Síntesi i estudi de les propietats de fotoluminescència de nanoestructures de sulfur de cadmi

S’han sintetitzat estructures de CdS a partir de sílice mesoporosa (SBA-15, SBA-16 i KIT-6) pel mètode del motlle rígid i s’han caracteritzat mitjançant TEM i XRD, i se n’han estudiat les seves propietats de fotoluminescència mitjançant CSLM. Els resultats obtinguts s’han comparat amb una mostra de CdS comercial. El CdS SBA-15 ha conservat l’estructura mesoporosa del motlle, mentre que en els altres dos casos s’ha mantingut només de manera parcial. Per a totes les mostres, la mida de partícula es troba en el rang dels 3-10 nm. Pel que fa a l’estructura, s’ha detectat fase wurtzita i zinc-blenda, en diferents percentatges entre elles. La formació de wurtzita pot ser en part responsable del col·lapse parcial de la mesostructura. El pic de màxima emissió en l’espectre de fotoluminescència es desplaça entre les diferents estructures segons la mida de partícula. L’amplada del pic varia en funció del grau de dispersió de partícula, de manera que s’observa que aquesta és inferior en les mostres sintetitzades respecte de la comercial.

Rigid flat webs on the projective plane

This paper studies global webs on the projective plane with vanishing curvature. The study is based on an interplay of local and global arguments. The main local ingredient is a criterium for the regularity of the curvature at the neighborhood of a generic point of the discriminant. The main global ingredient, the Legendre transform, is an avatar of classical projective duality in the realm of differential equations. We show that the Legendre transform of what we call reduced convex foliations are webs with zero curvature, and we exhibit a countable infinity family of convex foliations which give rise to a family of webs with zero curvature not admitting non-trivial deformations with zero curvature.

Estudio y construcción de herramientas de validación HTML

The Final Year Project consists of two essentially different parts, which share acommon theme: HTML code validation. The first of these two parts focuses on the study of the validation process. It supplies a brief introduction to the evolution of HTML and XHTML, the new tags introduced in HTML5 and the most common errors found in today's websites. Already developed HTML validation tools are analyzed and examined in detail in order to compare their features and evaluate their performances. Lastly, a comparison of the parsing process in the most common browsers found nowadays is provided. In the second part of the project the focus of the project is shifted towards the development of a XHTML5 validation tool. The input is a XHTML5 file whose content may or may not comply with the W3C specification, and therefore, may or may not be a valid XHTML5 document. The output provided by this tool will be a fixed XHTML5 document and an error log returned in the form of a XML file. Information as to the course of action pursued to fix the error and its location will also be included.

Analysis of Pleistocene paleodrainage evolution in the Po Basin (Italy) by multivariate statistical techniques

In order to obtain a high-resolution Pleistocene stratigraphy, eleven continuouslycored boreholes, 100 to 220m deep were drilled in the northern part of the PoPlain by Regione Lombardia in the last five years. Quantitative provenanceanalysis (QPA, Weltje and von Eynatten, 2004) of Pleistocene sands was carriedout by using multivariate statistical analysis (principal component analysis, PCA,and similarity analysis) on an integrated data set, including high-resolution bulkpetrography and heavy-mineral analyses on Pleistocene sands and of 250 majorand minor modern rivers draining the southern flank of the Alps from West toEast (Garzanti et al, 2004; 2006). Prior to the onset of major Alpine glaciations,metamorphic and quartzofeldspathic detritus from the Western and Central Alpswas carried from the axial belt to the Po basin longitudinally parallel to theSouthAlpine belt by a trunk river (Vezzoli and Garzanti, 2008). This scenariorapidly changed during the marine isotope stage 22 (0.87 Ma), with the onset ofthe first major Pleistocene glaciation in the Alps (Muttoni et al, 2003). PCA andsimilarity analysis from core samples show that the longitudinal trunk river at thistime was shifted southward by the rapid southward and westward progradation oftransverse alluvial river systems fed from the Central and Southern Alps.Sediments were transported southward by braided river systems as well as glacialsediments transported by Alpine valley glaciers invaded the alluvial plain.Kew words: Detrital modes; Modern sands; Provenance; Principal ComponentsAnalysis; Similarity, Canberra Distance; palaeodrainage

Bernstein's problem on weighted polynomial approximation

We formulate a necessary and sufficient condition for polynomials to be dense in a space of continuous functions on the real line, with respect to Bernstein's weighted uniform norm. Equivalently, for a positive finite measure [lletra "mu" minúscula de l'alfabet grec] on the real line we give a criterion for density of polynomials in Lp[lletra "mu" minúscula de l'alfabet grec entre parèntesis].

