Cette thèse traite de méthodes « model-based » pour résoudre des problèmes d'apprentissage par renforcement. On considère un agent confronté à une suite de décisions et un environnement dont l'état varie selon les décisions prises par l'agent. Ce dernier reçoit tout au long de l'interaction des récompenses qui dépendent à la fois de l'action prise et de l'état de l'environnement. L'agent ne connaît pas le modèle d'interaction et a pour but de maximiser la somme des récompenses reçues à long terme. Nous considérons différents modèles d'interactions : les processus de décisions markoviens, les processus de décisions markoviens partiellement observés et les modèles de bandits. Pour ces différents modèles, nous proposons des algorithmes qui consistent à construire à chaque instant un ensemble de modèles permettant d'expliquer au mieux l'interaction entre l'agent et l'environnement. Les méthodes dites « model-based » que nous élaborons se veulent performantes tant en pratique que d'un point de vue théorique. La performance théorique des algorithmes est calculée en terme de regret qui mesure la différence entre la somme des récompenses reçues par un agent qui connaîtrait à l'avance le modèle d'interaction et celle des récompenses cumulées par l'algorithme. En particulier, ces algorithmes garantissent un bon équilibre entre l'acquisition de nouvelles connaissances sur la réaction de l'environnement (exploration) et le choix d'actions qui semblent mener à de fortes récompenses (exploitation). Nous proposons deux types de méthodes différentes pour contrôler ce compromis entre exploration et exploitation. Le premier algorithme proposé dans cette thèse consiste à suivre successivement une stratégie d'exploration, durant laquelle le modèle d'interaction est estimé, puis une stratégie d'exploitation. La durée de la phase d'exploration est contrôlée de manière adaptative ce qui permet d'obtenir un regret logarithmique dans un processus de décision markovien paramétrique même si l'état de l'environnement n'est que partiellement observé. Ce type de modèle est motivé par une application d'intérêt en radio cognitive qu'est l'accès opportuniste à un réseau de communication par un utilisateur secondaire. Les deux autres algorithmes proposés suivent des stratégies optimistes : l'agent choisit les actions optimales pour le meilleur des modèles possibles parmi l'ensemble des modèles vraisemblables. Nous construisons et analysons un tel algorithme pour un modèle de bandit paramétrique dans un cas de modèles linéaires généralisés permettant ainsi de considérer des applications telles que la gestion de publicité sur internet. Nous proposons également d'utiliser la divergence de Kullback-Leibler pour la construction de l'ensemble des modèles vraisemblables dans des algorithmes optimistes pour des processus de décision markoviens à espaces d'états et d'actions finis. L'utilisation de cette métrique améliore significativement le comportement de des algorithmes optimistes en pratique. De plus, une analyse du regret de chacun des algorithmes permet de garantir des performances théoriques similaires aux meilleurs algorithmes de l'état de l'art.

In light of increasing harmful algal blooms and the need to protect human health and aquatic resources, proactive management approaches merit further study. For this purpose I conducted field samplings to characterize plankton community composition and laboratory experiments to test some approaches to new management schemes in the lower Rincon Delta. On site measurements and microscopic analysis showed that environmental parameters and plankton community composition varied considerably among sampling stations and sampling dates. A recent modeling study suggested that manipulation of freshwater inflow to estuaries might prevent phytoplankton blooms and enhance secondary productivity. To test this theory I conducted three semi-continuous design and flow-through incubation design experiments using natural plankton assemblages. I investigated the effect of two different pulsing regimes of inflow and nutrient loading on zooplankton densities, and phytoplankton biomass and diversity. Despite differences in zooplankton structure and phytoplankton community composition between the two experiment designs, the results confirmed that pulsed inflows might alter plankton dynamics. My findings showed that 3-day pulse treatments consistently supported greater zooplankton densities and higher phytoplankton species diversity when compared to 1-day pulse treatments. In addition, accumulation of phytoplankton biovolume remained low during 3-day pulse treatments. Differences in zooplankton performance between 3-day pulse and 1-day pulse inflow treatments were likely due to the ability of phytoplankton to uptake and store greater amounts of nutrients under conditions of 3-day pulse inflow. This resulted in food of higher quality for zooplankton, and might have supported greater zooplankton population growth rates. Additionally, in an attempt to understand the mechanisms leading to high biodiversity in aquatic ecosystems, I built a resource-storage model and studied the effects of resource-storage on competition of multiple phytoplankton species on multiple abiotic resources. I compared this model with a well-established multi-species competition model. My results showed that for certain species combinations a resource-storage-based model can generate dissimilar outcomes when compared to a model without resource-storage.

