1,230 research outputs found
Constant mean curvature solutions of the Einstein-scalar field constraint equations on asymptotically hyperbolic manifolds
We follow the approach employed by Y. Choquet-Bruhat, J. Isenberg and D.
Pollack in the case of closed manifolds and establish existence and
non-existence results for the Einstein-scalar field constraint equations on
asymptotically hyperbolic manifolds.Comment: 15 page
Vegetation and climate changes in the forest of Campinas, São Paulo State, Brazil, during the last 25,000 cal yr BP
A paleoenvironmental reconstruction was performed in a Riparian Forest near Campinas to improve knowledge of paleoclimate and paleoenvironment in the State of São Paulo, Brazil. A sediment core of 182 cm depth was collected in a swamp located within a Cerrado/Seasonal Semi-deciduous ecotone forest. Te chronological frame is given by eight radiocarbon dating methods. Pollen and stable isotope analyses (d 13C and d 15N) were performed all along the core. Modern pollen rain is based on fve surface samples collected along the Riparian Forest. Results show a sequence of changes in vegetation and climate between 25 and 13 cal kyr before present (BP), and from 4 cal kyr BP to the present time, with a hiatus between 11 and 4 kyr cal BP. Drier climatic conditions characterized the late Pleistocene and early Holocene, although they had moisture peaks able to maintain an open forest. Te Riparian Forest became fully installed from 4 cal kyr BP onward. Our results are in agreement with other regional studies and contribute to build a regional frame for past climatic conditions at the latitude of São Paulo.493CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQCOORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPESFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESPSem informaçãoSem informação2010/16507-
The Ricci flow of asymptotically hyperbolic mass and applications
We consider the evolution of the asymptotically hyperbolic mass under the
curvature-normalized Ricci flow of asymptotically hyperbolic, conformally
compactifiable manifolds. In contrast to asymptotically flat manifolds, for
which ADM mass is constant during Ricci flow, we show that the mass of an
asymptotically hyperbolic manifold of dimension n>2 decays smoothly to zero
exponentially in the flow time. From this, we obtain a no-breathers theorem and
a Ricci flow based, modified proof of the scalar curvature rigidity of
zero-mass asymptotically hyperbolic manifolds. We argue that the nonconstant
time evolution of the asymptotically hyperbolic mass is natural in light of a
conjecture of Horowitz and Myers, and is a test of that conjecture. Finally, we
use a simple parabolic scaling argument to produce a heuristic "derivation" of
the constancy of ADM mass under asymptotically flat Ricci flow, starting from
our decay formula for the asymptotically hyperbolic mass under the
curvature-normalized flow.Comment: Revised in accord with referee comments, typos and minor errors
corrected, Appendix B added, footnotes in-lined; version accepted for
publicatio
Semi-Supervised Learning with Graphs: Covariance Based Superpixels for Hyperspectral Image Classification
In this paper, we present a graph-based semi-supervised framework for
hyperspectral image classification. We first introduce a novel superpixel
algorithm based on the spectral covariance matrix representation of pixels to
provide a better representation of our data. We then construct a superpixel
graph, based on carefully considered feature vectors, before performing
classification. We demonstrate, through a set of experimental results using two
benchmarking datasets, that our approach outperforms three state-of-the-art
classification frameworks, especially when an extremely small amount of
labelled data is used.Case Studentship with the NP
The Early Bird Catches The Term: Combining Twitter and News Data For Event Detection and Situational Awareness
Twitter updates now represent an enormous stream of information originating
from a wide variety of formal and informal sources, much of which is relevant
to real-world events. In this paper we adapt existing bio-surveillance
algorithms to detect localised spikes in Twitter activity corresponding to real
events with a high level of confidence. We then develop a methodology to
automatically summarise these events, both by providing the tweets which fully
describe the event and by linking to highly relevant news articles. We apply
our methods to outbreaks of illness and events strongly affecting sentiment. In
both case studies we are able to detect events verifiable by third party
sources and produce high quality summaries
The Luminous Type Ic SN 1992ar at z=0.145
We present spectroscopic and photometric observations of SN1992ar, the more
distant SN in the Calan/Tololo Survey. We compare its spectrum with those of
nearby Type Ia and Ic SNe and conclude that the latter type is a better match
to SN 1992ar. Using K-corrections based on the spectra of well observed Type Ic
and Ia SNe we compute different possible rest frame light curves of SN 1992ar
and compare them with those of representative SNe of each type observed in the
nearby universe. From the photometry and the spectra, we are able to conclude
that SN 1992ar cannot be matched by any known example of a Type Ia SN. Even
though the data set collected is fairly complete (one spectrum and 10
photometric points), it is not possible to decide whether SN 1992ar was a fast
Type Ic SN, like SN 1994I, or a slow one, like SN 1983V. The absolute V
magnitudes at maximum implied by each of these possibilities are -19.2 and
-20.2, respectively. The latter would make SN 1992ar one of the brightest SNe
on record. SN 1992ar, hence, illustrates the problem of contamination faced by
the high z Type Ia SNe samples whose luminosity distances are used to determine
the cosmological parameters of the Universe. We present observational criteria
to distinguish the two SN types when the SiII 6355 line is redshifted out of
the sensitivity range of typical CCD detectors, and discuss the effect that
these luminous Type Ic SNe would have on the measured cosmological parameters,
if not removed from the High-z Type Ia SN samples.Comment: 40 pages, 9 figures. Accepted for publication in Astrophysical
Journa
Asymptotically Hyperbolic Non Constant Mean Curvature Solutions of the Einstein Constraint Equations
We describe how the iterative technique used by Isenberg and Moncrief to
verify the existence of large sets of non constant mean curvature solutions of
the Einstein constraints on closed manifolds can be adapted to verify the
existence of large sets of asymptotically hyperbolic non constant mean
curvature solutions of the Einstein constraints.Comment: 19 pages, TeX, no figure
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