2,636 research outputs found
Hamiltonian Dynamics of Linearly Polarized Gowdy Models Coupled to Massless Scalar Fields
The purpose of this paper is to analyze in detail the Hamiltonian formulation
for the compact Gowdy models coupled to massless scalar fields as a necessary
first step towards their quantization. We will pay special attention to the
coupling of matter and those features that arise for the three-handle and
three-sphere topologies that are not present in the well studied three torus
case -in particular the polar constraints that come from the regularity
conditions on the metric. As a byproduct of our analysis we will get an
alternative understanding, within the Hamiltonian framework, of the appearance
of initial and final singularities for these models.Comment: Final version to appear in Classical and Quantum Gravit
A Generalization of Chetaev's Principle for a Class of Higher Order Non-holonomic Constraints
The constraint distribution in non-holonomic mechanics has a double role. On
one hand, it is a kinematic constraint, that is, it is a restriction on the
motion itself. On the other hand, it is also a restriction on the allowed
variations when using D'Alembert's Principle to derive the equations of motion.
We will show that many systems of physical interest where D'Alembert's
Principle does not apply can be conveniently modeled within the general idea of
the Principle of Virtual Work by the introduction of both kinematic constraints
and variational constraints as being independent entities. This includes, for
example, elastic rolling bodies and pneumatic tires. Also, D'Alembert's
Principle and Chetaev's Principle fall into this scheme. We emphasize the
geometric point of view, avoiding the use of local coordinates, which is the
appropriate setting for dealing with questions of global nature, like
reduction.Comment: 27 pages. Journal of Mathematical Physics (to zappear
Symmetries in Classical Field Theory
The multisymplectic description of Classical Field Theories is revisited,
including its relation with the presymplectic formalism on the space of Cauchy
data. Both descriptions allow us to give a complete scheme of classification of
infinitesimal symmetries, and to obtain the corresponding conservation laws.Comment: 70S05; 70H33; 55R10; 58A2
Avances de investigación en frutales tropicales promisorios para el departamento de Meta.
El estudio presenta la recolección de germoplasma divididos en 4 grupos de la siguiente manera: en el grupo 1 se presentan los frutales tradicionales no cultivados en el departamento del Meta, en el grupo 2 se presentan los frutales tropicales promisorios poco conocidos en el mercado como arazá, borojó, chontaduro, cocona y carambolo, en el grupo 3 se presentan los frutales con alta potencialidad en el mercado nacional e internacional como el langostino y el camu-camú y en el grupo 4 se presentan los frutales poco conocidos en los mercados y que no se cultivan en huertos o son de reciente introducción a los Llanos. También se presentan las recomendaciones para la realización de germoplasma de frutales promisorios y exóticos para los Llanos Orientales, entre otros
Testing the comet nature of main belt comets. The spectra of 133P/Elst-Pizarro and 176P/LINEAR
We present the visible spectrum of MBCs 133P/Elst-Pizarro and 176P/LINEAR, as
well as three Themis family asteroids: (62) Erato, (379), Huenna and (383)
Janina, obtained in 2007 using three telescopes at "El Roque de los Muchachos"'
Observatory, in La Palma, Spain, and the 8m Kueyen (UT2) VLT telescope at Cerro
Paranal, Chile.
The spectra of 133P and 176P resemble best those of B-type asteroid and are
very similar to those of Themis family members and are significantly different
from the spectrum of comet 162P/Siding-Spring and most of the observed cometary
nuclei. CN gas emission is not detected in the spectrum of 133P. We determine
an upper limit for the CN production rate Q(CN) = mol/s,
three orders of magnitude lower than the Q(CN) of Jupiter family comets
observed at similar heliocentric distances.
The spectra of 133P/Elst-Pizarro and 176P/LINEAR confirm that they are likely
members of the Themis family of asteroids, fragments that probably retained
volatiles, and unlikely have a cometary origin in the trans-neptunian belt or
the Oort cloud.Comment: Paper sumbmited to A&A. 7 pages and 6 figure
XANES and EXAFS study of the local order in nanocrystalline yttria-stabilized zirconia
The local order around Zr and Y atoms of nanocrystalline yttria-stabilized zirconia (YSZ) powders with
different grain sizes has been investigated by x-ray absorption spectroscopies. The samples were prepared by
means of mechanical alloying with or without subsequent sintering treatment and also by milling commercial
YSZ. Our study is motivated by the interest in the electrical properties of grain boundaries and the controversy about the level of disorder in the intergrain regions in nanocrystalline YSZ. The x-ray absorption near edge
structure (XANES) analysis indicates that the local order of all the sintered samples is independent of the grain
size. This is confirmed by the analysis of the extended x-ray absorption fine structure, which points out also that,
in contrast to that found in sintered samples, the local order around the cation in the samples milled without
further sintering treatment extends only to the first coordination shell. Finally, the results of ab initio Zr K-edge XANES calculations lead us to conclude that the observed changes of the shape of the white line are not related to a phase transformation but reflects the short-range order present in the as-milled samples
Human bony labyrinth is an indicator of population history and dispersal from Africa.