This thesis deals with machine learning techniques for the extraction of structure and the analysis of the vertebrate olfactory pathway based on related methods. Some of its main contributions are summarized below. We have performed a systematic investigation for classification in biomedical images with the goal of recognizing a material in these images by its texture. This investigation included (i) different measures for evaluating the importance of image descriptors (features), (ii) methods to select a feature set based on these evaluations, and (iii) classification algorithms. Image features were evaluated according to their estimated relevance for the classification task and their redundancy with other features. For this purpose, we proposed a framework for relevance and redundancy measures and, within this framework, we proposed two new measures. These were the value difference metric and the fit criterion. Both measures performed well in comparison with other previously used ones for evaluating features. We also proposed a Hopfield network as a method for feature selection, which in experiments gave one of the best results relative to other previously used approaches. We proposed a genetic algorithm for clustering and tested it on several realworld datasets. This genetic algorithm was novel in several ways, including (i) the use of intra-cluster distance as additional optimization criterion, (ii) an annealing procedure, and (iii) adaptation of mutation rates. As opposed to many conventional clustering algorithms, our optimization framework allowed us to use different cluster validation measures including those which do not rely on cluster centroids. We demonstrated the use of the clustering algorithm experimentally with several cluster validity measures as optimization criteria. We compared the performance of our clustering algorithm to that of the often-used fuzzy c-means algorithm on several standard machine learning datasets from the University of California/Urvine (UCI) and obtained good results. The organization of representations in the brain has been observed at several stages of processing to spatially decompose input from the environment into features that are somehow relevant from a behavioral or perceptual standpoint. For the perception of smells, the analysis of such an organization, however, is not as straightforward because of the missing metric. Some studies report spatial clusters for several combinations of physico-chemical properties in the olfactory bulb at the level of the glomeruli. We performed a systematic study of representations based on a dataset of activity-related images comprising more than 350 odorants and covering the whole spatial array of the first synaptic level in the olfactory system. We found clustered representations for several physico-chemical properties. We compared the relevance of these properties to activations and estimated the size of the coding zones. The results confirmed and extended previous studies on olfactory coding for physico-chemical properties. Particularly of interest was the spatial progression by carbon chain that we found. We discussed our estimates of relevance and coding size in the context of processing strategies. We think that the results obtained in this study could guide the search into olfactory coding primitives and the understanding of the stimulus space. In a second study on representations in the olfactory bulb, we grouped odorants together by perceptual categories, such as floral and fruity. By the application of the same statistical methods as in the previous study, we found clustered zones for these categories. Furthermore, we found that distances between spatial representations were related to perceptual differences in humans as reported in the literature. This was possibly the first time that such an analysis had been done. Apart from pointing towards a spatial decomposition by perceptual dimensions, results indicate that distance relationships between representations could be perceptually meaningful. In a third study, we modeled axon convergence from olfactory receptor neurons to the olfactory bulb. Sensory neurons were stimulated by a set of biologically-relevant odors, which were described by a set of physico-chemical properties that covaried with the neural and glomerular population activity in the olfactory bulb. Convergence was mediated by the covariance between olfactory neurons. In our model, we could replicate the formation of glomeruli and concentration coding as reported in the literature, and further, we found that the spatial relationships between representational zones resulting from our model correlated with reported perceptual differences between odor categories. This shows that natural statistics, including similarity of physico-chemical structure of odorants, can give rise to an ordered arrangement of representations at the olfactory bulb level where the distances between representations are perceptually relevant. / <p>QC 20120224</p>

