Darbas skirtas hiperbolinio tipo I rūšies beveik kontaktinėms metrinėms struktūroms, arba -struktūroms, egzistuojančioms hiperbolinio tipo A-erdvės hiperpaviršiuose. / Working for the hyperbolic type of class I almost contact metric structures existing in the hypersurfaces of hyperbolic type A-space.

This thesis deals with an implementation of algorithm for computation of generator of almost split sequences ending at an indecomposable nonprojective module of path algebra over finite quiver. Algorithm is implemented in algebra system GAP (Groups, Algorithms, Programming) with additional package QPA (Quivers and Path Algebras). Powered by TCPDF (www.tcpdf.org)

Étant donné une variété symplectique $(M,\omega)$, il existe toujours des structures presque complexes $\omega$-compatibles positives. La question qui nous intéresse est de trouver des méthodes de sélection de certaines de ces structures. Des réponses ont déjà été données par V. Apostolov et T.Draghici, J.G. Evans, et J. Keller et M. Lejmi. Nous nous intéressons ici principalement à des méthodes de sélection définies en termes du tenseur de Nijenhuis. De manière très générale, lorsqu'on veut sélectionner certaines données géométriques, on peut aborder le problème de différentes manières. L’une d’entre elles consiste à regarder la décomposition en composantes irréductibles de certains tenseurs naturellement associés à la structure considérée et poser des conditions sur certaines composantes. Nous avons montré que le tenseur de Nijenhuis est irréductible sous l'action du groupe unitaire. Cette irréductibilité ne nous permet pas d'imposer d'autre condition linéaire à ce tenseur que son annulation, qui correspond aux variétés de Kähler. Une autre méthode possible de sélection est d’imposer des conditions à certaines distributions liées au problème. Nous avons étudié des distributions liées au tenseur de Nijenhuis. Nous nous sommes intéressés ici aux dimensions et propriétés d’involutivité possibles de ces distributions. Nous donnons des exemples invariants sous l’action d’un groupe, construits sur des groupes symplectiques ou sur des fibrés de twisteurs sur une variété riemannienne. La dernière méthode envisagée dans ce travail est la considération de fonctionnelles définies à partir des données. Pour construire une fonctionnelle la plus simple possible en termes du tenseur de Nijenhuis, nous intégrons une fonction polynomiale du second degré en les composantes du tenseur de Nijenhuis. On montre qu’un tel polynôme est toujours un multiple de la norme au carré de ce tenseur. La fonctionnelle obtenue est celle étudiée par Evans. Elle est a priori peu intéressante pour notre problème de sélection car il a prouvé qu’on peut trouver des exemples de variétés symplectiques n’admettant aucune structure kählérienne mais telle que l’infimum de la fonctionnelle soit nul / Given a symplectic manifold $(M,\omega)$, there always exist almost complex $\omega,$-compatible positive structures. The problem studied in this thesis is to find methods to select some of these structures. Answers have already been suggested by V. Apostolov and T.Draghici, J. G. Evans, and J. Keller and M. Lejmi. We are mainly interested here in selection methods defined in terms of the Nijenhuis tensor. The problem of selecting geometric objects can be tackled in various ways. One of them is to decompose into irreducible components some tensors naturally associated with the structure, and to impose conditions on some of those components. We prove that the Nijenhuis tensor is irreducible under the action of the unitary group. This irreducibility does not allow to impose any linear condition on the Nijenhuis tensor, except the vanishing of it, which corresponds to Kähler manifolds. Another possible method of selection is to impose conditions on distributions related to the problem. We study distributions defined by the Nijenhuis tensor. Our results concern the possible dimensions and properties of involutivity of these distributions. We give examples which are invariant under the action of a group, on some symplectic groups and on twisted bundles over some Riemannian manifolds. The last method considered in this work consists in looking for extremals of functionals defined from the data. To construct the simplest functional defined in terms of the Nijenhuis tensor, we integrate a polynomial function of the second degree into the components of this tensor. All such polynomials are multiple of the square of the norm of this tensor. This functional is the one studied by Evans; the drawback for our selection problem is that there exist examples of compact symplectic manifolds which do not admit any K\"ahler structure but such that the infimum of the functional is zero

