1,083 research outputs found
The Apparent Fractal Conjecture
This short communication advances the hypothesis that the observed fractal
structure of large-scale distribution of galaxies is due to a geometrical
effect, which arises when observational quantities relevant for the
characterization of a cosmological fractal structure are calculated along the
past light cone. If this hypothesis proves, even partially, correct, most, if
not all, objections raised against fractals in cosmology may be solved. For
instance, under this view the standard cosmology has zero average density, as
predicted by an infinite fractal structure, with, at the same time, the
cosmological principle remaining valid. The theoretical results which suggest
this conjecture are reviewed, as well as possible ways of checking its
validity.Comment: 6 pages, LaTeX. Text unchanged. Two references corrected. Contributed
paper presented at the "South Africa Relativistic Cosmology Conference in
Honour of George F. R. Ellis 60th Birthday"; University of Cape Town,
February 1-5, 199
Dust content solutions for the Alcubierre warp drive spacetime
The Alcubierre metric is a spacetime geometry where a massive particle inside
a spacetime distortion, called warp bubble, is able to travel at velocities
arbitrarily higher than the velocity of light, a feature known as the warp
drive. This is a consequence of general relativity, which allows global
superluminal velocities but restricts local speeds to subluminal ones as
required by special relativity. In this work we solved the Einstein equations
for the Alcubierre warp drive spacetime geometry considering the dust matter
distribution as source, since the Alcubierre metric was not originally advanced
as a solution of the Einstein equations, but as a spacetime geometry proposed
without a source gravity field. We found out that all Einstein equations
solutions of this geometry containing pressureless dust lead to vacuum
solutions. We also concluded that these solutions connect the Alcubierre metric
to the Burgers equation, which describes shock waves moving through an inviscid
fluid. Our results also indicated that these shock waves behave as plane waves.Comment: 15 pages, 1 figure. LaTeX. Accepted for publication in the European
Physical Journal
Forecasting Large Realized Covariance Matrices: The Benefits of Factor Models and Shrinkage
We propose a model to forecast large realized covariance matrices of returns,
applying it to the constituents of the S\&P 500 daily. To address the curse of
dimensionality, we decompose the return covariance matrix using standard
firm-level factors (e.g., size, value, and profitability) and use sectoral
restrictions in the residual covariance matrix. This restricted model is then
estimated using vector heterogeneous autoregressive (VHAR) models with the
least absolute shrinkage and selection operator (LASSO). Our methodology
improves forecasting precision relative to standard benchmarks and leads to
better estimates of minimum variance portfolios
Phylogenetic relationships of Chanidae (Teleostei: Gonorynchiformes) as impacted by Dastilbe moraesi, from the Sanfranciscana basin, Early Cretaceous of Brazil
Fossil gonorynchiform fishes range from the Lower Cretaceous to the early Miocene, and are represented by a few dozen living
species. The order is currently divided into two major clades: Gonorynchoidei, which includes the families Gonorynchidae
and Kneriidae, and Chanoidei, encompassing a single family, Chanidae, with a single recent species, the Indo-Pacific Chanos
chanos, and several fossil taxa. Chanidae includes some poorly known taxa, such as Dastilbe moraesi, described from the
Aptian (Lower Cretaceous) of the Areado Formation, Sanfranciscana basin, Brazil. This species is currently considered to be
a junior synonym of the type species of its genus, Dastilbe crandalli, from Santana Formation, Aptian, northeastern Brazil.
The analysis of abundant D. moraesi specimens revealed several new morphological features, many of which had previously
been misinterpreted. Dastilbe moraesi was incorporated into a gonorynchiform character matrix as revised and modified for
the Chanidae. We obtained a single most parsimonious tree in which D. moraesi is distinct and phylogenetically apart from D.
crandalli. According our analysis, D. moraesi forms a sister pair with Chanos, a clade which is closely related to Tharrhias,
all composing the tribe ChaniniGonorynchiformes fósseis ocorrem desde do Cretáceo inferior ao Mioceno inferior, e são representados por alguns
representantes viventes. A ordem está dividida atualmente em dois clados principais: Gonorynchoidei, que inclui as famĂlias
Gonorynchidae e Kneriidae, e Chanoidei, compreendendo uma Ăşnica famĂlia, Chanidae, com uma Ăşnica espĂ©cie vivente,
Chanos chanos, do Indo-PacĂfico, alĂ©m de vários representantes fĂłsseis. Chanidae inclui alguns táxons problemáticos,
tais como Dastilbe moraesi, descrito do Aptiano (Cretáceo Inferior) da Formação Areado, bacia Sanfranciscana, Brasil.