The labyrinth seal is a non-contact annular type sealing device used to reduce the internal leakage of the working fluid which is caused by the pressure difference between each stage in a turbomachine. Reducing the leakage mass flow rate of the working fluid through the labyrinth seal is desirable because it improves the efficiency of the turbomachine. The carry-over coefficient, based on the divergence angle of the jet, changed with flow parameters with fixed seal geometry while earlier models expressed the carry-over coefficient solely as a function of seal geometry. For both compressible and incompressible flows, the Reynolds number based on clearance was the only flow parameter which could influence the carry-over coefficient. In the case of incompressible flow based on the simulations for various seal geometries and operating conditions, for a given Reynolds number, the carry-over coefficient strongly depended on radial clearance to tooth width ratio. Moreover, in general, the lower the Reynolds number, the larger is the divergence angle of the jet and this results in a smaller carry-over coefficient at lower Reynolds numbers. However, during transition from laminar to turbulent, the carry-over coefficient reduced initially and once the Reynolds number attained a critical value, the carry-over coefficient increased again. In the case of compressible flow, the carry-over coefficient had been slightly increased if radial clearance to tooth width ratio and radial clearance to tooth pitch ratio were increased. Further, the carry-over coefficient did not considerably change if only radial clearance to tooth width ratio was decreased. The discharge coefficient for compressible and incompressible flows depended only on the Reynolds number based on clearance. The discharge coefficient of the tooth in a single cavity labyrinth seal was equivalent to that in a multiple tooth labyrinth seal indicating that flow downstream had negligible effect on the discharge coefficient. In particular, for compressible fluid under certain flow and seal geometric conditions, the discharge coefficient did not increase with an increase in the Reynolds number. It was correlated to the pressure ratio, Pr. Moreover, it was also related to the fact that the flow of the fluid through the constriction became compressible and the flow eventually became choked. At low pressure ratios (less than 0.7), Saikishan’s incompressible model deviated from CFD simulation results. Hence, the effects of compressibility became significant and both the carry-over coefficient compressibility factor and the discharge coefficient compressibility factor needed to be considered and included into the leakage model. The carry-over coefficient compressibility factor, phi, had two linear relationships with positive and negative slopes regarding the pressure ratios. This result was not associated with the seal geometry because the seal geometry ratios for each instance were located within the nearly same ranges. Further, the phi-Pr relationship was independent of the number of teeth regardless of single and multiple cavity labyrinth seals. The discharge coefficient compressibility factor, psi, was a linear relationship with pressure ratios across the tooth as Saikishan predicted. However, in certain flow and seal geometric conditions, Saikishan’s model needed to be modified for the deviation appearing when the pressure ratios were decreased. Hence, a modified psi-Pr relationship including Saikishan’s model was presented in order to compensate for the deviation between the simulations and his model.

In light of increasing harmful algal blooms and the need to protect human health and aquatic resources, proactive management approaches merit further study. For this purpose I conducted field samplings to characterize plankton community composition and laboratory experiments to test some approaches to new management schemes in the lower Rincon Delta. On site measurements and microscopic analysis showed that environmental parameters and plankton community composition varied considerably among sampling stations and sampling dates. A recent modeling study suggested that manipulation of freshwater inflow to estuaries might prevent phytoplankton blooms and enhance secondary productivity. To test this theory I conducted three semi-continuous design and flow-through incubation design experiments using natural plankton assemblages. I investigated the effect of two different pulsing regimes of inflow and nutrient loading on zooplankton densities, and phytoplankton biomass and diversity. Despite differences in zooplankton structure and phytoplankton community composition between the two experiment designs, the results confirmed that pulsed inflows might alter plankton dynamics. My findings showed that 3-day pulse treatments consistently supported greater zooplankton densities and higher phytoplankton species diversity when compared to 1-day pulse treatments. In addition, accumulation of phytoplankton biovolume remained low during 3-day pulse treatments. Differences in zooplankton performance between 3-day pulse and 1-day pulse inflow treatments were likely due to the ability of phytoplankton to uptake and store greater amounts of nutrients under conditions of 3-day pulse inflow. This resulted in food of higher quality for zooplankton, and might have supported greater zooplankton population growth rates. Additionally, in an attempt to understand the mechanisms leading to high biodiversity in aquatic ecosystems, I built a resource-storage model and studied the effects of resource-storage on competition of multiple phytoplankton species on multiple abiotic resources. I compared this model with a well-established multi-species competition model. My results showed that for certain species combinations a resource-storage-based model can generate dissimilar outcomes when compared to a model without resource-storage.