Modification of the Perrine-Baum diagram to improve the calculation of high voltage electric lines

Through the history of Electrical Engineering education, vectorial and phasorial diagrams have been used as a fundamental learning tool. At present, computational power has replaced them by long data lists, the result of solving equation systems by means of numerical methods. In this sense, diagrams have been shifted to an academic background and although theoretically explained, they are not used in a practical way within specific examples. This fact may be against the understanding of the complex behavior of the electrical power systems by students. This article proposes a modification of the classical Perrine-Baum diagram construction to allowing both a more practical representation and a better understanding of the behavior of a high-voltage electric line under different levels of load. This modification allows, at the same time, the forecast of the obsolescence of this behavior and line’s loading capacity. Complementary, we evaluate the impact of this tool in the learning process showing comparative undergraduate results during three academic years

La teoría de Morales-Ramis y el algoritmo de Kovacic

La teor\'\ı a de Morales–Ramis es la teor\'\ı a de Galois en el contextode los sistemas din\'amicos y relaciona dos tipos diferentes de integrabilidad:integrabilidad en el sentido de Liouville de un sistema hamiltonianoe integrabilidad en el sentido de la teor\'\ı a de Galois diferencial deuna ecuaci\'on diferencial. En este art\'\i culo se presentan algunas aplicacionesde la teor\'\i a de Morales–Ramis en problemas de no integrabilidadde sistemas hamiltonianos cuya ecuaci\'on variacional normal a lo largode una curva integral particular es una ecuaci\'on diferencial lineal desegundo orden con coeficientes funciones racionales. La integrabilidadde la ecuaci\'on variacional normal es analizada mediante el algoritmode Kovacic.

Experimental design on the simplex

Optimum experimental designs depend on the design criterion, the model andthe design region. The talk will consider the design of experiments for regressionmodels in which there is a single response with the explanatory variables lying ina simplex. One example is experiments on various compositions of glass such asthose considered by Martin, Bursnall, and Stillman (2001).Because of the highly symmetric nature of the simplex, the class of models thatare of interest, typically Scheff´e polynomials (Scheff´e 1958) are rather differentfrom those of standard regression analysis. The optimum designs are also ratherdifferent, inheriting a high degree of symmetry from the models.In the talk I will hope to discuss a variety of modes for such experiments. ThenI will discuss constrained mixture experiments, when not all the simplex is availablefor experimentation. Other important aspects include mixture experimentswith extra non-mixture factors and the blocking of mixture experiments.Much of the material is in Chapter 16 of Atkinson, Donev, and Tobias (2007).If time and my research allows, I would hope to finish with a few comments ondesign when the responses, rather than the explanatory variables, lie in a simplex.ReferencesAtkinson, A. C., A. N. Donev, and R. D. Tobias (2007). Optimum ExperimentalDesigns, with SAS. Oxford: Oxford University Press.Martin, R. J., M. C. Bursnall, and E. C. Stillman (2001). Further results onoptimal and efficient designs for constrained mixture experiments. In A. C.Atkinson, B. Bogacka, and A. Zhigljavsky (Eds.), Optimal Design 2000,pp. 225–239. Dordrecht: Kluwer.Scheff´e, H. (1958). Experiments with mixtures. Journal of the Royal StatisticalSociety, Ser. B 20, 344–360.1

From severance pay to self-insurance: Effects of severance payments savings accounts in Colombia

In 1990 Colombia replaced its traditional system of severance paymentswith a new system of severance payments savings accounts (SPSAs). Althoughseverance payments often are justified on the grounds that they provideinsurance against earnings loss, they also increase costs for employersand distort employment decisions. The impact of severance payments dependslargely on how much of the costs to employers can be shifted to workers.The theoretical analysis in this paper shows that, in contrast to atraditional system of severance payments, the system of SPSAs facilitatesthe shifting of severance payments costs to workers in the form of lowerwages. Empirical results using the Colombian National Household Surveysindicate that the introduction of SPSAs shifted around 80% of the totalseverance payments contributions to wages and had a positive effect onweekly hours. Results using the 1997 Colombian Living Standards MeasurementSurvey suggest that, although SPSAs in part replaced employer insurancewith self-insurance, SPSAs continue to play a consumption smoothing rolefor the non-employed.

