21,727 research outputs found
Isometric shifts and metric spaces
Let M be a complete metric space. It is proved that if the space or
scalar-valued bounded continuous functions on M admits an isometric shift, then
M is separable.Comment: 10 pages, LaTeX, no figure
Automatic Detection of Trends in Dynamical Text: An Evolutionary Approach
This paper presents an evolutionary algorithm for modeling the arrival dates
of document streams, which is any time-stamped collection of documents, such as
newscasts, e-mails, IRC conversations, scientific journals archives and weblog
postings. This algorithm assigns frequencies (number of document arrivals per
time unit) to time intervals so that it produces an optimal fit to the data.
The optimization is a trade off between accurately fitting the data and
avoiding too many frequency changes; this way the analysis is able to find fits
which ignore the noise. Classical dynamic programming algorithms are limited by
memory and efficiency requirements, which can be a problem when dealing with
long streams. This suggests to explore alternative search methods which allow
for some degree of uncertainty to achieve tractability. Experiments have shown
that the designed evolutionary algorithm is able to reach the same solution
quality as those classical dynamic programming algorithms in a shorter time. We
have also explored different probabilistic models to optimize the fitting of
the date streams, and applied these algorithms to infer whether a new arrival
increases or decreases {\em interest} in the topic the document stream is
about.Comment: 22 pages, submitted to Journal of Information Retrieva
Entropy in locally-de Sitter spacetimes
As quotient spaces, Minkowski and de Sitter are fundamental spacetimes in the
sense that they are known "a priori", independently of Einstein equation. They
represent different non-gravitational backgrounds for the construction of
physical theories. If general relativity is constructed on a de Sitter
spacetime, the underlying kinematics will no longer be ruled by Poincar\'e, but
by the de Sitter group. In this case the definition of diffeomorphism changes,
producing concomitant changes in the notions of energy and entropy. These
changes are explicitly discussed for the case of the Schwarzschild solution, in
which the black hole and the de Sitter horizons show up as a unique entangled
system. Such entanglement, together with energy conservation, create a
constraint between the black hole activity and the evolution of the de Sitter
radius, providing a new scenario for the study of cosmology.Comment: 11 pages. V2: presentation changes aiming at clarifying the text.
Version accepted for publication in Int. J. Mod. Phys.
Flat deformations of
In this paper we study projective flat deformations of projective spaces. We
prove that the singular fibers of projective flat deformations of projective
spaces appear either in codimension 1 or over singular points of the base. We
also describe projective flat deformations of projective spaces with smooth
total space, and discuss flatness criteria.Comment: 6 page
NectaRSS, an RSS feed ranking system that implicitly learns user preferences
In this paper a new RSS feed ranking method called NectaRSS is introduced.
The system recommends information to a user based on his/her past choices. User
preferences are automatically acquired, avoiding explicit feedback, and ranking
is based on those preferences distilled to a user profile. NectaRSS uses the
well-known vector space model for user profiles and new documents, and compares
them using information-retrieval techniques, but introduces a novel method for
user profile creation and adaptation from users' past choices. The efficiency
of the proposed method has been tested by embedding it into an intelligent
aggregator (RSS feed reader), which has been used by different and
heterogeneous users. Besides, this paper proves that the ranking of newsitems
yielded by NectaRSS improves its quality with user's choices, and its
superiority over other algorithms that use a different information
representation method.Comment: Submitted to First Monday. 16 page
Gravitational wave background from neutron star phase transition
We study the generation of a stochastic gravitational wave (GW) background
produced by a population of neutron stars (NSs) which go over a hadron-quark
phase transition in its inner shells. We obtain, for example, that the NS phase
transition, in cold dark matter scenarios, could generate a stochastic GW
background with a maximum amplitude of , in the
frequency band for stars forming at
redshifts of up to We study the possibility of detection of this
isotropic GW background by correlating signals of a pair of Advanced LIGO
observatories.Comment: 17 pages, 5 figure
de Sitter-invariant special relativity and the dark energy problem
The replacement of the Poincar\'e-invariant Einstein special relativity by a
de Sitter-invariant special relativity produces concomitant changes in all
relativistic theories, including general relativity. A crucial change in the
latter is that both the background de Sitter curvature and the gravitational
dynamical curvature turns out to be included in a single curvature tensor. This
means that the cosmological term no longer explicitly appears in Einstein
equation, and is consequently not restricted to be constant. In this paper, the
Newtonian limit of such theory is obtained, and the ensuing Newtonian Friedmann
equations are show to provide a good account of the dark energy content of the
present-day universe.Comment: 14 pages. Accepted for publication in Class. Quantum Grav. V2: a few
corrections and small presentation change
On the volume of singular-hyperbolic sets
An attractor for a 3-vector field is singular-hyperbolic if all
its singularities are hyperbolic and it is partially hyperbolic with volume
expanding central direction. We prove that singular-hyperbolic
attractors, for some , always have zero volume, thus extending an
analogous result for uniformly hyperbolic attractors. The same result holds for
a class of higher dimensional singular attractors. Moreover, we prove that if
is a singular-hyperbolic attractor for then either it has zero
volume or is an Anosov flow. We also present examples of
singular-hyperbolic attractors with positive volume. In addition, we show that
generically we have volume zero for robust classes of
singular-hyperbolic attractors.Comment: 19 pages, 3 figures; references updated and minor correction
de Sitter invariant special relativity and galaxy rotation curves
Owing to the existence of an invariant length at the Planck scale, Einstein
special relativity breaks down at that scale. A possible solution to this
problem is arguably to replace the Poincar\'e invariant Einstein special
relativity by a de Sitter invariant special relativity. In addition to
reconciling Lorentz symmetry with the existence of an invariant length, such
replacement produces concomitant changes in all relativistic theories,
including general relativity, which becomes what we have called 'de Sitter
modified general relativity'. In this paper, the Newtonian limit of this theory
is used to study the circular velocity of stars around the galactic center. It
is shown that the de Sitter modified Newtonian force---which includes
corrections coming from the underlying local kinematics---could possibly
explain the rotation curve of galaxies without the necessity of supposing the
existence of a dark matter halo.Comment: 12 pages, 1 figure. V2: presentation changes aiming at clarifying the
text, 13 pages, 1 figure; matches published version. arXiv admin note: text
overlap with arXiv:1704.0212
Stacked triangular lattice: Percolation properties
The stacked triangular lattice has the shape of a triangular prism. In spite
of being considered frequently in solid state physics and materials science,
its percolation properties have received few attention. We investigate several
non-universal percolation properties on this lattice using Monte Carlo
simulation. We show that the percolation threshold is
for bonds and
for sites. The number of clusters at
the threshold per site is and
. The stacked triangular lattice is a
convenient choice to study the RGB model [Sci. Rep. {\bf 2}, 751 (2012)]. We
present results on this model and its scaling behavior at the percolation
threshold
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