6,354 research outputs found
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
Nuclear shadowing from exclusive quarkonium photoproduction at the BNL RHIC and CERN LHC
The photonuclear production of vector mesons in ultraperipheral heavy ion
collisions is investigated within the collinear approach using different
parameterizations for the nuclear gluon distribution. The integrated cross
section and the rapidity distribution for the () process are computed for energies of RHIC and LHC. A comparison with
the recent PHENIX data on coherent production of mesons is also
presented. We demonstrate that the study of the exclusive quarkonium
photoproduction can be used to constrain the nuclear effects in the gluon
distribution.Comment: 8 pages, 4 figures, 2 tables. Version to be published in Physical
Review
Evaluation of Red Clover Populations Selected for Persistence and Dry Matter Yield in Rio Grande Do Sul, Brasil
Red clover is one of the most important temperate legume species and it is used to alleviate the lack of forage in natural grasslands during the cool season in Rio Gra do Sul, Brasil. However, the lack of persistence presented by this species has not allowed its full utilization. Hence, a breeding program was started to select plants with more persistence and with good dry matter (DM) yield. This work was carried out at the “Estação Experimental Agronômica”, Universidade Federal do Rio Grande do Sul. Three selected red clover populations were evaluated in relation to the best available commercial check, cv. Quiñqueli, in a mixture with annual ryegrass. There were significant differences among the populations in relation to persistence and DM yield of its components. Two of the selected populations were more persistent than the check after the first season of growth, pointing to the possibility of making progress in this trait while maintaining the yield potential
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
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
The ground state properties of the spin-1/2 transverse Ising chain with periodically varying bonds and fields
Using continued fractions we study the ground state properties of the
spin-1/2 Ising chain in a transverse field with periodically varying
interaction strengths and external fields. We consider in detail the chain
having the period of modulation of interactions equals 2 and compare the
results obtained with those corresponding to the spin-1/2 isotropic XY chain in
a transverse field. In contrast to the behaviour of the transverse XY chain,
the transverse Ising chain does not exhibit a step-like magnetization vs. field
dependence caused by the alternation of bonds, its susceptibility exhibits a
logarithmic singularity at the field determined by interaction strengths, and
it is stable with respect to spin-Peierls dimerization.Comment: 11 pages, latex, 4 figure
Forgetting in Answer Set Programming with Anonymous Cycles
FORGET (PTDC/CCI-INF/32219/2017). NOVA LINCS (UID/CEC/04516/2019).It is now widely accepted that the operation of forgetting in the context of Answer Set Programming [10, 18] is best characterized by the so-called strong persistence, a property that requires that all existing relations between the atoms not to be forgotten be preserved. However, it has been shown that strong persistence cannot always be satisfied. What happens if we must nevertheless forget? One possibility that has been explored before is to consider weaker versions of strong persistence, although not without a cost: some relations between the atoms not to be forgotten are broken in the process. A different alternative is to enhance the logical language so that all such relations can be maintained after the forgetting operation. In this paper, we borrow from the recently introduced notion of fork [1] – a conservative extension of Equilibrium Logic and its monotonic basis, the logic of Here-and-There – which has been shown to be sufficient to overcome the problems related to satisfying strong persistence. We map this notion into the language of logic programs, enhancing it with so-called anonymous cycles, and we introduce a concrete syntactical forgetting operator over this enhanced language that we show to always obey strong persistence.publishe
Avaliação da formação de compostos fenĂłlicos em resĂduos contendo benzeno apĂłs aplicação de processo fenton.
Projeto/Plano de Ação: 16.300.30004-00
Peripheral heavy ion collisions as a probe of the nuclear gluon distribution
At high energies a quark-gluon plasma is expected to be formed in heavy ion
collisions at RHIC and LHC. The theoretical description of these processes is
directly associated to a complete knowledge of the details of medium effects in
the nuclear gluon distribution. In this paper we analyze the possibility to
constraint the behavior of this distribution considering peripheral heavy ion
collisions. We reanalyze the photoproduction of heavy quarks for the deduction
of the in-medium gluon distribution using three current parameterizations for
this parton distribution. Moreover, we show that the elastic photoproduction of
vector mesons is a potential process to probe the nuclear gluon distribution.Comment: 8 figures, accepted for publication in Physicsl review
Estirpes de rizóbio eficientes na fixação biológica de nitrogênio para leguminosas com potencial de uso na recuperação de áreas mineradas.
Este trabalho teve como objetivo selecionar estirpes de rizóbio eficientes na fixação biológica de nitrogênio para leguminosas herbáceas de crescimento rápido e sistema radicular profuso Tephrosia adunca e Mimosa xanthocentra.bitstream/CNPAB-2010/35739/1/cot115.pdfParceria; UFRRJ
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