Crystallographic studies of magnetite and titanomaghemite

The crystal structure of natural magnetite has been investigated on the basis of previously published X-ray intensity data and a newly acquired, more extensive data base. Both investigations show that the structure does not conform to the centrosymmetrical space group Fd3m, as is normally assumed, but the non-centrosymmetrical space group F43m. The structure refinement provides values for the atom positions, anisotropic thermal parameters and bond lengths. A study of Friedel related pairs of X-ray intensities shows that Friedel's law is violated in magnetite, further confirming that the space group is non-centrosymmetrical. It was found that the octahedral site cations in magnetite do not occupy special positions at the centres of the octahedral interstices as they should under the space group Fd3m, but are displaced along <111 > directions leading to F43m symmetry. A mechanism is known for the origin of these displacements and the likelihood of similar displacements occurring in other natural and synthetic spinels is discussed. The crystal structure of a natural titanomaghemite was determined by a combination of X-ray diffraction and Mõssbauer spectroscopy. This was confirmed as possessing a primitive cubic Bravais lattice with the space group P4332 and the structural formula: Fe3+.0.96 0 0.04 [Fe2+0.23 Fe3+0.99 Ti4+0.42 0 0.37 ] 042 - where 0 represents a cation vacancy. As the above formula shows, there are cation vacancies on both tetrahedral arrl octahedral sites, the majority being restricted to octahedral sltes. No tetrahedral site Fe2+ or Ti4+ was observed. Values for the atom positions, anisotropic thermal parameters and bond lengths have been determined for this particular specimen.

Cross-correlation-aided transport in stochastically driven accretion flows

The origin of linear instability resulting in rotating sheared accretion flows has remained a controversial subject for a long time. While some explanations of such non-normal transient growth of disturbances in the Rayleigh stable limit were available for magnetized accretion flows, similar instabilities in the absence of magnetic perturbations remained unexplained. This dichotomy was resolved in two recent publications by Chattopadhyay and co-workers [Mukhopadhyay and Chattopadhyay, J. Phys. A 46, 035501 (2013)1751-811310.1088/1751-8113/46/3/035501; Nath, Phys. Rev. E 88, 013010 (2013)PLEEE81539-375510.1103/PhysRevE.88.013010] where it was shown that such instabilities, especially for nonmagnetized accretion flows, were introduced through interaction of the inherent stochastic noise in the system (even a "cold" accretion flow at 3000 K is too "hot" in the statistical parlance and is capable of inducing strong thermal modes) with the underlying Taylor-Couette flow profiles. Both studies, however, excluded the additional energy influx (or efflux) that could result from nonzero cross correlation of a noise perturbing the velocity flow, say, with the noise that is driving the vorticity flow (or equivalently the magnetic field and magnetic vorticity flow dynamics). Through the introduction of such a time symmetry violating effect, in this article we show that nonzero noise cross correlations essentially renormalize the strength of temporal correlations. Apart from an overall boost in the energy rate (both for spatial and temporal correlations, and hence in the ensemble averaged energy spectra), this results in mutual competition in growth rates of affected variables often resulting in suppression of oscillating Alfven waves at small times while leading to faster saturations at relatively longer time scales. The effects are seen to be more pronounced with magnetic field fluxes where the noise cross correlation magnifies the strength of the field concerned. Another remarkable feature noted specifically for the autocorrelation functions is the removal of energy degeneracy in the temporal profiles of fast growing non-normal modes leading to faster saturation with minimum oscillations. These results, including those presented in the previous two publications, now convincingly explain subcritical transition to turbulence in the linear limit for all possible situations that could now serve as the benchmark for nonlinear stability studies in Keplerian accretion disks.

Some properties of g- and P-spaces

A γ-space with a strictly positive measure is separable. An example of a non-separable γ−space with c.c.c. is given. A P−space with c.c.c. is countable and discrete.