The dispersal of modern humans from Africa is now well documented with genetic data that track population history, as well as gene flow between populations. Phenetic skeletal data, such as cranial and pelvic morphologies, also exhibit a dispersal-from-Africa signal, which, however, tends to be blurred by the effects of local adaptation and in vivo phenotypic plasticity, and that is often deteriorated by postmortem damage to skeletal remains. These complexities raise the question of which skeletal structures most effectively track neutral population history. The cavity system of the inner ear (the so-called bony labyrinth) is a good candidate structure for such analyses. It is already fully formed by birth, which minimizes postnatal phenotypic plasticity, and it is generally well preserved in archaeological samples. Here we use morphometric data of the bony labyrinth to show that it is a surprisingly good marker of the global dispersal of modern humans from Africa. Labyrinthine morphology tracks genetic distances and geography in accordance with an isolation-by-distance model with dispersal from Africa. Our data further indicate that the neutral-like pattern of variation is compatible with stabilizing selection on labyrinth morphology. Given the increasingly important role of the petrous bone for ancient DNA recovery from archaeological specimens, we encourage researchers to acquire 3D morphological data of the inner ear structures before any invasive sampling. Such data will constitute an important archive of phenotypic variation in present and past populations, and will permit individual-based genotype-phenotype comparisons
Diagnóstico al sector de alimentos balanceados para animales en Colombia
La Facultad de Administración de la Universidad del Rosario viene adelantando
desde el año 2004 una serie de estudios orientados a comprender,
de una mejor manera, la perdurabilidad empresarial. Una diversidad de
sectores han sido analizados: financiero, salud, comercio, confecciones y
aeronáutico, entre otros.
La principal herramienta implementada ha sido el Análisis Estructural
de Sectores Estratégicos (AESE), cuya metodología permite percibir, de
una mejor manera, el estado del sector a través de cuatro pruebas: hacinamiento,
panorama competitivo, análisis de fuerzas de mercado y estudio
de competidores.
En la primera fase de la investigación se realizaron estudios en diferentes
sectores económicos; la siguiente fase consistirá en realizar un análisis
cruzado entre sectores y entre países, el cual permitirá aportar elementos
para la generación de una teoría sobre la perdurabilidad empresarial. De
manera paralela, se seguirán realizando estudios de sectores con estudiantes
de pregrado y posgrado.
El presente estudio fue desarrollado, conjuntamente, con estudiantes de
la Especialización en Gerencia de Negocios Internacionales del convenio Universidad del Rosario-Universidad CES, y es resultado del trabajo elaborado
para la asignatura Análisis Estratégico, coordinada por el profesor
Hugo Alberto Rivera, director de la línea de investigación en perdurabilidad
empresarial.
El sector estudiado es el de las empresas de alimentos balanceados para
animales, base del sector pecuario en Colombia; estas se encargan de convertir
materias primas de origen agrícola en alimento para la producción de
animales.
La producción de alimentos balanceados ha mantenido una dinámica
importante en el país durante los últimos años, jalonada, principalmente, por
el incremento en el consumo de carne de pollo, y, en menor proporción, el de
otras líneas de producción (peces y codornices, entre otros). Las turbulencias
indicadas en el presente informe exponen la complejidad, dinamismo e incertidumbre
que se tiene al momento de reflejar la toma de decisiones en cuanto
a inversión, ventas, fusiones e integraciones. Esto se debe a que el sector
estudiado es un sector donde la incertidumbre sobre los eventos futuros está
dada por factores exógenos que no son atribuibles al control humano, sino al
control de la naturaleza; para esto, las empresas ajustan sus condiciones de
producción y estrategias de venta de acuerdo a las necesidades de sus consumidores
finales, evidenciando así un alto componente de investigación y
desarrollo junto a la versatilidad de adaptación a los cambios del entorno,
y desencadenando, a su vez, una imitación de las empresas pequeñas frente
a las tradicionales y representativas del sector.
Para poder realizar un correcto análisis, es necesario partir de las diferentes
especies que tienen explotación pecuaria en Colombia, agrupándolas
en grandes grupos representativos con la finalidad de reconocer las potencialidades
de cada uno de los actores y poder hacer una evaluación con un
mayor grado de certeza.
A lo largo del documento, se podrá evidenciar una lectura crítica, basada
en los datos analizados y con opiniones fundamentadas en la lectura
del mercado y en la literatura proporcionada por las agremiaciones y entes
reguladores
Neutron polarizabilities investigated by quasi-free Compton scattering from the deuteron
Measuring Compton scattered photons and recoil neutrons in coincidence,
quasi-free Compton scattering by the neutron has been investigated at MAMI
(Mainz) at in an energy range from 200 to 400 MeV.
From the data a polarizability difference of in units of has been
determined. In combination with the polarizability sum deduced from photo absorption data, the neutron electric and
magnetic polarizabilities, and ,
are obtained
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