Simply connected Alexandrov 4-manifolds with positive or nonnegative curvature and torus actions

We point out that a 4-dimensional topological manifold with an Alexandrov metric (of curvature bounded below) and with an effective, isometric action of the circle or the 2-torus is locally smooth. This observation implies that the topological and equivariant classifications of compact, simply connected Riemannian 4-manifolds with positive or nonnegative sectional curvature and an effective isometric action of a circle or a 2-torus also hold if we consider Alexandrov manifolds instead of Riemannian manifolds.Comment: 10 pages, title changed, results are now proved for Alexandrov 4-manifolds, added examples, simplified proof

On maximal curves having classical Weierstrass gaps

We study geometrical properties of maximal curves having classical Weierstrass gaps.Comment: 9 pages, Latex2

On maximal curves

We study arithmetical and geometrical properties of maximal curves, that is, curves defined over the finite field F_{q^2} whose number of F_{q^2}-rational points reaches the Hasse-Weil upper bound. Under a hypothesis on non-gaps at a rational point, we prove that maximal curves are F_{q^2}-isomorphic to y^q + y = x^m, for some $m \in Z^+$. As a consequence we show that a maximal curve of genus g=(q-1)^2/4 is F_{q^2}-isomorphic to the curve y^q + y = x^{(q+1)/2}.Comment: LaTex2e, 17 pages; this article is an improved version of the paper alg-geom/9603013 (by Fuhrmann and Torres

Barriers to Entry in Monopoly Markets: Automobile Distribution in Brazil

O Objetivo deste Trabalho é Analisar os Efeitos da Entrada de uma Segunda Concessionária de Automóveis em Mercados Previamente Monopolizados. para Tanto, Construiu-Se um Banco de Dados com a Localização de Concessionárias de Automóveis em Microrregiões e Características Demográficas e Econômicas Destas Microrregiões. a Partir Desse Banco de Dados e de Modelos de Escolha Binária, Foram Identificadas Variáveis que Condicionam a Existência e o Número de Concessionárias em Microrregiões. Utilizando-Se de um Modelo Adaptado de Bresnahan e Reiss (1990), Foram Estimados os Custos Fixos de Entrada de Concessionárias em Mercados Monopolizados. os Resultados Obtidos Sugerem que as Barreiras À Entrada não são Significativas, o que Aumenta a Probabilidade de que a Cláusula de Exclusividade nos Contratos de Concessão não Cause Danos À Concorrência no Mercado Brasileiro de Distribuição de Automóveis

Institutions, Credit rationing and housing development

One of the basic principles that allow a smooth operation of the markets is the equilibrium between supply and demand. According to this principle, when demand exceeds supply, the price mechanism will try to bring the system back into equilibrium. When this thinking is applied to the housing market, it leads to the conclusion that any inequality in housing supply or demand is transitory. Nonetheless, the fact that a considerable share of the population live in precarious homes for generations seems to speak against the virtues of market mechanisms in the resolution of housing disequilibria. Stiglitz and Weiss (1981) argue that in the face of asymmetric information, under some conditions the equilibrium of the credit market can be marked by rationing. Asymmetric information – working through the effects of adverse selection and of incentive – has impacts on the return function of bank loans, which leads to interest rates used in housing loans to be different from those that balance supply and demand for credit, causing credit rationing. Literature of the New Institutional Economics (NIE) in turn points out the fact that institutions can reduce the degree of uncertainty by lessening the effects of asymmetric information. Regarding the housing market, the degree of property rights, as well as the mortgage institution which acts as a contract enforcement tool, provide the credit market with information on the quality of the borrower and thus broaden the social scope of this market. The purpose of this article is to understand how the equilibrium in the housing market is influenced by credit rationing and to what extend institutional development affects this scarcity and the interest rates of housing loans. The model developed in this article, which combines the tradition of dynamic models of housing investment with the premises of the New Institutional Economics and the considerations of Stiglitz and Weiss (1981) and (1992) on rationing in the credit market, allows us to identify the role of institutions on housing development.Asymmetric information, Rationed credit market, Property rights, Mortgage foreclosure costs, Housing development

Flux switching alternators for small wind generation

This paper considers the application of flux switching alternators for small wind generation. First, after a brief presentation of the small wind energy systems, a description and the fundamentals of flux switching machines is given, then the state of art of flux switching generators is presented. Finally a critical assessment is shown, considering the main advantages and drawbacks of this type of machines as alternator for use in small wind generation systems.Peer ReviewedPostprint (published version