702 research outputs found
On the k-Symplectic, k-Cosymplectic and Multisymplectic Formalisms of Classical Field Theories
The objective of this work is twofold: First, we analyze the relation between
the k-cosymplectic and the k-symplectic Hamiltonian and Lagrangian formalisms
in classical field theories. In particular, we prove the equivalence between
k-symplectic field theories and the so-called autonomous k-cosymplectic field
theories, extending in this way the description of the symplectic formalism of
autonomous systems as a particular case of the cosymplectic formalism in
non-autonomous mechanics. Furthermore, we clarify some aspects of the geometric
character of the solutions to the Hamilton-de Donder-Weyl and the
Euler-Lagrange equations in these formalisms. Second, we study the equivalence
between k-cosymplectic and a particular kind of multisymplectic Hamiltonian and
Lagrangian field theories (those where the configuration bundle of the theory
is trivial).Comment: 25 page
Quantum Conductance Steps in Solutions of Multiwalled Carbon Nanotubes
We have prepared solutions of multiwalled carbon nanotubes in Aroclor 1254, a
mixture of polychlorinated biphenyls. The solutions are stable at room
temperature. Transport measurements were performed using a scanning--tunneling
probe on a sample prepared by spin--coating of the solution on gold substrates.
Conductance steps were clearly seen. An histogram of a high number of traces
shows maximum peaks at integer values of the conductance quantum , demonstrating ballistic transport at room temperature along the carbon
nanotube over distances longer than .Comment: 4 pages and 2 figure
¿Existe un sesgo en la participación y visibilidad de las mujeres en ecología? Una comparación entre los congresos ibérico y suizo
Women are still under-represented in science and technology because there is a bias in the different evaluation processes from the admission of oral communications at conferences to the access to more qualified positions. In this study, we compared the participation and visibility of female ecologists in the IV Iberian Ecological Conference (CIE4) with the Annual Swiss Conference on Ecology, Evolution, Systematics, Biogeography and Conservation (Biology16) where a method of blind evaluation was applied.
In both conferences, and after active selection by conference committees, male delegates presented proportionately more regular oral contributions than female ones, although these differences were marginally significant. In the Biology16, this bias was due to lower female applicants for oral contributions, and in the CIE4, to a lower selection rate of female oral contributions. In the CIE4, the higher male presence in oral contributions was consistent with male greater visibility, whereas female visibility was higher in the Biology16. This different visibility was caused by contrasting selection for invited speakers; being male researches preferentially invited at the CIE4, but female ones at the Biology16.
Implementing a blind review system of contributions and active policies to promote the participation of women as invited speakers may reduce the differences in visibility and could contribute in the medium term to eliminate the bias against women in the selection of oral contributions at the Iberian conferences
Non-standard connections in classical mechanics
In the jet-bundle description of first-order classical field theories there
are some elements, such as the lagrangian energy and the construction of the
hamiltonian formalism, which require the prior choice of a connection. Bearing
these facts in mind, we analyze the situation in the jet-bundle description of
time-dependent classical mechanics. So we prove that this connection-dependence
also occurs in this case, although it is usually hidden by the use of the
``natural'' connection given by the trivial bundle structure of the phase
spaces in consideration. However, we also prove that this dependence is
dynamically irrelevant, except where the dynamical variation of the energy is
concerned. In addition, the relationship between first integrals and
connections is shown for a large enough class of lagrangians.Comment: 17 pages, Latex fil
Invariant Forms and Automorphisms of Locally Homogeneous Multisymplectic Manifolds
It is shown that the geometry of locally homogeneous multisymplectic
manifolds (that is, smooth manifolds equipped with a closed nondegenerate form
of degree > 1, which is locally homogeneous of degree k with respect to a local
Euler field) is characterized by their automorphisms. Thus, locally homogeneous
multisymplectic manifolds extend the family of classical geometries possessing
a similar property: symplectic, volume and contact. The proof of the first
result relies on the characterization of invariant differential forms with
respect to the graded Lie algebra of infinitesimal automorphisms, and on the
study of the local properties of Hamiltonian vector fields on locally
multisymplectic manifolds. In particular it is proved that the group of
multisymplectic diffeomorphisms acts (strongly locally) transitively on the
manifold. It is also shown that the graded Lie algebra of infinitesimal
automorphisms of a locally homogeneous multisymplectic manifold characterizes
their multisymplectic diffeomorphisms.Comment: 25 p.; LaTeX file. The paper has been partially rewritten. Some
terminology has been changed. The proof of some theorems and lemmas have been
revised. The title and the abstract are slightly modified. An appendix is
added. The bibliography is update
Biobased polymers derived from itaconic acid bearing clickable groups with potent antibacterial activity and negligible hemolytic activity.
Herein, we report, for the first time, the synthesis of clickable polymers derived from biobased itaconic
acid, which was then used for the preparation of novel cationic polymers with antibacterial properties and
low hemotoxicity via click chemistry. Itaconic acid (IA) was subjected to chemical modification by incorporating
clickable alkyne groups on the carboxylic acids. The resulting monomer with pendant alkyne
groups was easily polymerized and copolymerized with dimethyl itaconate (DMI) by radical polymerization.
