42,666 research outputs found
Nonsingular density profiles of dark matter halos and Strong gravitational lensing
We use the statistics of strong gravitational lenses to investigate whether
mass profiles with a flat density core are supported. The probability for
lensing by halos modeled by a nonsingular truncated isothermal sphere (NTIS)
with image separations greater than a certain value (ranging from zero to ten
arcseconds) is calculated. NTIS is an analytical model for the postcollapse
equilibrium structure of virialized objects derived by Shapiro, Iliev & Raga.
This profile has a soft core and matches quite well with the mass profiles of
dark matter-dominated dwarf galaxies deduced from their observed rotation
curves. It also agrees well with the NFW (Navarro-Frenk-White) profile at all
radii outside of a few NTIS core radii. Unfortunately, comparing the results
with those for singular lensing halos (NFW and SIS+NFW) and strong lensing
observations, the probabilities for lensing by NTIS halos are far too low. As
this result is valid for any other nonsingular density profiles (with a large
core radius), we conclude that nonsingular density profiles (with a large core
radius) for CDM halos are ruled out by statistics of strong gravitational
lenses.Comment: 17 pages, 4 figures, ApJ accepted. Final version matches the proofs.
A curve in figure 2 is corrected, conclusions unchange
Transient dynamics for sequence processing neural networks: effect of degree distributions
We derive a analytic evolution equation for overlap parameters including the
effect of degree distribution on the transient dynamics of sequence processing
neural networks. In the special case of globally coupled networks, the
precisely retrieved critical loading ratio is obtained,
where is the network size. In the presence of random networks, our
theoretical predictions agree quantitatively with the numerical experiments for
delta, binomial, and power-law degree distributions.Comment: 11 pages, 6 figure
The relationship between web enjoyment and student perceptions and learning using a web-based tutorial
Web enjoyment has been regarded as a component of system experience. However, there has been little targeted research considering the role of web enjoyment alone in student learning using web-based systems. To address this gap, this study aims to examine the influence of web enjoyment on learning performance and perceptions by controlling system experience as a variable in the study. 74 students participated in the study, using a web-based tutorial covering subject matter in the area of 'Computation and algorithms'. Their learning performance was assessed with a pre-test and a post-test and their learning perceptions were evaluated with a questionnaire. The results indicated that there are positive relationships between the levels of web enjoyment and perceived usefulness and non-linear navigation for users with similar, significant levels of system experience. The implications of these findings in relation to web-based learning are explored and ways in which the needs of students who report different levels of web enjoyment might be met are discussed
Hidden and Generalized Conformal Symmetry of Kerr-Sen Spacetimes
It is recently conjectured that generic non-extremal Kerr black hole could be
holographically dual to a hidden conformal field theory in two dimensions.
Moreover, it is known that there are two CFT duals (pictures) to describe the
charged rotating black holes which correspond to angular momentum and
electric charge of the black hole. Furthermore these two pictures can be
incorporated by the CFT duals (general picture) that are generated by
modular group. The general conformal structure can be
revealed by looking at charged scalar wave equation in some appropriate values
of frequency and charge. In this regard, we consider the wave equation of a
charged massless scalar field in background of Kerr-Sen black hole and show in
the "near region", the wave equation can be reproduced by the Casimir operator
of a local hidden conformal
symmetry. We can find the exact agreement between macroscopic and microscopic
physical quantities like entropy and absorption cross section of scalars for
Kerr-Sen black hole. We then find an extension of vector fields that in turn
yields an extended local family of hidden conformal symmetries, parameterized by one
parameter. For some special values of the parameter, we find a copy of
hidden conformal algebra for the charged
Gibbons-Maeda-Garfinkle-Horowitz-Strominger black hole in the strong deflection
limit.Comment: 16 pages, new material and results added, extensive improvements in
interpretation of results, references adde
Divergence and Shannon information in genomes
Shannon information (SI) and its special case, divergence, are defined for a
DNA sequence in terms of probabilities of chemical words in the sequence and
are computed for a set of complete genomes highly diverse in length and
composition. We find the following: SI (but not divergence) is inversely
proportional to sequence length for a random sequence but is length-independent
for genomes; the genomic SI is always greater and, for shorter words and longer
sequences, hundreds to thousands times greater than the SI in a random sequence
whose length and composition match those of the genome; genomic SIs appear to
have word-length dependent universal values. The universality is inferred to be
an evolution footprint of a universal mode for genome growth.Comment: 4 pages, 3 tables, 2 figure
Structural constraints in complex networks
We present a link rewiring mechanism to produce surrogates of a network where
both the degree distribution and the rich--club connectivity are preserved. We
consider three real networks, the AS--Internet, the protein interaction and the
scientific collaboration. We show that for a given degree distribution, the
rich--club connectivity is sensitive to the degree--degree correlation, and on
the other hand the degree--degree correlation is constrained by the rich--club
connectivity. In particular, in the case of the Internet, the assortative
coefficient is always negative and a minor change in its value can reverse the
network's rich--club structure completely; while fixing the degree distribution
and the rich--club connectivity restricts the assortative coefficient to such a
narrow range, that a reasonable model of the Internet can be produced by
considering mainly the degree distribution and the rich--club connectivity. We
also comment on the suitability of using the maximal random network as a null
model to assess the rich--club connectivity in real networks.Comment: To appear in New Journal of Physics (www.njp.org
Machine learning the relationship between Debye temperature and superconducting transition temperature
Recently a relationship between the Debye temperature and the
superconducting transition temperature of conventional superconductors
has been proposed [npj Quantum Materials , 59 (2018)]. The
relationship indicates that for phonon-mediated BCS
superconductors, with being a pre-factor of order . In order to
verify this bound, we train machine learning (ML) models with 10,330 samples in
the Materials Project database to predict . By applying our ML models
to 9,860 known superconductors in the NIMS SuperCon database, we find that the
conventional superconductors in the database indeed follow the proposed bound.
We also perform first-principles phonon calculations for HS and
LaH at 200 GPa. The calculation results indicate that these
high-pressure hydrides essentially saturate the bound of versus
.Comment: 10 pages, 5 figure
Estudo de métodos para a anålise simultanea de antioxidantes por HPLC.
O objetivo deste estudo foi o de aprimorar os mĂ©todos analĂticos usados para quantificar os homĂłlogos de vitamina E e γ-orizanol para amostras de arroz integral e branco polido e, desta maneira, fornecer ferramenta para estudos sobre os efeitos genĂ©ticos, ambientais, processamentos de pĂłs-colheita, armazenamento e cocção, sobre os nĂveis destes compostos fitoquĂmicos
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