Análise cefalométrica: ortodontistas x radiologistas

Cephalometric analysis is the mensuration of linear and angular measures through demarcation points as distances and lines on teleradiography, and is considered of fundamental importance for diagnosis and orthodontic planning. In this manner, the objective of this research was to compare cephalometric measurements obtained by dentists and radiologists from the analysis of the same radiograph, in a computerized cephalometric analysis program. All research participants marked 18 cephalometric points on a 14-inch notebook computer, as directed by the program itself (Radiocef 2®). From there, they generated 14 cephalometric parameters including skeletal, dental-skeletal, dental and soft tissue. In order to verify the intra-examiner agreement, 10 professionals from each group repeated the marking of the points with a minimum interval of eight days between the two markings. The intra-group variability was calculated based on the coefficients of variation (CV). The comparison between groups was performed using the Student t-test for normally distributed variables, and using the Mann-Whitney test for those with non-normal distribution. In the group of orthodontists, the measurements of Pog and 1-NB, SL, S-Ls Line, S-Li Line and 1.NB showed high internal variability. In the group of radiologists, the same occurred with the values of Pog and 1-NB, S-Ls Line, S-Li Line and 1.NA. In the comparison between groups, all the analyzed linear values and two angular values showed statistically significant differences between radiologists and dentists (p <0.05). According to the results, the interexaminer error in cephalometric analysis requires more attention, but does not come from a specific class of specialists, being either dentists or radiologists.

Critère de validation croisée pour le choix des modèles des petits domaines au niveau des unités

Ce mémoire s’intéresse à l’étude du critère de validation croisée pour le choix des modèles relatifs aux petits domaines. L’étude est limitée aux modèles de petits domaines au niveau des unités. Le modèle de base des petits domaines est introduit par Battese, Harter et Fuller en 1988. C’est un modèle de régression linéaire mixte avec une ordonnée à l’origine aléatoire. Il se compose d’un certain nombre de paramètres : le paramètre β de la partie fixe, la composante aléatoire et les variances relatives à l’erreur résiduelle. Le modèle de Battese et al. est utilisé pour prédire, lors d’une enquête, la moyenne d’une variable d’intérêt y dans chaque petit domaine en utilisant une variable auxiliaire administrative x connue sur toute la population. La méthode d’estimation consiste à utiliser une distribution normale, pour modéliser la composante résiduelle du modèle. La considération d’une dépendance résiduelle générale, c’est-à-dire autre que la loi normale donne une méthodologie plus flexible. Cette généralisation conduit à une nouvelle classe de modèles échangeables. En effet, la généralisation se situe au niveau de la modélisation de la dépendance résiduelle qui peut être soit normale (c’est le cas du modèle de Battese et al.) ou non-normale. L’objectif est de déterminer les paramètres propres aux petits domaines avec le plus de précision possible. Cet enjeu est lié au choix de la bonne dépendance résiduelle à utiliser dans le modèle. Le critère de validation croisée sera étudié à cet effet.

An alternative benchmarking approach for electricity utility regulation using maximum entropy

The main purpose of this study is to present an alternative benchmarking approach that can be used by national regulators of utilities. It is widely known that the lack of sizeable data sets limits the choice of the benchmarking method and the specification of the model to set price controls within incentive-based regulation. Ill-posed frontier models are the problem that some national regulators have been facing. Maximum entropy estimators are useful in the estimation of such ill-posed models, in particular in models exhibiting small sample sizes, collinearity and non-normal errors, as well as in models where the number of parameters to be estimated exceeds the number of observations available. The empirical study involves a sample data used by the Portuguese regulator of the electricity sector to set the parameters for the electricity distribution companies in the regulatory period of 2012-2014. DEA and maximum entropy methods are applied and the efficiency results are compared.

Change-point detection in environmental time series based on the informational approach

In this study, the Schwarz Information Criterion (SIC) is applied in order to detect change-points in the time series of surface water quality variables. The application of change-point analysis allowed detecting change-points in both the mean and the variance in series under study. Time variations in environmental data are complex and they can hinder the identification of the so-called change-points when traditional models are applied to this type of problems. The assumptions of normality and uncorrelation are not present in some time series, and so, a simulation study is carried out in order to evaluate the methodology’s performance when applied to non-normal data and/or with time correlation.

