632 research outputs found

    Almost complete and equable heteroclinic networks

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    Heteroclinic connections are trajectories that link invariant sets for an autonomous dynamical flow: these connections can robustly form networks between equilibria, for systems with flow-invariant spaces. In this paper we examine the relation between the heteroclinic network as a flow-invariant set and directed graphs of possible connections between nodes. We consider realizations of a large class of transitive digraphs as robust heteroclinic networks and show that although robust realizations are typically not complete (i.e. not all unstable manifolds of nodes are part of the network), they can be almost complete (i.e. complete up to a set of zero measure within the unstable manifold) and equable (i.e. all sets of connections from a node have the same dimension). We show there are almost complete and equable realizations that can be closed by adding a number of extra nodes and connections. We discuss some examples and describe a sense in which an equable almost complete network embedding is an optimal description of stochastically perturbed motion on the network

    Distribution of knowledge production in the chemical sciences in the US

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    Thesis (M.B.A.)--Massachusetts Institute of Technology, Sloan School of Management, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 43-44).A citation analysis was carried out to gain an understanding of the geographical and institutional distribution of highly cited articles in the chemical sciences in the US over the last thirty years. The contribution of US chemistry departments was determined by quantifying the number of highly cited articles published by individual authors or groups of authors from the same department. Articles stemming from collaborative research across schools were not considered. The results show that a dilution in intradepartmental knowledge production has occurred both on a geographical and institutional level. Three chemistry departments have emerged as strong producers of high impact articles over the last thirty years: the University of North Carolina, Texas A&M University and the University of Utah. In terms of aggregate numbers of highly cited articles these three schools are in the top ten of over seventy schools which were evaluated; their chemistry departments are en par in terms of scientific impact with those from Ivy League schools like Stanford University, Harvard University and the California Institute of Technology. While the literature reports increasing concentration for the US research base, the present analysis shows a dilution in chemical knowledge production when collaborative efforts across departments and schools are excluded. This finding suggests that the increase in concentration in the US science base is not a uniform trend when studied on a more granular level.by Peter A. Lohse.M.B.A

    Investments in the human capital of the socially disadvantaged children: Effects on redistribution

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    Recently, early investments in the human capital of children from socially disadvantaged environments have attracted a great deal of attention. Programs of such early intervention, aiming at children's health and well-being, are spreading considerably in the U.S. and are currently tested in several European countries. In a discrete version of the Mirrlees model with a parents' and a children's generation we show the intra-generational and the inter-generational redistributional consequences of such intervention programs. It turns out that the parents' generation always loses when such intervention programs are implemented. Among the children's generation it is the rich who always benefit. Despite the expectation that early intervention puts the poor descendants in a better position, our analysis reveals that the poor among the children's generation may even be worse off if the effect of early intervention on their productivity is not large enough. --Early Intervention,welfare,redistribution,taxation

    Phase Diagram of Vertically Shaken Granular Matter

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    A shallow, vertically shaken granular bed in a quasi 2-D container is studied experimentally yielding a wider variety of phenomena than in any previous study: (1) bouncing bed, (2) undulations, (3) granular Leidenfrost effect, (4) convection rolls, and (5) granular gas. These phenomena and the transitions between them are characterized by dimensionless control parameters and combined in a full experimental phase diagram.Comment: 11 pages, 14 figures, submitted to "Physics of Fluids

    Virtuelle Eingliederungsarbeitsmärkte: Ein Weg aus der Arbeitslosigkeitsfalle

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    Die große Koalition in Berlin sieht sich gewaltigen Herausforderungen auf dem Gebiet des Arbeitsmarktes und der Sozialpolitik gegenüber. Entgegen allen Hoffnungen und Anstrengungen verharrt die Unterbeschäftigung weiter auf hohem Niveau. Inwiefern geraten Arbeitslose in die Arbeitslosigkeitsfalle? Wie könnte der Arbeitsmarkt für Geringqualifizierte besser organisiert werden? --

    Spontaneous Ratchet Effect in a Granular Gas

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    The spontaneous clustering of a vibrofluidized granular gas is employed to generate directed transport in two different compartmentalized systems: a "granular fountain" in which the transport takes the form of convection rolls, and a "granular ratchet" with a spontaneous particle current perpendicular to the direction of energy input. In both instances, transport is not due to any system-intrinsic anisotropy, but arises as a spontaneous collective symmetry breaking effect of many interacting granular particles. The experimental and numerical results are quantitatively accounted for within a flux model.Comment: 4 pages, 5 figures; Fig. 4 has been reduced in size and qualit

    Effects of Extreme Drought on Vegetation of a Lava Flow on Mauna Loa, Hawai'i

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    Effects of an extreme drought were examined along an elevational gradient on Mauna Loa Volcano, Hawai'i. The composition, vigor, and survivorship of plants were examined on a 2400-yr-old pahoehoe lava flow at three elevations: 1755,2000, and 2195 m above sea level. Three plant species, Coprosma ernodeoides A. Gray, Styphelia tameiameiae (Cham. & ScWechtend.) F. v. Muell., and Vaccinium reticulatum Sm., were encountered most frequently at the three sites. Greatest mortality occurred at the site at 2000 m elevation, where the drought caused a shift from a slight excess of precipitation over evaporation to a large excess of evaporation. Occasional severe droughts may play an important part in shaping primary succession in this region

    Crystal Nucleation by Laser-Induced Cavitation\ud

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    High-speed and high-resolution photography have been used to investigate the relationship between creation, expansion, and collapse of a vapor cavity induced by a 6 ns laser pulse and the subsequent nucleation of crystals. A thin layer of supersaturated aqueous solutions of (NH4)2SO4 and KMnO4 was confined between two glass plates with a separation of 50 and 100 μm. The expansion and collapse of the laser-induced vapor bubble occurred over a total time scale of 200 μs, while the first identifiable crystal appears one second after the laser pulse. Crystals were observed to form on a ring with a diameter of 70 μm centered in the focal point of the laser. The ring is preceded by an optical disturbance observed through the cavity around 30–50 μs after the laser pulse and vapor cavity formation. This ring-shaped optical disturbance originates from changes in refractive index induced by crystal nuclei formation. The formation of the nuclei most probably coincides with the formation of the bubble, when the rate of evaporation and the supersaturation are at their maxima. Apparently, it takes the nuclei around 30–50 μs to grow to a particle size with a visible optical disturbanc
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