5,775 research outputs found
A note on the cylindrical collapse of counter-rotating dust
We find analytical solutions describing the collapse of an infinitely long
cylindrical shell of counter-rotating dust. We show that--for the classes of
solutions discussed herein--from regular initial data a curvature singularity
inevitably develops, and no apparent horizons form, thus in accord with the
spirit of the hoop conjecture.Comment: 8 pages, LaTeX, ijmpd macros (included), 1 eps figure; accepted for
publication in Int. J. Mod. Phys.
No-horizon theorem for spacetimes with spacelike G1 isometry groups
We consider four-dimensional spacetimes which obey the
Einstein equations , and admit a global spacelike
isometry group. By means of dimensional reduction and local
analyis on the reduced (2+1) spacetime, we obtain a sufficient condition on
which guarantees that cannot contain apparent
horizons. Given any (3+1) spacetime with spacelike translational isometry, the
no-horizon condition can be readily tested without the need for dimensional
reduction. This provides thus a useful and encompassing apparent horizon test
for -symmetric spacetimes. We argue that this adds further evidence
towards the validity of the hoop conjecture, and signals possible violations of
strong cosmic censorship.Comment: 8 pages, LaTeX, uses IOP package; published in Class. Quantum Gra
Strong curvature singularities in quasispherical asymptotically de Sitter dust collapse
We study the occurrence, visibility, and curvature strength of singularities
in dust-containing Szekeres spacetimes (which possess no Killing vectors) with
a positive cosmological constant. We find that such singularities can be
locally naked, Tipler strong, and develop from a non-zero-measure set of
regular initial data. When examined along timelike geodesics, the singularity's
curvature strength is found to be independent of the initial data.Comment: 16 pages, LaTeX, uses IOP package, 2 eps figures; accepted for
publication in Class. Quantum Gra
Automatic detection of Acacia longifolia invasive species based on UAV-acquired aerial imagery
The Acacia longifolia species is known for its rapid growth and dissemination, causing loss of biodiversity in the affected areas. In order to avoid the uncontrolled spread of this species, it is important to effectively monitor its distribution on the agroforestry regions. For this purpose, this paper proposes the use of Convolutional Neural Networks (CNN) for the detection of Acacia longifolia, from images acquired by an unmanned aerial vehicle. Two models based on the same CNN architecture were elaborated. One classifies image patches into one of nine possible classes, which are later converted into a binary model; this model presented an accuracy of and in the validation and training sets, respectively. The second model was trained directly for binary classification and showed an accuracy of and for the validation and test sets, respectively. The results show that the use of multiple classes, useful to provide the aerial vehicle with richer semantic information regarding the environment, does not hamper the accuracy of Acacia longifolia detection in the classifier’s primary task. The presented system also includes a method for increasing classification’s accuracy by consulting an expert to review the model’s predictions on an automatically selected sub-set of the samples.info:eu-repo/semantics/publishedVersio
Absence of trapped surfaces and singularities in cylindrical collapse
The gravitational collapse of an infinite cylindrical thin shell of generic
matter in an otherwise empty spacetime is considered. We show that geometries
admitting two hypersurface orthogonal Killing vectors cannot contain trapped
surfaces in the vacuum portion of spacetime causally available to geodesic
timelike observers. At asymptotic future null infinity, however, congruences of
outgoing radial null geodesics become marginally trapped, due to convergence
induced by shear caused by the interaction of a transverse wave component with
the geodesics. The matter shell itself is shown to be always free of trapped
surfaces, for this class of geometries. Finally, two simplified matter models
are analytically examined. For one model, the weak energy condition is shown to
be a necessary condition for collapse to halt; for the second case, it is a
sufficient condition for collapse to be able to halt.Comment: 26 pages, revtex4, 1 eps figure; matches version to appear in Phys.
