19,256 research outputs found
On Effective Constraints for the Riemann-Lanczos System of Equations
There have been conflicting points of view concerning the Riemann--Lanczos
problem in 3 and 4 dimensions. Using direct differentiation on the defining
partial differential equations, Massa and Pagani (in 4 dimensions) and Edgar
(in dimensions n > 2) have argued that there are effective constraints so that
not all Riemann tensors can have Lanczos potentials; using Cartan's criteria of
integrability of ideals of differential forms Bampi and Caviglia have argued
that there are no such constraints in dimensions n < 5, and that, in these
dimensions, all Riemann tensors can have Lanczos potentials. In this paper we
give a simple direct derivation of a constraint equation, confirm explicitly
that known exact solutions of the Riemann-Lanczos problem satisfy it, and argue
that the Bampi and Caviglia conclusion must therefore be flawed. In support of
this, we refer to the recent work of Dolan and Gerber on the three dimensional
problem; by a method closely related to that of Bampi and Caviglia, they have
found an 'internal identity' which we demonstrate is precisely the three
dimensional version of the effective constraint originally found by Massa and
Pagani, and Edgar.Comment: 9pages, Te
Canonical Filtrations of Gorenstein Injective Modules
The principle "Every result in classical homological algebra should have a
counterpart in Gorenstein homological algebra" is given in [3]. There is a
remarkable body of evidence supporting this claim (cf. [2] and [3]). Perhaps
one of the most glaring exceptions is provided by the fact that tensor products
of Gorenstein projective modules need not be Gorenstein projective, even over
Gorenstein rings. So perhaps it is surprising that tensor products of
Gorenstein injective modules over Gorenstein rings of finite Krull dimension
are Gorenstein injective.
Our main result is in support of the principle. Over commutative, noetherian
rings injective modules have direct sum decompositions into indecomposable
modules. We will show that Gorenstein injective modules over Gorenstein rings
of finite Krull dimension have filtrations analogous to those provided by these
decompositions. This result will then provide us with the tools to prove that
all tensor products of Gorenstein injective modules over these rings are
Gorenstein injective.Comment: 9 pages; It has been accepted for publication in Proceedings of the
American Mathematical Societ
Density Functional Theory of a Curved Liquid-Vapour Interface: Evaluation of the rigidity constants
It is argued that to arrive at a quantitative description of the surface
tension of a liquid drop as a function of its inverse radius, it is necessary
to include the bending rigidity k and Gaussian rigidity k_bar in its
description. New formulas for k and k_bar in the context of density functional
theory with a non-local, integral expression for the interaction between
molecules are presented. These expressions are used to investigate the
influence of the choice of Gibbs dividing surface and it is shown that for a
one-component system, the equimolar surface has a special status in the sense
that both k and k_bar are then the least sensitive to a change in the location
of the dividing surface. Furthermore, the equimolar value for k corresponds to
its maximum value and the equimolar value for k_bar corresponds to its minimum
value. An explicit evaluation using a short-ranged interaction potential
between molecules, shows that k is negative with a value around minus 0.5-1.0
kT and that k_bar is positive with a value which is a bit more than half the
magnitude of k. Finally, for dispersion forces between molecules, we show that
a term proportional to log(R)/R^2 replaces the rigidity constants and we
determine the (universal) proportionality constants.Comment: 28 pages; 5 figures; accepted for publication in J. Phys.: Condens.
Matter (2013
Gobierno Corporativo en Costa Rica
(Disponible en idioma inglĂ©s Ăşnicamente) En este trabajo se analizan las prácticas de conducciĂłn empresarial en Costa Rica. En primer lugar, se calculan las medidas de conducciĂłn empresarial constitutivas empleando datos especĂficos de 87 compañĂas costarricenses, y se analizan sus efectos en su desempeño; aquĂ, la media de la conducciĂłn empresarial constitutiva de compañĂas cotizadas en bolsa es igual a 56. 14. En segundo lugar, se presentan nuevos elementos de juicio sobre las medidas constitutivas de la conducciĂłn empresarial de jure y de facto al nivel de compañĂa y sobre sus repercusiones en el desempeño de las mismas. Los resultados indican que la conducciĂłn empresarial de facto es mejor que la conducciĂłn empresarial de jure. Estos resultados sugieren que las compañĂas deben aplicar un conjunto de medidas adicionales para compensar la debilidad del entorno jurĂdico. Además, se presentan indicios de que una mejor gestiĂłn de conducciĂłn empresarial y medidas constitutivas se vinculan con un desempeño superior de la empresa. En tercer lugar, en este trabajo se analizan los factores que controlan en Ăşltima instancia a las empresas cotizadas y no cotizadas en bolsa, y se establecen vĂnculos entre ellas y sus polĂticas de dividendos. Las empresas familiares son corrientes en Costa Rica y no necesariamente se vinculan con un mejor desempeño empresarial. En cuarto lugar, se estudia la evoluciĂłn de la propiedad de compañĂas en Costa Rica. Por Ăşltimo se analizan cambios del CĂłdigo de Comercio desde el punto de vista de la buena conducciĂłn empresarial.