Atividades de investigação mediadas pelas TICE : estudo das conceções, interações e discussões desenvolvidas por crianças e jovens dentro de um contexto de ensino de ciências ativo e colaborativo

A temática de discussão neste trabalho é o Ensino de Ciências através da Investigação (ENCI) [inquiry-based science education (IBSE)] que utilisa das Tecnologias de Informação e Comunicação (TIC) para o ensino de Ciências (TICEC). Deste modo, esta pesquisa tem o objetivo de identificar as conceções sobre a Natureza da Ciência e Tecnologia, além de compreender as interações, relações e discussões de crianças e jovens ao realizarem atividades de investigação através das TIC, dentro de um contexto de ensino de Ciências ativo e colaborativo. Organizamos esta investigação em três partes com diferentes estudos para alcançar este objetivo. A primeira parte apresenta estudos, a partir de uma revisão sistemática, de como se ensina e aprende Ciências utilizando as TICEC. Realizamos ainda uma revisão sobre o papel da argumentação no ensino de Ciências e elaboramos um instrumento para analizar a argumentação dos participantes numa atividade investigativa baseada em TICs. A segunda parte deste trabalho caracteriza a metodologia utilizada e os elementos da investigação: aspetos metodológicos (pesquisa qualitativa e estudo de caso), cenário (espaço não-formal de ensino de ciências ativo e colaborativo), sujeitos (crianças e jovens provenientes de um meio económico vulnerável), instrumentos de coleta de dados (questionário, filmagem de entrevistas, focus group e observações) e análise de dados (sistema de categorias). Também é apresentada a elaboração de uma atividade de investigação de ensino de Ciências (AIEC) mediada por recursos digitais, denominada de Módulo Temático Virtual (MTV). A terceira parte caracteriza-se pela apresentação dos resultados. No primeiro momento identificamos as conceções dos participantes sobre a Natureza da Ciência (Nature of Science – NOS), Natureza da Tecnologia (Nature of Technology - NOT) e o papel dos cientistas na sociedade. O último estudo vem caracterizar as interações, relações e discussões desenvolvidas pelos participantes quando realizam AIEC através das TICEC. Verificamos que dentro de um contexto ativo de ensino de Ciências e mediado pelas TIC, os participantes tendem a manifestar quatro aspetos ou domínios: social, técnico, afetivo e cognitivo. Os quatros domínios referidos caracterizam os processos que surgem durante o desenvolvimento de atividades científicas num cenário específico complementar à educação tradicional e são indicadores de como planear e analisar o ensino de Ciências quando se utiliza recurso digitais.

Nonlinear evolution of harmonically forced perturbations on a wingtip vortex

Wingtip vortices are created by flying airplanes due to lift generation. The vortex interaction with the trailing aircraft has sparked researchers’ interest to develop an efficient technique to destroy these vortices. Different models have been used to describe the vortex dynamics and they all show that, under real flight conditions, the most unstable modes produce a very weak amplification. Another linear instability mechanism that can produce high energy gains in short times is due to the non-normality of the system. Recently, it has been shown that these non-normal perturbations also produce this energy growth when they are excited with harmonic forcing functions. In this study, we analyze numerically the nonlinear evolution of a spatially, pointwise and temporally forced perturbation, generated by a synthetic jet at a given radial distance from the vortex core. This type of perturbation is able to produce high energy gains in the perturbed base flow (10^3), and is also a suitable candidate for use in engineering applications. The flow field is solved for using fully nonlinear three-dimensional direct numerical simulation with a spectral multidomain penalty method model. Our novel results show that the nonlinear effects are able to produce locally small bursts of instability that reduce the intensity of the primary vortex.

Aprendizagem das ciências em espaços não-formais: visita de estudo na rota das energias renováveis