The feed molar ratio of comonomers was varied to precisely tune the content of alkyne groups in
the copolymers and the amphiphilic balance. Subsequently, an azide with a thiazole group, which is a
component of the vitamin thiamine (B1), was attached onto the polymers by copper-catalyzed azidealkyne
cycloaddition (CuAAC) click chemistry leading to triazole linkages. N-Alkylation reactions of the
thiazole and triazole groups with methyl and butyl iodides provide the corresponding itaconate derivatives
with pendant azolium groups. The copolymers with variable cationic charge densities and hydrophobic/
hydrophilic balances, depending on the comonomer feed ratio, display potent antibacterial activity
against Gram-positive bacteria, whereas the activity was almost null against Gram-negative bacteria.
Hemotoxicity assays demonstrated that the copolymers exhibited negligible hemolysis and excellent
selectivity, more than 1000-fold, for Gram-positive bacteria over human red blood cells.post-print1945 K
PLA and PBAT-based electrospun fibers functionalized with antibacterial bio-based polymers
Antimicrobial fibers based on biodegradable polymers, poly(lactic acid) (PLA), and poly(butylene adipate-co-terephthalate) (PBAT) are prepared by electrospinning. For this purpose, a biodegradable/bio-based polyitaconate containing azoles groups (PTTI) is incorporated at 10 wt.% into the electrospinning formulations. The resulting fibers functionalized with azole moieties are uniform and free of beads. Then, the accessible azole groups are subjected to N-alkylation, treatment that provides cationic azolium groups with antibacterial activity at the surface of fibers. The positive charge density, roughness, and wettability of the cationic fibers are evaluated and compared with flat films. It is confirmed that these parameters exert an important effect on the antimicrobial properties, as well as the length of the alkylating agent and the hydrophobicity of the matrix. The quaternized PLA/PTTI fibers exhibit the highest efficiency against the tested bacteria, yielding a 4-Log reduction against S. aureus and 1.7-Log against MRSA. Then, biocompatibility and bioactivity of the fibers are evaluated in terms of adhesion, morphology and viability of fibroblasts. The results show no cytotoxic effect of the samples, however, a cytostatic effect is appreciated, which is ascribed to the strong electrostatic interactions between the positive charge at the fiber surface and the negative charge of the cell membranes
Higher-order Mechanics: Variational Principles and other topics
After reviewing the Lagrangian-Hamiltonian unified formalism (i.e, the
Skinner-Rusk formalism) for higher-order (non-autonomous) dynamical systems, we
state a unified geometrical version of the Variational Principles which allows
us to derive the Lagrangian and Hamiltonian equations for these kinds of
systems. Then, the standard Lagrangian and Hamiltonian formulations of these
principles and the corresponding dynamical equations are recovered from this
unified framework.Comment: New version of the paper "Variational principles for higher-order
dynamical systems", which was presented in the "III Iberoamerican Meeting on
Geometry, Mechanics and Control" (Salamanca, 2012). The title is changed. A
detailed review is added. Sections containing results about variational
principles are enlarged with additional comments, diagrams and summarizing
results. Bibliography is update
Grupo de estudos entre alunos ingressantes (calouros) e veteranos do instituto de química da UFG: análise do processo de reformulação
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Lógica mediática y comercialismo periodístico. Un análisis longitudinal de encuadres en la cobertura electoral
La cobertura de las elecciones ha puesto desde hace varios años menos énfasis en los aspectos sustanciales de la política relativos a los temas públicos o propuestas de política, para resaltar un tratamiento dominado por una lógica mediática, con elementos de interés humano, conflicto y escándalo. Este fenómeno se ha relacionado con la creciente comercialización de los medios informativos y en particular la prensa de referencia, que, sometida a un decremento constante de lectoría, alta competitividad en Internet y acaparamiento de intereses corporativos, ha intentado apelar a mercados de lectores poco politizados a través de dichos tratamientos. Para verificar este supuesto realizamos análisis de contenido longitudinal de las últimas cuatro elecciones presidenciales mexicanas (1994, 2000, 2006 y 2012) utilizando encuadres de tipo temático, que representan el contenido sustancioso, y encuadres de tipo estratégico, que indican la presencia de una lógica mediática. Los hallazgos apuntan al incremento tan sólo del encuadre de conflicto, a la vez que se preserva un encuadre centrado en el proceso electoral antes que en los temas de campaña, de manera que no se constata una relación lineal entre un comercialismo ascendente y los encuadres vinculados con la lógica mediática.In recent years, election coverage has deemphasized the substance of politics (content related to issues or policy) to focus on a media logic treatment, as in the case of stories on human interest, conflict, and scandal. The latter has been connected to the increasing commercialism of news media, the printed press in particular; which - besides a constant decrease of readership, high competition from the Internet, and corporate acquisition - has tried to appeal to a less politicized market through that kind of treatment. In order to prove this assumption, we conducted a longitudinal content analysis of the last four Mexican presidential elections (1994, 2000, 2006 and 2012). The aim was to evaluate – on one hand - the presence of thematic meta-frames that represent the political substance, and – on other – the strategic frames, that manifest a media logic in action. The findings pointed out an increase only in the conflict frame, whilst the political process frame was steady across the elections, and higher than the issue frame. Our data does not allow us to establish neither a significant nor linear relationship between an ongoing commercialism of the press and the frames linked to media logic
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