La localisation d'un véhicule via les techniques de SLAM (Simultaneous Localization And Mapping pour cartographie et localisation simultanées) a connu un essor important durant les 20 dernières années. Pourtant, peu d'approches ont tenté d'étendre ces algorithmes à une flotte de véhicules malgré les nombreuses applications potentielles. C'est ici l'objectif de cette thèse. Pour ce faire, une approche de SLAM monoculaire pour un seul véhicule a d'abord été développée. Celle-ci propose de coupler un filtre de Kalman étendu avec une représentation cartésienne des amers afin de produire des cartes de faible densité mais de qualité. En effet, l'extension à plusieurs véhicules nécessite des échanges permanents par l'intermédiaire de communications sans fil. Avec peu d'amers dans les cartes, notre approche s'accommode bien du nombre de véhicules de la flotte. Des capteurs peu onéreux ont aussi été privilégiés (une unique caméra et un odomètre) afin de réduire le coût d'une extension multivéhicule. Des correctifs ont été proposés afin d'éviter les problèmes de divergence induits par les choix précédents. Des expérimentations ont montré que la solution de SLAM produite était légère et rapide tout en fournissant une localisation de qualité. La dérive, inhérente à tout algorithme de SLAM, a également fait l'objet d'une analyse. Celle-ci a été intégrée au SLAM par l'intermédiaire d'une architecture dédiée et d'un modèle dynamique. Le but est de pouvoir rendre consistante la localisation fournie par le SLAM, même en l'absence d'estimation de la dérive. Cela permet d'effectuer des fermetures de boucle ou encore d'intégrer des informations géo-référencées de manière naturelle tout en conservant l'intégrité de la solution. En multivéhicule, cet aspect est un point clef puisque chaque véhicule dérive différemment des autres. Il est donc important de le prendre en compte. Enfin, le SLAM a été étendu à plusieurs véhicules. Une structure générique a été prévue afin que notre approche monoculaire puisse être remplacée par n'importe quel algorithme de SLAM. Notre architecture décentralisée évite la consanguinité des données (le fait de compter deux fois une même information) et gère les défaillances réseau, que cela soit des ruptures de communication ou encore des latences dans la réception des données. La partie statique du modèle de dérive permet également de prendre en compte le fait que les positions initiales des véhicules d'une flotte puissent être inconnues. L'intégrité est ainsi maintenue en permanence. Enfin, notre approche étant entièrement décentralisée, elle a pu être testée et validée en simulation et avec des expérimentations réelles dans diverses configurations (convoi en colonne ou en ligne, avec 2 ou 3 véhicules).

The visual system and good ocular motor control play an important role in the effective development of gross motor, sport, fine motor and academic skills (Erhardt et al., 1988:84; Desrocher, 1999:36; Orfield, 2001:114). Various researchers report a link between ocular motor problems and attention–deficit hyperactivity disorder (ADHD) (Cheatum & Hammond, 2000:263; Farrar et al., 2001:441; Gould et al., 2001:633; Armstrong & Munoz, 2003:451; Munoz et al., 2003:510; Borsting et al., 2005:588; Hanisch et al., 2005:671; Mason et al., 2005:1345; Loe et al., 2009:432). A few studies were carried out to analyse the links between ADHD and ocular motor control with regard to matters like visual attention, visual perception and ocular motor control like eye movement outside the normal fixation point, but no studies have been reported on the status of the ocular motor control of South African populations, and the effect of visual–motor intervention on the ocular motor control or visual–motor integration of learners with ADHD. The aim of the study was twofold, namely firstly to determine the ocular motor control functions and status of visual–motor integration of a selected group of 6– tot 8–year–old learners with ADHD in Brakpan, South Africa, while the second aim was to determine whether a visualmotor– based intervention programme can improve the ocular motor control and status of the visual–motor integration of a selected group of 6– to 8–year–old learners with ADHD in Brakpan, South Africa. Statistica for Windows 2010 was used to analyse the data. The Sensory Input Screening measuring instrument and the Quick Neurological Screening Test II (QNST–II) were used to assess the ocular motor control functions (fixation, ocular alignment, visual tracking and