20,945 research outputs found
On Ptolemaic metric simplicial complexes
We show that under certain mild conditions, a metric simplicial complex which
satisfies the Ptolemy inequality is a CAT(0) space. Ptolemy's inequality is
closely related to inversions of metric spaces. For a large class of metric
simplicial complexes, we characterize those which are isometric to Euclidean
space in terms of metric inversions.Comment: 13 page
Toward a dynamical systems analysis of neuromodulation
This work presents some first steps toward a
more thorough understanding of the control systems
employed in evolutionary robotics. In order
to choose an appropriate architecture or to construct
an effective novel control system we need
insights into what makes control systems successful,
robust, evolvable, etc. Here we present analysis
intended to shed light on this type of question
as it applies to a novel class of artificial neural
networks that include a neuromodulatory mechanism:
GasNets.
We begin by instantiating a particular GasNet
subcircuit responsible for tuneable pattern generation
and thought to underpin the attractive
property of “temporal adaptivity”. Rather than
work within the GasNet formalism, we develop an
extension of the well-known FitzHugh-Nagumo
equations. The continuous nature of our model
allows us to conduct a thorough dynamical systems
analysis and to draw parallels between this
subcircuit and beating/bursting phenomena reported
in the neuroscience literature.
We then proceed to explore the effects of different
types of parameter modulation on the system
dynamics. We conclude that while there are
key differences between the gain modulation used
in the GasNet and alternative schemes (including
threshold modulation of more traditional synaptic
input), both approaches are able to produce
tuneable pattern generation. While it appears, at
least in this study, that the GasNet’s gain modulation
may not be crucial to pattern generation ,
we go on to suggest some possible advantages it
could confer
Complex surfaces plated by thin-film deposition in one operation
Ion plating deposits thin film on complex surface in one operation. The ionized materials follow electric lines of force to all points on the objects, uniformly plating the surface from all sides simultaneously
Host-Source Country linkages as determinants of foreign acquisitions by Indian MNEs
This experimental paper explains foreign acquisitions by Indian multinational enterprises by reference to location specific factors in both the source country (India) and host countries together with variables required designed to capture the distance between India and the host country, both geographic and psychic. The paper finds that country specific advantages play an important role in explaining Indian foreign acquisitions. The general model performs well and Indian institutional and domestic capital variables add explanatory value
Is Accra a superstar city ?
A recent study of house price behavior in U.S. cities by Gyourko, Mayer, and Sinai (2006) raises questions about so-called superstar cities in which housing is so inelastically supplied that it becomes unaffordable, as higher-income families outbid residents. We consider the case of Accra, Ghana, in this light, estimating the elasticity of housing supply and discussing the implications for growth and income distribution. There is not a great deal of data available to examine trends in Accra, so our method is indirect. First, we use a variant of the traditional monocentric city model to calculate the elasticity of Accra's housing supply relative to those of other similarly-sized African cities. This suggests that housing supply responsiveness is much higher elsewhere. This muted supply responsiveness is consistent with the observed higher housing prices. Second, we estimate a number of traditional housing demand equations and reduced form equations. Placing a number of restrictions on the equations allows us to infer Accra's housing supply elasticity. Taken together, our approaches suggest that lower-income families in Accra have such poor housing conditions because the market is extremely unresponsive to demand.Although the outcomes we have traced-high housing prices and low quality-are not unusual relative to the other developed country superstar cities, they are extreme. The welfare costs are considerable, so much so that in addition to direct housing market effects, these policies also appear to have potentially significant implications for the achievement of more equitable growth.Economic Theory&Research,Housing&Human Habitats,Banks&Banking Reform,,Public Sector Management and Reform
Characterization of erosion of metallic materials under cavitation attack in a mineral oil
Cavitation erosion and erosion rates of eight metallic materials representing three crystal structures were studied using a 20-kHz ultrasonic magnetostrictive oscillator in viscous mineral oil. The erosion rates of the metals with an fcc matrix were 10 to 100 times higher than that of an hcp-matrix titanium alloy. The erosion rates of iron and molybdenum, with bcc matrices, were higher than that of the titanium alloy but lower than those of the fcc metals. Scanning electron microscopy indicates that the cavitation pits are initially formed at the grain boundaries and precipitates and that the pits that formed at the triple points grew faster than the others. Transcrystalline craters formed by cavitation attack over the surface of grains and roughened the surfaces by multiple slip and twinning. Surface roughness measurements show that the pits that formed over the grain boundaries deepended faster than other pits. Computer analysis revealed that a geometric expression describes the nondimensional erosion curves during the time period 0.5 t(0) t 2.5 t(0), where t(0) is the incubation period. The fcc metals had very short incubation periods; the titanium alloy had the longest incubation period
Characterization of erosion of metallic materials under cavitation attack in a mineral oil
Cavitation erosion and erosion rates of eight metallic materials representing three crystal structures were studied. The erosion experiments were conducted with a 20-kHz ultrasonic magnetostrictive oscillator in a viscous mineral oil. The erosion rates of the metals with an fcc matrix were 10 to 100 times higher than that of an hop-matrix titanium alloy. The erosion rates of iron and molybdenum, with bcc matrices, were higher than that of the titanium alloy but lower than those of those of the fcc materials. Studies with scanning electron microscopy indicated that the cavitation pits were initially formed at the grain boundaries and precipitates and that the pits formed at the junction of grain boundaries grew faster than the others. Transcrystalline craters formed by cavitation attack over the surface of grains and roughened the surfaces by multiple slip and twinning. Surface roughness measurements showed that the pits that formed over the grain boundaries deepened faster than pits. Computer analysis revealed that a geometric expression describes the nondimensional erosion curves during the time period 0.5 t (sub 0) t 2.5 t (sub 0), where t (sub 0) is the incubation period. The fcc metals had very short incubation periods; the titanium alloy had the longest incubation period
Cavitation Erosion of Copper, Brass, Aluminum and Titanium Alloys in Mineral Oil
The variations of the mean depth of penetration, the mean depth rate of penetration, MDRP, the pit diameter 2a and depth h due to cavitation attack on Al 6061-T6, Cu, brass of composition Cu-35Zn-3Pb and Ti-5A1-2.5Sn are presented. The experiments are conducted in a mineral oil of viscosity 110 CS using a magnetostrictive oscillator of 20 kHz frequency. Based on MDRP on the materials, it is found that Ti-5Al-2.5Sn exhibits cavitation erosion resistance which is two orders of magnitude higher than the other three materials. The values of h/a are the largest for copper and decreased with brass, titanium, and aluminum. Scanning electron microscope studies show that extensive slip and cross slip occurred on the surface prior to pitting and erosion. Twinning is also observed on copper and brass
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