Esta espécie é atualmente considerada um sinônimo júnior da espécie-tipo de seu gênero, Dastilbe crandalli, da Formação
Santana, Aptiano do nordeste do Brasil. A análise de abundante material de D. moraesi revelou várias novas caracterĂsticas
anatĂ´micas, muitas das quais haviam sido previamente mal interpretadas. Dastilbe moraesi foi incorporado em uma matriz
revisada de caracteres da famĂlia Chanidae. NĂłs obtivemos uma Ăşnica árvore mais parcimoniosa na qual D. moraesi Ă© distinto
e filogeneticamente distante de D. crandalli. De acordo com nossa análise, D. moraesi é o grupo-irmão de Chanos, um clado
intimamente relacionado a Tharrhias, com todos compondo a tribo ChaniniThis study was supported by CNPq (process # 401818/2010-1) and
project CGL2013-42643P, Ministerio de Ciencia e Innovación de Españ
Assessing the Potential of Sensors Based on LbL and Sputtering Films
Funding text This research was funded by the project PCIF/GFC/0078/2018: influence of forest VOCs (volatile organic compounds) on extreme fire behavior at Fundação para a Ciência e a Tecnologia (FCT). The authors also thankful for the research center grants no. UIDB/FIS/04559/2020 and no. UIDP/FIS/04559/2020 (LIBPhys) from FCT/MCTES, Portugal.A new theory suggests that flammable gases generated by heated vegetation, in particular the volatile organic compounds (VOC) common to Mediterranean plants, may, under certain topographic and wind conditions, accumulate in locations where, after the arrival of the ignition source, they rapidly burst into flames as explosions. Hence, there is a need for the development of a system that can monitor the development of these compounds. In this work, a sensor's array is proposed as a method for monitoring the amount of eucalyptol and α-pinene, the major VOC compounds of the Eucalyptus and Pine trees. The detection of the target compounds was assessed using the impedance spectroscopy response of thin films. Combinations of layers of polyelectrolytes, such as poly(allylamine hydrochloride) (PAH), polyethyleneimine (PEI), poly(sodium 4-sytrenesulfonate) (PSS) graphene oxide (GO), and non/functionalized multiwall nanotubes (MWCNT-COOH or MWCNT), namely, PAH/GO, PEI/PSS, PEI/GO, PAH/MWCNT, PAH/MWCNT-COOH, films, and TiO2 and ZnO sputtered films, were deposited onto ceramic supports coated with gold interdigitated electrodes. The results showed that concentrations of the target VOCs, within the range of 68 to 999 ppmv, can be easily distinguished by analyzing the impedance spectra, particularly in the case of the ZnO- and PAH/GO-film-based sensors, which showed the best results in the detection of the target compounds. Through principal component analysis (PCA), the best set of features attained for the ZnO and PAH/GO based sensor devices revealed a linear trend of the PCA's first principal component with the concentration within the range 109 and 807 ppmv. Thus, the values of sensitivity to eucalyptol and α-pinene concentrations, which were (2.2 ± 0.3) × 10-4 and (5.0 ± 0.7) × 10-5 per decade, respectively, as well as resolutions of 118 and 136 ppbv, respectively, were identified.publishersversionpublishe
Anterior Hepatic Transection for Caudate Lobectomy
Resection of the caudate lobe (segment I- dorsal sector, segment IX- right paracaval region, or both) is often technically difficult due to the lobe’s location deep in the hepatic parenchyma and because it is adjacent to the major hepatic vessels (e.g., the left and middle hepatic veins)
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