convergence–divergence), while the Beery Developmental Test of Visual–Motor Integration (VMI–4de weergawe) was used to determine the status of the learners’ visual–motor integration (VMI), visual perception (VP) and motor coordination (MC). The Disruptive Behaviour Scale, a checklist for ADHD (Bester, 2006), was used as measuring instrument to identify the learners with ADHD. Fifty–six learners (31 boys, 25 girls, with an average age of 7,03 years +0,65) participated in the pre–test and were divided into an ADHD (n=39) and a non–ADHD (n=16) group for aim one. Two–way tables were used to determine the percentage of ocular motor control deficits in the learners with and without ADHD, and an independent t–test was used to analyse the visual–motor integration of these learners. The Pearson Chi–squared test was used to determine the practical significance of differences in VMI and VP (d>0,05). The results of the study reveal that the majority of learners displayed ocular motor control deficits, regardless of whether they were classified with ADHD or not. The biggest percentage of learners fell into Class 2 (moderate deficits), particularly with regard to horizontal (68,57%; 52,63%; w=0,16) and vertical tracking (65,71%; 73,68%), as well as convergence–divergence (80%; 78,95%; w=0,11). However, it appears that ADHD learners experience more serious problems (Class 3) with visual tracking than learners without ADHD (both eyes: 22,86%; compared to 10,53% (w=0,22); right eye: 11,43% compared to 0% (p=0,05; w=0,34); left eye: 14,29% compared to 0% (p=0,02; w=0,38)). Learners with and without ADHD displayed a practically significant difference with respect to visual perception (d=0,37) and motor coordination (d=0,5) compared to learners without ADHD (who achieved better results). For aim 2 the subjects were divided into three groups. A pre–test–post–test design compiled from an availability sample of three groups (intervention group with ADHD (n=20); control group with ADHD (n=10) and control group without ADHD (n=17)) was used for this part of the study. The intervention group participated in a nine–week (3x/week and for 45 minutes) visualmotor– based intervention programme in which the ocular motor control functions section was applied for about 5 minutes per learner. Forty–seven learners (25 boys and 22 girls) with an average age of 6,95 years (+0,69) constituted the experimental group, while a control group with ADHD with an average age of 7,2 years (+0,79) and a control group without ADHD with an average age of 7,12 years (+0,60) did not receive any intervention and just participated in the pre– and post–test opportunity. A two–way cross–tabulation table was used to determine the changes in ocular motor control functions. These results mainly revealed that practically significant changes occurred in all three groups, be it improvement or deterioration in the various classes of ocular motor control. It appears that as far as horizontal and vertical visual tracking is concerned, and with convergence–divergence, more subjects were moved back from Class 3 (serious cases) to Class 1 (no deficits) and 2 (moderate deficits) in particular than in the other two groups that had received no intervention. Independent t–testing was used to analyse intragroup differences in the visual–motor integration subdivisions, while a covariance analysis (ANCOVA) (corrected for pre–test differences) was used to determine adjusted average post–test difference values. These results revealed that the motor coordination of the intervention group improved more than that of the control group with ADHD (p=0,18). This can lead to the conclusion that the intervention programme did have an effect on this specific skill. Abstract The overall indications of the results are that learners with ADHD have a general tendency to achieve poorer results in ocular motor control tests and with skills involving visual–motor integration, visual perception and motor coordination than learners without ADHD. Although only a minor improvement was identified in the experimental group after participation in the intervention programme, it is recommended with regard to motor coordination in particular that a similar programme be compiled for ADHD learners that focuses more specifically on the ocular motor control needs of each learner, and that it be presented on a more individual basis in order to accomplish greater improvement. / Thesis (M.A. (Kinderkinetics))--North-West University, Potchefstroom Campus, 2012.