Nos programas actuais defende-se um ensino contextualizado numa perspectiva CTSA - Ciência, Tecnologia, Sociedade e Ambiente, e a diversificação de estratégias e actividades é recomendada. Numa perspectiva construtivista, a aprendizagem fora da sala de aula, aprendizagem em espaços não-formais - as visitas de estudo - são consideradas como uma das estratégias com elevado potencial pedagógico. No presente estudo foram realizadas e avaliadas duas visitas de estudo no âmbito da disciplina de Física e Química do 10° ano de escolaridade, tendo a primeira sido feita como estudo-piloto ao Centro de Ciência Viva de Constância e uma segunda como estudo principal, Na Rota das Energias Renováveis, à Barragem do Alqueva, Central Solar Fotovoltaica da Amareleja, Parque de Natureza de Noudar e Herdade do Esporão. Realizou-se uma pré-visita aos locais a visitar, estabeleceram-se contactos privilegiados com as instituições visitadas, preparam-se materiais didácticos, um Roteiro e um Guia de Campo para cada local visitado, foram elaborados e apresentados aos alunos questionários sobre o seu grau de satisfação com as visitas e realizaram-se ainda entrevistas em grupo com alguns alunos para recolher as suas impressões. Os alunos realizaram trabalhos de divulgação da visita tendo sido disponibilizada, no "site" da escola, informação detalhada sobre a visita. Neste estudo analisou-se em que medida uma aprendizagem contextualizada num espaço não-formal foi promotora de um envolvimento produtivo dos alunos. As aprendizagens realizadas situaram-se no domínio cognitivo, social ou atitudinal e os materiais didácticos, como o Guia de Campo tiveram um papel relevante para essas aprendizagens. A motivação dos alunos para a disciplina melhorou durante e após a visita. A colaboração com as instituições visitadas mostrou-se importante no estabelecimento de uma interface entre o espaço formal e o não-formal. A realização de visitas de estudo é uma componente importante dos curricula e deve ser encarada com naturalidade a sua realização frequente, como parte de uma estratégia para motivar os alunos para a disciplina e aumentar a sua capacidade de aprender os conteúdos. ABSTRACT: Nowadays programs defend teaching model that follows a perspective based in Science, Technology, Society and Environment, with special focus in the diversification of strategies. Outside of classroom teaching, in non-formal spaces, as in field trips, is considered one of the strategies with the highest pedagogic potential. ln this study, two field trips were made based on the Physics and Chemistry program for the 1Oth grade, with the first one as the pilot visit to Centro de Ciência Viva de Constância, and the second one as the main study, ln the Path of Renewable Energies, to Alqueva’s dam, Amareleja's Solar Photovoltaic Center, Noudar's Nature Park and Herdade do Esporão. A pre-visit was made for each one of the places to visit, contacts were made, materials were created, an itinerary and a Field Guide for both field trips, satisfaction questionaries were made to the students and the teacher also interviewed some students for appreciations about the visit. The students created posters about the field trip, and information was also showed in the school website. With this study, the benefits of a contextualized learning in a non-formal space were demonstrated. These acquisitions of knowledge were in various domains, such as cognition, social behavior and the didactic materials had a relevant role in these acquirements. Moreover, the students' motivation for the subject in question improved significantly during and after the field trip. The cooperation with the visited institutions revealed to be important in the establishment of an interface between the formal and the non-formal spaces. Making field trips is an important component in the curricula and should be encouraged and seen as a natural part of a strategy of motivation of the students for a certain subject, increasing its ability to learn matters.

Assessing the fit of unidimensional IRT models for binary data under model misspecification

Model misspecification affects the classical test statistics used to assess the fit of the Item Response Theory (IRT) models. Robust tests have been derived under model misspecification, as the Generalized Lagrange Multiplier and Hausman tests, but their use has not been largely explored in the IRT framework. In the first part of the thesis, we introduce the Generalized Lagrange Multiplier test to detect differential item response functioning in IRT models for binary data under model misspecification. By means of a simulation study and a real data analysis, we compare its performance with the classical Lagrange Multiplier test, computed using the Hessian and the cross-product matrix, and the Generalized Jackknife Score test. The power of these tests is computed empirically and asymptotically. The misspecifications considered are local dependence among items and non-normal distribution of the latent variable. The results highlight that, under mild model misspecification, all tests have good performance while, under strong model misspecification, the performance of the tests deteriorates. None of the tests considered show an overall superior performance than the others. In the second part of the thesis, we extend the Generalized Hausman test to detect non-normality of the latent variable distribution. To build the test, we consider a seminonparametric-IRT model, that assumes a more flexible latent variable distribution. By means of a simulation study and two real applications, we compare the performance of the Generalized Hausman test with the M2 limited information goodness-of-fit test and the Likelihood-Ratio test. Additionally, the information criteria are computed. The Generalized Hausman test has a better performance than the Likelihood-Ratio test in terms of Type I error rates and the M2 test in terms of power. The performance of the Generalized Hausman test and the information criteria deteriorates when the sample size is small and with a few items.