O consumo per capita de pescados no Brasil é relativamente baixo quando comparado com as outras proteínas de origem animais. Do lado da oferta, este fenômeno está relacionado com diversos fatores como, por exemplo, a sobre pesca, a baixa produção nacional, a distância entre centros produtores e consumidores etc. Este trabalho aborda os fatores que estão ligados à sua demanda, tais como: a influência dos preços e da renda da população no seu consumo. Inicialmente, caracterizou-se o problema da oferta de pescados no Brasil. Em seguida, foi feita uma revisão sobre a teoria econômica e o método de cálculo da função e de suas elasticidades para a forma funcional conhecida por Almost Ideal Demand System - AIDS. A partir dos microdados da Pesquisa de Orçamento Familiar - POF 2002-2003, fez-se uma análise descritiva da demanda por pescados no Brasil. Finalmente, foram estimadas as funções demanda e calcularam-se as elasticidades para dois tipos de agrupamentos: um que considerou 5 grupos de proteínas animais e o outro com 7 grupos de alimentos. Estas funções foram estimadas para o Brasil e para duas macro-regiões: Norte-Nordeste e Centro-Sul. Os principais resultados são: o consumo per capita de pescados é baixo porque poucos domicílios consomem pescados. O consumo de pescado da Região Norte-Nordeste é significativamente diferente do padrão observado na Região Centro-Sul do país. Os principais produtos substitutos aos pescados no país são as proteínas mais elaboradas e não as carnes mais tradicionais como a de aves e as vermelhas. Os supermercados são os pontos de vendas mais utilizados pelos consumidores de pescados de renda mais elevada, principalmente na Região Centro-Sul, mas os pequenos estabelecimentos comerciais também possuem grande importância na comercialização de pescados para o consumidor final, principalmente, na Região Norte-Nordeste. / Per capita consumption of fish in Brazil is relatively small as compared to other animal proteins. On the supply side, this phenomenon can be explained by several factors such as: low national fish production, the distance between fish supply regions and the main consumptions centers etc. This study analyses the influence of prices and population income on the demand of fish in Brazil. First, the problem of fish supply in Brazil is characterized. It is followed by reviews of the relevant economic theory and the methods of the function and the elasticity calculations for a functional form known by Almost Ideal Demand System - AIDS. A descriptive analysis of fish demand in Brazil using the microdata called the Familiar Budget Research - POF 2002-2003 is presented. Finally, demand functions and their elasticities are calculated for two different cases: one considering 5 groups of animal proteins and other with 7 groups of food categories. These functions are estimated for Brazil as a whole and two macro-regions: Northnortheast and Center-South. The main results are: per capita consumption of fish is low in Brazil because few households consume fish. The pattern of fish consumption in the North-Northeast Region is different as compared to the Center-South. The main substitutes for fish are the processed proteins and not the traditional types of meat as chicken and red meat. For high income households located mainly in Center-South Region, fish are mainly purchased in supermarkets. However, small commercial establishments are still important in the fish retail market, especially in the North-Northeast.

O consumo per capita de pescados no Brasil é relativamente baixo quando comparado com as outras proteínas de origem animais. Do lado da oferta, este fenômeno está relacionado com diversos fatores como, por exemplo, a sobre pesca, a baixa produção nacional, a distância entre centros produtores e consumidores etc. Este trabalho aborda os fatores que estão ligados à sua demanda, tais como: a influência dos preços e da renda da população no seu consumo. Inicialmente, caracterizou-se o problema da oferta de pescados no Brasil. Em seguida, foi feita uma revisão sobre a teoria econômica e o método de cálculo da função e de suas elasticidades para a forma funcional conhecida por Almost Ideal Demand System - AIDS. A partir dos microdados da Pesquisa de Orçamento Familiar - POF 2002-2003, fez-se uma análise descritiva da demanda por pescados no Brasil. Finalmente, foram estimadas as funções demanda e calcularam-se as elasticidades para dois tipos de agrupamentos: um que considerou 5 grupos de proteínas animais e o outro com 7 grupos de alimentos. Estas funções foram estimadas para o Brasil e para duas macro-regiões: Norte-Nordeste e Centro-Sul. Os principais resultados são: o consumo per capita de pescados é baixo porque poucos domicílios consomem pescados. O consumo de pescado da Região Norte-Nordeste é significativamente diferente do padrão observado na Região Centro-Sul do país. Os principais produtos substitutos aos pescados no país são as proteínas mais elaboradas e não as carnes mais tradicionais como a de aves e as vermelhas. Os supermercados são os pontos de vendas mais utilizados pelos consumidores de pescados de renda mais elevada, principalmente na Região Centro-Sul, mas os pequenos estabelecimentos comerciais também possuem grande importância na comercialização de pescados para o consumidor final, principalmente, na Região Norte-Nordeste. / Per capita consumption of fish in Brazil is relatively small as compared to other animal proteins. On the supply side, this phenomenon can be explained by several factors such as: low national fish production, the distance between fish supply regions and the main consumptions centers etc. This study analyses the influence of prices and population income on the demand of fish in Brazil. First, the problem of fish supply in Brazil is characterized. It is followed by reviews of the relevant economic theory and the methods of the function and the elasticity calculations for a functional form known by Almost Ideal Demand System - AIDS. A descriptive analysis of fish demand in Brazil using the microdata called the Familiar Budget Research - POF 2002-2003 is presented. Finally, demand functions and their elasticities are calculated for two different cases: one considering 5 groups of animal proteins and other with 7 groups of food categories. These functions are estimated for Brazil as a whole and two macro-regions: Northnortheast and Center-South. The main results are: per capita consumption of fish is low in Brazil because few households consume fish. The pattern of fish consumption in the North-Northeast Region is different as compared to the Center-South. The main substitutes for fish are the processed proteins and not the traditional types of meat as chicken and red meat. For high income households located mainly in Center-South Region, fish are mainly purchased in supermarkets. However, small commercial establishments are still important in the fish retail market, especially in the North-Northeast.