The visual system and good ocular motor control play an important role in the effective development of gross motor, sport, fine motor and academic skills (Erhardt et al., 1988:84; Desrocher, 1999:36; Orfield, 2001:114). Various researchers report a link between ocular motor problems and attention–deficit hyperactivity disorder (ADHD) (Cheatum & Hammond, 2000:263; Farrar et al., 2001:441; Gould et al., 2001:633; Armstrong & Munoz, 2003:451; Munoz et al., 2003:510; Borsting et al., 2005:588; Hanisch et al., 2005:671; Mason et al., 2005:1345; Loe et al., 2009:432). A few studies were carried out to analyse the links between ADHD and ocular motor control with regard to matters like visual attention, visual perception and ocular motor control like eye movement outside the normal fixation point, but no studies have been reported on the status of the ocular motor control of South African populations, and the effect of visual–motor intervention on the ocular motor control or visual–motor integration of learners with ADHD. The aim of the study was twofold, namely firstly to determine the ocular motor control functions and status of visual–motor integration of a selected group of 6– tot 8–year–old learners with ADHD in Brakpan, South Africa, while the second aim was to determine whether a visualmotor– based intervention programme can improve the ocular motor control and status of the visual–motor integration of a selected group of 6– to 8–year–old learners with ADHD in Brakpan, South Africa. Statistica for Windows 2010 was used to analyse the data. The Sensory Input Screening measuring instrument and the Quick Neurological Screening Test II (QNST–II) were used to assess the ocular motor control functions (fixation, ocular alignment, visual tracking and convergence–divergence), while the Beery Developmental Test of Visual–Motor Integration (VMI–4de weergawe) was used to determine the status of the learners’ visual–motor integration (VMI), visual perception (VP) and motor coordination (MC). The Disruptive Behaviour Scale, a checklist for ADHD (Bester, 2006), was used as measuring instrument to identify the learners with ADHD. Fifty–six learners (31 boys, 25 girls, with an average age of 7,03 years +0,65) participated in the pre–test and were divided into an ADHD (n=39) and a non–ADHD (n=16) group for aim one. Two–way tables were used to determine the percentage of ocular motor control deficits in the learners with and without ADHD, and an independent t–test was used to analyse the visual–motor integration of these learners. The Pearson Chi–squared test was used to determine the practical significance of differences in VMI and VP (d>0,05). The results of the study reveal that the majority of learners displayed ocular motor control deficits, regardless of whether they were classified with ADHD or not. The biggest percentage of learners fell into Class 2 (moderate deficits), particularly with regard to horizontal (68,57%; 52,63%; w=0,16) and vertical tracking (65,71%; 73,68%), as well as convergence–divergence (80%; 78,95%; w=0,11). However, it appears that ADHD learners experience more serious problems (Class 3) with visual tracking than learners without ADHD (both eyes: 22,86%; compared to 10,53% (w=0,22); right eye: 11,43% compared to 0% (p=0,05; w=0,34); left eye: 14,29% compared to 0% (p=0,02; w=0,38)). Learners with and without ADHD displayed a practically significant difference with respect to visual perception (d=0,37) and motor coordination (d=0,5) compared to learners without ADHD (who achieved better results). For aim 2 the subjects were divided into three groups. A pre–test–post–test design compiled from an availability sample of three groups (intervention group with ADHD (n=20); control group with ADHD (n=10) and control group without ADHD (n=17)) was used for this part of the study. The intervention group participated in a nine–week (3x/week and for 45 minutes) visualmotor– based intervention programme in which the ocular motor control functions section was applied for about 5 minutes per learner. Forty–seven learners (25 boys and 22 girls) with an average age of 6,95 years (+0,69) constituted the experimental group, while a control group with ADHD with an average age of 7,2 years (+0,79) and a control group without ADHD with an average age of 7,12 years (+0,60) did not receive any intervention and just participated in the pre– and post–test opportunity. A two–way cross–tabulation table was used to determine the changes in ocular motor control functions. These results mainly revealed that practically significant changes occurred in all three groups, be it improvement or deterioration in the various classes of ocular motor control. It appears that as far as horizontal and vertical visual tracking is concerned, and with convergence–divergence, more subjects were moved back from Class 3 (serious cases) to Class 1 (no deficits) and 2 (moderate deficits) in particular than in the other two groups that had received no intervention. Independent t–testing was used to analyse intragroup differences in the visual–motor integration subdivisions, while a covariance analysis (ANCOVA) (corrected for pre–test differences) was used to determine adjusted average post–test difference values. These results revealed that the motor coordination of the intervention group improved more than that of the control group with ADHD (p=0,18). This can lead to the conclusion that the intervention programme did have an effect on this specific skill. Abstract The overall indications of the results are that learners with ADHD have a general tendency to achieve poorer results in ocular motor control tests and with skills involving visual–motor integration, visual perception and motor coordination than learners without ADHD. Although only a minor improvement was identified in the experimental group after participation in the intervention programme, it is recommended with regard to motor coordination in particular that a similar programme be compiled for ADHD learners that focuses more specifically on the ocular motor control needs of each learner, and that it be presented on a more individual basis in order to accomplish greater improvement. / Thesis (M.A. (Kinderkinetics))--North-West University, Potchefstroom Campus, 2012.