Rev. D (in press
Anaerobic decolorization of an azo dye by a mixed culture
Wool dyeing wastewater contains xenobiotic compounds that can be removed by biotechnological processes. Studies on various dyes showed that anaerobic processes are suitable to alter azo dyes as a first step of the biodegradation process. These compounds are reduced by anaerobic consortia to aromatic amines and its ultimate degradation can be achieved by a further aerobic treatment. Studies on degradation rate of an wool acid dye were performed in batch systems inoculated with anaerobic biomass. A commercial diazo dye, Acid Red 73, was added to the synthetic medium in which glucose was used as sole carbon source. Results indicated that the Acid Red 73 was partially degraded by a mixed culture of anaerobic bacteria and a decolorization of 90% was obtained. Kinetics studies on removal of the colour showed that the decolorization rate was several times faster than the degradation rate of glucose for a range of dye concentrations between 60 mg/L and 400 mg/L. A first order kinetic model was used for dye concentrations up to 200 mg/L. For higher concentrations a model similar to the Michaelis-Menten equation was better fitted to the experimental data.(undefined
Phases of massive scalar field collapse
We study critical behavior in the collapse of massive spherically symmetric
scalar fields. We observe two distinct types of phase transition at the
threshold of black hole formation. Type II phase transitions occur when the
radial extent of the initial pulse is less than the Compton
wavelength () of the scalar field. The critical solution is that
found by Choptuik in the collapse of massless scalar fields. Type I phase
transitions, where the black hole formation turns on at finite mass, occur when
. The critical solutions are unstable soliton stars with
masses \alt 0.6 \mu^{-1}. Our results in combination with those obtained for
the collapse of a Yang-Mills field~{[M.~W. Choptuik, T. Chmaj, and P. Bizon,
Phys. Rev. Lett. 77, 424 (1996)]} suggest that unstable, confined solutions to
the Einstein-matter equations may be relevant to the critical point of other
matter models.Comment: 5 pages, RevTex, 4 postscript figures included using psfi
Crossover to the KPZ equation
We characterize the crossover regime to the KPZ equation for a class of
one-dimensional weakly asymmetric exclusion processes. The crossover depends on
the strength asymmetry () and it occurs at
. We show that the density field is a solution of an
Ornstein-Uhlenbeck equation if , while for it is
an energy solution of the KPZ equation. The corresponding crossover for the
current of particles is readily obtained.Comment: Published by Annales Henri Poincare Volume 13, Number 4 (2012),
813-82
Training deep neural density estimators to identify mechanistic models of neural dynamics
Mechanistic modeling in neuroscience aims to explain observed phenomena in terms of underlying causes. However, determining which model parameters agree with complex and stochastic neural data presents a significant challenge. We address this challenge with a machine learning tool which uses deep neural density estimators-- trained using model simulations-- to carry out Bayesian inference and retrieve the full space of parameters compatible with raw data or selected data features. Our method is scalable in parameters and data features, and can rapidly analyze new data after initial training. We demonstrate the power and flexibility of our approach on receptive fields, ion channels, and Hodgkin-Huxley models. We also characterize the space of circuit configurations giving rise to rhythmic activity in the crustacean stomatogastric ganglion, and use these results to derive hypotheses for underlying compensation mechanisms. Our approach will help close the gap between data-driven and theory-driven models of neural dynamics
Photolytic release at different wavelengths of tetrapeptide AAPV from a pyrenylmethyl conjugate
Comunicação em painel no 2nd Symposium on Medicinal Chemistry, Braga, Portugal, 8 Maio 2015The present work describes the photolysis of ester conjugate NH2-Ala-Ala-Pro-Val-Pym, with Pym being the pyrenylmethylene group, at different wavelengths of irradiation in different solvents and simulated physiological environment. The photolysis of the ester conjugate and the release of the free tetrapeptide was monitored by HPLC with UV detection, with collection of kinetic data.Fundação para a Ciência e Tecnologia(FCT, Portugal) for financial support to the
NMR portuguese network (PTNMR, Bruker Avance III 400-Univ. Minho), FCT
and FEDER (European Fund for Regional Development)-COMPETE
-QREN-EU for financial support to Research Centre of Chemistry, CQ/UM [PEst
-C/QUI/UI0686/2013 (FCOMP-01-0124-FEDER-037302)]info:eu-repo/semantics/publishedVersio
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