541 research outputs found
Non-integrability of the mixmaster universe
We comment on an analysis by Contopoulos et al. which demonstrates that the
governing six-dimensional Einstein equations for the mixmaster space-time
metric pass the ARS or reduced Painlev\'{e} test. We note that this is the case
irrespective of the value, , of the generating Hamiltonian which is a
constant of motion. For we find numerous closed orbits with two
unstable eigenvalues strongly indicating that there cannot exist two additional
first integrals apart from the Hamiltonian and thus that the system, at least
for this case, is very likely not integrable. In addition, we present numerical
evidence that the average Lyapunov exponent nevertheless vanishes. The model is
thus a very interesting example of a Hamiltonian dynamical system, which is
likely non-integrable yet passes the reduced Painlev\'{e} test.Comment: 11 pages LaTeX in J.Phys.A style (ioplppt.sty) + 6 PostScript figures
compressed and uuencoded with uufiles. Revised version to appear in J Phys.
Homoclinic chaos in the dynamics of a general Bianchi IX model
The dynamics of a general Bianchi IX model with three scale factors is
examined. The matter content of the model is assumed to be comoving dust plus a
positive cosmological constant. The model presents a critical point of
saddle-center-center type in the finite region of phase space. This critical
point engenders in the phase space dynamics the topology of stable and unstable
four dimensional tubes , where is a saddle direction and
is the manifold of unstable periodic orbits in the center-center sector.
A general characteristic of the dynamical flow is an oscillatory mode about
orbits of an invariant plane of the dynamics which contains the critical point
and a Friedmann-Robertson-Walker (FRW) singularity. We show that a pair of
tubes (one stable, one unstable) emerging from the neighborhood of the critical
point towards the FRW singularity have homoclinic transversal crossings. The
homoclinic intersection manifold has topology and is constituted
of homoclinic orbits which are bi-asymptotic to the center-center
manifold. This is an invariant signature of chaos in the model, and produces
chaotic sets in phase space. The model also presents an asymptotic DeSitter
attractor at infinity and initial conditions sets are shown to have fractal
basin boundaries connected to the escape into the DeSitter configuration
(escape into inflation), characterizing the critical point as a chaotic
scatterer.Comment: 11 pages, 6 ps figures. Accepted for publication in Phys. Rev.
A Comparative Study between Two Regression Methods on LiDAR Data: A Case Study
Airborne LiDAR (Light Detection and Ranging) has become
an excellent tool for accurately assessing vegetation characteristics in
forest environments. Previous studies showed empirical relationships between
LiDAR and field-measured biophysical variables. Multiple linear
regression (MLR) with stepwise feature selection is the most common
method for building estimation models. Although this technique has provided
very interesting results, many other data mining techniques may
be applied. The overall goal of this study is to compare different methodologies
for assessing biomass fractions at stand level using airborne Li-
DAR data in forest settings. In order to choose the best methodology, a
comparison between two different feature selection techniques (stepwise
selection vs. genetic-based selection) is presented. In addition, classical
MLR is also compared with regression trees (M5P). The results when
each methodology is applied to estimate stand biomass fractions from
an area of northern Spain show that genetically-selected M5P obtains
the best results
Impact of sarcopenia on outcomes of locally advanced esophageal cancer patients treated with neoadjuvant chemoradiation followed by surgery
Lubricating Bacteria Model for Branching growth of Bacterial Colonies
Various bacterial strains (e.g. strains belonging to the genera Bacillus,
Paenibacillus, Serratia and Salmonella) exhibit colonial branching patterns
during growth on poor semi-solid substrates. These patterns reflect the
bacterial cooperative self-organization. Central part of the cooperation is the
collective formation of lubricant on top of the agar which enables the bacteria
to swim. Hence it provides the colony means to advance towards the food. One
method of modeling the colonial development is via coupled reaction-diffusion
equations which describe the time evolution of the bacterial density and the
concentrations of the relevant chemical fields. This idea has been pursued by a
number of groups. Here we present an additional model which specifically
includes an evolution equation for the lubricant excreted by the bacteria. We
show that when the diffusion of the fluid is governed by nonlinear diffusion
coefficient branching patterns evolves. We study the effect of the rates of
emission and decomposition of the lubricant fluid on the observed patterns. The
results are compared with experimental observations. We also include fields of
chemotactic agents and food chemotaxis and conclude that these features are
needed in order to explain the observations.Comment: 1 latex file, 16 jpeg files, submitted to Phys. Rev.