On the similarity of Sturm-Liouville operators with non-Hermitian boundary conditions to self-adjoint and normal operators

We consider one-dimensional Schrödinger-type operators in a bounded interval with non-self-adjoint Robin-type boundary conditions. It is well known that such operators are generically conjugate to normal operators via a similarity transformation. Motivated by recent interests in quasi-Hermitian Hamiltonians in quantum mechanics, we study properties of the transformations and similar operators in detail. In the case of parity and time reversal boundary conditions, we establish closed integral-type formulae for the similarity transformations, derive a non-local self-adjoint operator similar to the Schrödinger operator and also find the associated “charge conjugation” operator, which plays the role of fundamental symmetry in a Krein-space reformulation of the problem.

Molecular phase space transport in water:non-stationary random walk model

Molecular transport in phase space is crucial for chemical reactions because it defines how pre-reactive molecular configurations are found during the time evolution of the system. Using Molecular Dynamics (MD) simulated atomistic trajectories we test the assumption of the normal diffusion in the phase space for bulk water at ambient conditions by checking the equivalence of the transport to the random walk model. Contrary to common expectations we have found that some statistical features of the transport in the phase space differ from those of the normal diffusion models. This implies a non-random character of the path search process by the reacting complexes in water solutions. Our further numerical experiments show that a significant long period of non-stationarity in the transition probabilities of the segments of molecular trajectories can account for the observed non-uniform filling of the phase space. Surprisingly, the characteristic periods in the model non-stationarity constitute hundreds of nanoseconds, that is much longer time scales compared to typical lifetime of known liquid water molecular structures (several picoseconds).

Infinite horizon MPC with non-minimal state space feedback

In the MPC literature, stability is usually assured under the assumption that the state is measured. Since the closed-loop system may be nonlinear because of the constraints, it is not possible to apply the separation principle to prove global stability for the Output feedback case. It is well known that, a nonlinear closed-loop system with the state estimated via an exponentially converging observer combined with a state feedback controller can be unstable even when the controller is stable. One alternative to overcome the state estimation problem is to adopt a non-minimal state space model, in which the states are represented by measured past inputs and outputs [P.C. Young, M.A. Behzadi, C.L. Wang, A. Chotai, Direct digital and adaptative control by input-output, state variable feedback pole assignment, International journal of Control 46 (1987) 1867-1881; C. Wang, P.C. Young, Direct digital control by input-output, state variable feedback: theoretical background, International journal of Control 47 (1988) 97-109]. In this case, no observer is needed since the state variables can be directly measured. However, an important disadvantage of this approach is that the realigned model is not of minimal order, which makes the infinite horizon approach to obtain nominal stability difficult to apply. Here, we propose a method to properly formulate an infinite horizon MPC based on the output-realigned model, which avoids the use of an observer and guarantees the closed loop stability. The simulation results show that, besides providing closed-loop stability for systems with integrating and stable modes, the proposed controller may have a better performance than those MPC controllers that make use of an observer to estimate the current states. (C) 2008 Elsevier Ltd. All rights reserved.

Detection and description of non-linear interdependence in normal multichannel human EEG data

Objectives: This study examines human scalp electroencephalographic (EEG) data for evidence of non-linear interdependence between posterior channels. The spectral and phase properties of those epochs of EEG exhibiting non-linear interdependence are studied. Methods: Scalp EEG data was collected from 40 healthy subjects. A technique for the detection of non-linear interdependence was applied to 2.048 s segments of posterior bipolar electrode data. Amplitude-adjusted phase-randomized surrogate data was used to statistically determine which EEG epochs exhibited non-linear interdependence. Results: Statistically significant evidence of non-linear interactions were evident in 2.9% (eyes open) to 4.8% (eyes closed) of the epochs. In the eyes-open recordings, these epochs exhibited a peak in the spectral and cross-spectral density functions at about 10 Hz. Two types of EEG epochs are evident in the eyes-closed recordings; one type exhibits a peak in the spectral density and cross-spectrum at 8 Hz. The other type has increased spectral and cross-spectral power across faster frequencies. Epochs identified as exhibiting non-linear interdependence display a tendency towards phase interdependencies across and between a broad range of frequencies. Conclusions: Non-linear interdependence is detectable in a small number of multichannel EEG epochs, and makes a contribution to the alpha rhythm. Non-linear interdependence produces spatially distributed activity that exhibits phase synchronization between oscillations present at different frequencies. The possible physiological significance of these findings are discussed with reference to the dynamical properties of neural systems and the role of synchronous activity in the neocortex. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.