Two-photon imaging through a multimode fiber
In this work we demonstrate 3D imaging using two-photon excitation through a
20 cm long multimode optical fiber (MMF) of 350 micrometers diameter. The
imaging principle is similar to single photon fluorescence through a MMF,
except that a focused femtosecond pulse is delivered and scanned over the
sample. In our approach, focusing and scanning through the fiber is
accomplished by digital phase conjugation using mode selection by time gating
with an ultra-fast reference pulse. The excited two-photon emission is
collected through the same fiber. We demonstrate depth sectioning by scanning
the focused pulse in a 3D volume over a sample consisting of fluorescent beads
suspended in a polymer. The achieved resolution is 1 micrometer laterally and
15 micrometers axially. Scanning is performed over an 80x80 micrometers field
of view. To our knowledge, this is the first demonstration of high-resolution
three-dimensional imaging using two-photon fluorescence through a multimode
fiber
The Elasticity of Substitution in Demand for Non-Tradable Goods in Costa Rica
Using quarterly (annual) information on consumption and prices of non-tradable goods for the period 1980-2002 (1981-2001), this paper estimates the elasticity of substitution in demand for non-tradable goods in Costa Rica. The unit root and cointegration properties of the time series are tested, and then controlling for exogenous variables, the elasticity of substitution belonging to the interval [1. 46, 2. 14] ([ 0. 22, 0. 28]) is estimated. These results are statistically robust.
De novo Development and Characterization of Tetranucleotide Microsatellite Loci Markers from a Southeastern Population of the House Finch (Haemorhous mexicanus)
Microsatellites are short tandem repeats (e.g. TAGATAGA) of base pairs in a species’ genome. High mutation rates in these regions produce variation in the number of repeats across individuals that can be utilized to study patterns of population- and landscape-level genetics and to determine parentage genetically. In this project our objective was to develop microsatellite markers for the House Finch, Haemorhous mexicanus. This species has become one of the most well-studied species of songbirds due to its unique geographical, evolutionary, and epidemiological history. Using mist-nets we captured birds on the Arkansas Tech University campus and collected blood samples to obtain genomic DNA. Samples were processed in The Field Museum’s Pritzker Laboratory for Molecular Systematics and Evolution, where we fragmented genomic DNA and isolated fragments that contained potential microsatellites using specially designed biotin labelled probes. These DNA fragments were transformed into competent E. coli cells which were then PCR-amplified and Sanger sequenced. After sequencing DNA fragments from approximately 500 E. coli colonies, we designed and characterized a set of 13 tetranucleotide microsatellite loci. The average number of alleles and heterozygosity found in 12 individuals from Arkansas was 8.69 and 0.80, respectively. This finalized set of microsatellites can be utilized by researchers to determine parentage and characterize genetic differences across House Finch populations
Examining affective structure in chickens: valence, intensity, persistence and generalization measured using a conditioned place preference test
When measuring animals’ valenced behavioural responses to stimuli, the Conditioned Place Preference (CPP) test goes a step further than many approach-based and avoidance-based tests by establishing whether a learned preference for, or aversion to, the location in which the stimulus was encountered can be generated. We designed a novel, four-chambered CPP test to extend the capability of the usual CPP paradigm to provide information on four key features of animals’ affective responses: valence, scale, persistence and generalization. Using this test, we investigated the affective responses of domestic chickens (Gallus gallus domesticus) to four potentially aversive stimuli: 1. Puffs of air; 2. Sight of (robotic) snake; 3. Sprays of water; 4. Sound of conspecific alarm calls. We found conditioned avoidance of locations associated with the air puffs and water sprays (Friedman’s χ2(3) = 13.323 p > .005; χ2(3) = 14.235 p > .005), but not with the snake and alarm calls. The scale of the learned avoidance was similar for the air puff and water spray stimuli, but persistence and generalization differed. We conclude that the four chambered CPP test can have a valuable role to play in making multi-feature measurements of stimulus-generated affective responses, and we highlight the value of such measurements for improving our understanding of the structure of affect in chickens and other animals
Are Groups more Rational than Individuals? A Review of Interactive Decision Making in Groups
Many decisions are interactive; the outcome of one party depends not only on its decisions or on acts of nature but also on the decisions of others. In the present article, we review the literature on decision making made by groups of the past 25 years. Researchers have compared the strategic behavior of groups and individuals in many games: prisoner’s dilemma, dictator, ultimatum, trust, centipede and principal-agent games, among others. Our review suggests that results are quite consistent in revealing that groups behave closer to the game-theoretical assumption of rationality and selfishness than individuals. We conclude by discussing future research avenues in this area.group decision making, interactive decision making, rationality, discontinuity effect
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