Esta tesis doctoral es una colección de tres artículos. Los capítulos 1 y 2, co-autorados con Joachim Voth, investigan por qué Europa en 1700 ya era más rico que el resto del mundo y por qué Inglaterra fue el primer país en industrializarse. Encontramos que las dinámicas de la población, en lugar del crecimiento de la productividad, fueron los promotores más importantes del desarrollo económico de Europa Occidental durante la temprana edad moderna (1450-1700). Calibramos un modelo probabilístico para representar Inglaterra en 1700 y encontramos que ingresos iniciales más altos unidos a limitaciones de fertilidad aumentaron la probabilidad de industrialización. En el tercer capítulo, presento un nuevo hecho estilizado y analizo su contribución al sesgo del cambio tecnológico hacia los trabajadores más cualificados: El porcentaje de trabajadores cualificados en la producción intermedia está altamente correlacionado con la proporción de trabajo cualificado en la producción final. Esto genera un efecto multiplicador que refuerza la demanda de trabajo cualificado a lo largo de la cadena de producción. El efecto es importante, explica más de un tercio del aumento de la demanda de trabajadores cualificados en la industria manufacturera de EE.UU. / This dissertation is a collection of three essays. Chapters 1 and 2, co-authored with Joachim Voth, investigate the question why Europe in 1700 was ahead of the rest of the world and why England was the first country to industrialize. We find that population dynamics, rather than productivity growth, were the most important drivers for Western Europe to overtake China in the early modern period (1450-1700). We calibrate a probabilistic model to match England in 1700 and find that higher initial per capita incomes together with fertility limitation increased its industrialization probabilities. In the third chapter, I present a novel stylized fact and analyze its contribution to the skill bias of technical change: The share of skilled labor embedded in intermediate inputs correlates strongly with the skill share employed in final production. This delivers a multiplier that reinforces skill demand along the production chain. The effect is large, accounting for more than one third of the observed skill upgrading in U.S. manufacturing.

This thesis fills in the gaps in the existing theory of wave phenomena in thick diffraction gratings at extreme angles of scattering, i.e. when the scattered wave propagates parallel or almost parallel to the grating boundaries. A consistent theory of a new type of Bragg scattering of bulk and guided optical modes in thick uniform and non-uniform, dissipative and non-dissipative, slanted periodic gratings has been developed. This type of scattering is called extremely asymmetrical scattering (EAS). One of the main distinctive features of EAS is the strong resonant increase of the scattered wave amplitude compared to the amplitude of the incident wave. Several unique combinations of strong resonances shaping a complex multi-resonant pattern of EAS in different types of gratings have been predicted and investigated theoretically and numerically. This includes the prediction of a new resonant wave effect in non-uniform gratings with varying phase – double-resonant EAS, the discovery of several sharp and strong resonances with respect to scattering angle in gratings with the scattered wave propagating almost parallel to the grating boundaries (grazing-angle scattering (GAS)) for the case of second-order scattering, and the prediction of a new type of eigenmode in gratings with second-order scattering (especially in gratings with large amplitude). In addition, several other important practical problems that may be crucial for the experimental observation and application of EAS and GAS have been solved. These are the determination of the tolerance of EAS to small grating imperfections, e.g., fluctuations of the grating amplitude, prediction of unusually high sensitivity of second-order EAS to small variations of mean structural parameters, determination of the effect of weak dissipation on EAS, etc. Physical reasons for the predicted resonances and effects are explained. In particular, the crucial role of the diffractional divergence for EAS and GAS has been revealed, especially for non-uniform gratings. Methods of analysis involve the approximate and rigorous approaches. The approximate method is based on understanding the role of the diffractional divergence in the geometry of EAS and the two-wave approximation (valid for any types of waves). The rigorous approach is based on the rigorous coupled-wave analysis (RCWA) and, in particular, the known enhanced T-matrix algorithm (by Moharam, et al.) that is numerically stable for narrow and wide gratings with arbitrary amplitude (valid only for bulk electromagnetic waves).