Explaining movements in the labor share

In this paper we study the evolution of the labor share in the OECD since 1970. We show it is essentially related to the capital-output ratio; that this relationship is shifted by factors like the price of imported materials or the skill mix; and that discrepancies between the marginal product of labor and the real wage (due to, e.g., product market power, union bargaining, and labor adjustment costs) cause departures from it. We provide estimates of the model with panel data on 14 industries and 14 countries for 1973-93 and use them to compute the evolution of the wage gap in Germany and the US.

Killing by lung cancer or by diabetes? The trade-off between smoking and obesity

As the prevalence of smoking has decreased to below 20%, health practitioners interest has shifted towards theprevalence of obesity, and reducing it is one of the major health challenges in decades to come. In this paper westudy the impact that the final product of the anti-smoking campaign, that is, smokers quitting the habit, had onaverage weight in the population. To these ends, we use data from the Behavioral Risk Factors Surveillance System,a large series of independent representative cross-sectional surveys. We construct a synthetic panel that allows us tocontrol for unobserved heterogeneity and we exploit the exogenous changes in taxes and regulations to instrumentthe endogenous decision to give up the habit of smoking. Our estimates, are very close to estimates issued in the 90sby the US Department of Health, and indicate that a 10% decrease in the incidence of smoking leads to an averageweight increase of 2.2 to 3 pounds, depending on choice of specification. In addition, we find evidence that the effectovershoots in the short run, although a significant part remains even after two years. However, when we split thesample between men and women, we only find a significant effect for men. Finally, the implicit elasticity of quittingsmoking to the probability of becoming obese is calculated at 0.58. This implies that the net benefit from reducingthe incidence of smoking by 1% is positive even though the cost to society is $0.6 billions.

The labor market effects of payroll taxes in a middle-income country: Evidence from Colombia

We use a panel of manufacturing plants from Colombia to analyze how the risein payroll tax rates over the 1980 s and 1990 s affected the labor market.Our estimates indicate that formal wages fall by between 1.4% and 2.3% as aresult of a 10% rise in payroll taxes. This 'less-than-full-shifting' islikely to be the result of weak linkages between benefits and taxes and thepresence of downward wage rigidities induced by a binding minimum wage inColombia. Because the costs of taxation are only partly shifted fromemployers to employees, employment should also fall. Our results indicatethat a 10% increase in payroll taxes lowered formal employment by between4% and 5%. In addition, we find less shifting and larger disemploymenteffects for production than non-production workers. These results suggestthat policies aimed at boosting the relative demand of low-skill workers byreducing social security taxes on those with low earnings may be effectivein a country like Colombia, especially if tax cuts are targeted to indirectbenefits.

Computing congruences of modular forms and Galois representations modulo prime powers

This article starts a computational study of congruences of modular forms and modular Galoisrepresentations modulo prime powers. Algorithms are described that compute the maximum integermodulo which two monic coprime integral polynomials have a root in common in a sensethat is defined. These techniques are applied to the study of congruences of modular forms andmodular Galois representations modulo prime powers. Finally, some computational results withimplications on the (non-)liftability of modular forms modulo prime powers and possible generalisationsof level raising are presented.

On the robustness of least-squares Monte Carlo (LSM) for pricing American derivatives

This paper analyses the robustness of Least-Squares Monte Carlo, a techniquerecently proposed by Longstaff and Schwartz (2001) for pricing Americanoptions. This method is based on least-squares regressions in which theexplanatory variables are certain polynomial functions. We analyze theimpact of different basis functions on option prices. Numerical resultsfor American put options provide evidence that a) this approach is veryrobust to the choice of different alternative polynomials and b) few basisfunctions are required. However, these conclusions are not reached whenanalyzing more complex derivatives.

Macaulay style formulas for sparse resultants

We present formulas for computing the resultant of sparse polyno- mials as a quotient of two determinants, the denominator being a minor of the numerator. These formulas extend the original formulation given by Macaulay for homogeneous polynomials.