Herein we consider material tensors that pertain to thin sheets or thin films, which we model as two-dimensional objects. We assume that the thin sheet in question carries a crystallographic texture characterized by an orientation distribution function defined on the rotation group SO(3), which is almost transversely-isotropic about the sheet normal so that mechanical and physical properties of the thin sheet have weak planar-anisotropy. We present a procedure by which a special orthonormal basis can be determined in each tensor subspace invariant under the action of the orthogonal group O(2). We call members of such special bases irreducible basis tensors under O(2). For the class of thin sheets in question, we derive a representation formula in which each tensor in any given tensor subspace Z is written as the sum of a transversely-isotropic term and a linear combination of orthonormal irreducible basis tensors in Z, where the coefficients are given explicitly in terms of texture coefficients and undetermined material parameters. In addition to the general representation formula, we present also the specialized form for subspaces of tensor products of second-order symmetric tensors, a type commonly found in mechanics of materials.

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This thesis contains three essays on topics in agricultural economics. Essays one and two share a focus on international trade and economic development, and essays two and three apply dynamic tools to agricultural economic policy issues.<p>Essay one analyses trade-related implications of a developing country's decision to adopt genetically-modified crop technology. A fixed-proportions model is constructed that evaluates the welfare implications of a range of adoption policies and export market responses. The model in this essay illustrates the importance of the prospective adopter formulating a projection of probable export market effects before making an adoption decision and of the role that high transaction costs may play in a developing country's adoption decision. The model also considers the effects of a new policy tool; a check-off style levy on genetically-modified technology in place of a technology-use agreement. A levy could be useful tool in developing countries, which are characterised by high transaction costs. <p>Essay two models the effects of emergency food aid on a recipient country's agricultural industry. This essay formulates a definition of needed aid in the context of a food emergency and constructs an optimal control model that solves a path of aid shipments that best meets that need. The effects of a range of food aid paths on recipient-country agricultural production are illustrated through numerical simulations. There are two key results. First, a non-optimal amount of aid can hinder a recipient-country's recovery from an exogenous food shock. Second, an exogenous shock can affect farmer revenue and therefore impact planting decisions. This effect must be considered in aid allocation policies. <p>Essay three uses time-series econometric techniques to develop a demand model that assesses the effectiveness of commodity advertising. This essay describes the importance of considering long-run and dynamic effects in demand systems, especially in the case of closely substitutable commodities. A demand system that tests for and accommodates dynamic and time-series properties is developed and applied to US meat data. The results of this model are compared to a traditional static demand system. The dynamic model produces econometrically and theoretically sound results and generates some more intuitively appealing estimates.

This thesis contains three essays on topics in agricultural economics. Essays one and two share a focus on international trade and economic development, and essays two and three apply dynamic tools to agricultural economic policy issues.<p>Essay one analyses trade-related implications of a developing country's decision to adopt genetically-modified crop technology. A fixed-proportions model is constructed that evaluates the welfare implications of a range of adoption policies and export market responses. The model in this essay illustrates the importance of the prospective adopter formulating a projection of probable export market effects before making an adoption decision and of the role that high transaction costs may play in a developing country's adoption decision. The model also considers the effects of a new policy tool; a check-off style levy on genetically-modified technology in place of a technology-use agreement. A levy could be useful tool in developing countries, which are characterised by high transaction costs. <p>Essay two models the effects of emergency food aid on a recipient country's agricultural industry. This essay formulates a definition of needed aid in the context of a food emergency and constructs an optimal control model that solves a path of aid shipments that best meets that need. The effects of a range of food aid paths on recipient-country agricultural production are illustrated through numerical simulations. There are two key results. First, a non-optimal amount of aid can hinder a recipient-country's recovery from an exogenous food shock. Second, an exogenous shock can affect farmer revenue and therefore impact planting decisions. This effect must be considered in aid allocation policies. <p>Essay three uses time-series econometric techniques to develop a demand model that assesses the effectiveness of commodity advertising. This essay describes the importance of considering long-run and dynamic effects in demand systems, especially in the case of closely substitutable commodities. A demand system that tests for and accommodates dynamic and time-series properties is developed and applied to US meat data. The results of this model are compared to a traditional static demand system. The dynamic model produces econometrically and theoretically sound results and generates some more intuitively appealing estimates.

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