema de Cauchy en el que la función de la ecuación diferencial ordinaria que define el problema es analítica en un cierto entorno, concepto que introduciremos en el primer capítulo. Teniendo en cuenta esta hipótesis de analiticidad, se quiere llegar a la conclusión de que el problema posee una única solución analítica, y, además, puede obtenerse su desarrollo de Taylor término a término. Por otro lado, gracias al teorema de Cauchy-Kowalewski, garantizaremos la existencia y unicidad de solución analítica del problema de Cauchy con la particularidad de que, en este caso, la ecuación que define el problema vendrá dada por una ecuación diferencial en derivadas parciales de primer orden. Todo ello bajo ciertas condiciones, la más importante es que los datos iniciales o de Cauchy, y los coeficientes de la ecuación en derivadas parciales son funciones analíticas, y, por tanto, la solución del problema también lo será. Para finalizar, en el último capítulo se verá, en primer lugar, cómo aplicar el resultado anterior sobre un problema de Cauchy definido por una EDP de orden superior al primero. Por otro lado, estudiaremos ciertas superficies donde no pueden darse lo datos iniciales del problema general de Cauchy, esto es, definido por una EDP en forma implícita. Veremos que estas superficies son las denominadas superficies características, y sobre ellas el problema no está bien planteado. Para llevar a cabo estos objetivos necesitamos conocer algunas nociones del cálculo infinitesimal, como son los conceptos de serie numérica y de potencias, y algunos resultados de convergencia de éstas; así como los conceptos de función analítica y función mayorante. También los teoremas de la función inversa e implícita, a los cuales tendremos que recurrir a lo largo del trabajo.Universidad de Sevilla. Grado en Matemática

Revisión metodológica de la enseñanza del francés para fines específicos

A background survey of the different methodologies used in specialized teaching of French is essential. Firstly, because we think that there is not enough theory in this respect, and secondly, because the use of up to date methodological mechanisms has to necessarily be based on the knowledge regarding previous research in this field and later development. This article is divided into two parts. The first part offers a panorama of the French functional teaching irruption. The second tackles the study of the four methodological stages, which have provided the guidelines for the teaching process ever since the beginning

Block freeze-concentration of coffee extract: Effect of freezing and thawing stages on solute recovery and bioactive compounds

Coffee extract was freeze-concentrated using the total block technique. The effects of four parameters were evaluated: the initial coffee mass fraction (5 and 15% w/w), the cooling temperature ( 10 and 20 C), the heating temperature (20 and 40 C) and the freezing direction (parallel and counter-flow to the thawing direction). The solid concentration was measured during the thawing stage to quantify the solute recovery and the concentration index for one stage of freeze concentration. The coffee mass fraction, the freezing direction and the cooling temperature significantly influenced the solute recovery. A concentration index between 1 and 2.3 was obtained in one cycle. The effect of block freeze concentration on the bioactive compound concentration and the antioxidant activity was measured. The coffee bioactive compounds were distributed in proportion to the total solid content in the ice and liquid. Therefore, block freeze concentration is an effective technique to preserve functional properties of coffee extracts.Postprint (published version