Antimicrobials: A Global Alliance for Optimizing Their Rational Use in Intra-Abdominal Infections (AGORA)
Intra-abdominal infections (IAI) are an important cause of morbidity and are frequently associated with poor prognosis, particularly in high-risk patients. The cornerstones in the management of complicated IAIs are timely effective source control with appropriate antimicrobial therapy. Empiric antimicrobial therapy is important in the management of intra-abdominal infections and must be broad enough to cover all likely organisms because inappropriate initial antimicrobial therapy is associated with poor patient outcomes and the development of bacterial resistance. The overuse of antimicrobials is widely accepted as a major driver of some emerging infections (such as C. difficile), the selection of resistant pathogens in individual patients, and for the continued development of antimicrobial resistance globally. The growing emergence of multi-drug resistant organisms and the limited development of new agents available to counteract them have caused an impending crisis with alarming implications, especially with regards to Gram-negative bacteria. An international task force from 79 different countries has joined this project by sharing a document on the rational use of antimicrobials for patients with IAIs. The project has been termed AGORA (Antimicrobials: A Global Alliance for Optimizing their Rational Use in Intra-Abdominal Infections). The authors hope that AGORA, involving many of the world\u27s leading experts, can actively raise awareness in health workers and can improve prescribing behavior in treating IAIs
A Comparative Study of Machine Learning Regression Methods on LiDAR Data: A Case Study
Light Detection and Ranging (LiDAR) is a remote sensor
able to extract vertical information from sensed objects. LiDAR-derived
information is nowadays used to develop environmental models for describing
fire behaviour or quantifying biomass stocks in forest areas. A
multiple linear regression (MLR) with previous stepwise feature selection
is the most common method in the literature to develop LiDAR-derived
models. MLR defines the relation between the set of field measurements
and the statistics extracted from a LiDAR flight. Machine learning has
recently been paid an increasing attention to improve classic MLR results.
Unfortunately, few studies have been proposed to compare the
quality of the multiple machine learning approaches. This paper presents
a comparison between the classic MLR-based methodology and common
regression techniques in machine learning (neural networks, regression
trees, support vector machines, nearest neighbour, and ensembles such
as random forests). The selected techniques are applied to real LiDAR
data from two areas in the province of Lugo (Galizia, Spain). The results
show that support vector regression statistically outperforms the rest of
techniques when feature selection is applied. However, its performance
cannot be said statistically different from that of Random Forests when
previous feature selection is skipped
The mixmaster universe: A chaotic Farey tale
When gravitational fields are at their strongest, the evolution of spacetime
is thought to be highly erratic. Over the past decade debate has raged over
whether this evolution can be classified as chaotic. The debate has centered on
the homogeneous but anisotropic mixmaster universe. A definite resolution has
been lacking as the techniques used to study the mixmaster dynamics yield
observer dependent answers. Here we resolve the conflict by using observer
independent, fractal methods. We prove the mixmaster universe is chaotic by
exposing the fractal strange repellor that characterizes the dynamics. The
repellor is laid bare in both the 6-dimensional minisuperspace of the full
Einstein equations, and in a 2-dimensional discretisation of the dynamics. The
chaos is encoded in a special set of numbers that form the irrational Farey
tree. We quantify the chaos by calculating the strange repellor's Lyapunov
dimension, topological entropy and multifractal dimensions. As all of these
quantities are coordinate, or gauge independent, there is no longer any
ambiguity--the mixmaster universe is indeed chaotic.Comment: 45 pages, RevTeX, 19 Figures included, submitted to PR
Do spinal cord-injured individuals with stronger sense of coherence use different psychological defense styles?
Objectives: Although the importance of sense of coherence (SOC) and psychological defense mechanisms (PDMs) in the process of coping has been demonstrated, it has not yet been clarified whether individuals with stronger SOC use specific PDMs.Study design: Cross-sectional. Setting: Iran. Methods: Demographic and injury-related variables including injury level, time since injury, American Spinal Cord Association (ASIA) Scale and Spinal cord independence measure-III were collected among individuals with spinal cord injury (SCI). SOC was assessed by the Short-form Sense of Coherence Scale. PDMs were identified using 40-version of the Defense Style Questionnaire. Results: Neurotic defense style was the most commonly used style especially. The overall most commonly used PDM was 'rationalization', which was used by 95. Individuals with stronger SOC used more mature style (P=0.001, r=0.52), particularly 'humor' and 'suppression' mechanisms (P<0.0001 and 0.024, respectively). There was a negative correlation between stronger SOC and the use of immature defenses including passive aggression (P=0.001, r=-0.51), acting out (P=0.001, r=-0.48), isolation (P=0.009, r=-0.50), autistic fantasy (P=0.010, r=-0.30) and somatization (P<0.0001, r=-0.62). Married individuals had significantly stronger SOC (P=0.01). Age, gender, age at the time of injury incidence, time since injury, ASIA score and cause of injury were not determinants of SOC. Conclusion: In this study, PDMs, which are more probable to be used by individuals with stronger SOC, have been identified. Mature defenses including 'humor' and 'suppression' are used by stronger SOC more often, whereas immature mechanisms are less likely to be used. © 2016 International Spinal Cord Society
Adiabatic invariants and Mixmaster catastrophes
We present a rigorous analysis of the role and uses of the adiabatic
invariant in the Mixmaster dynamical system. We propose a new invariant for the
global dynamics which in some respects has an improved behaviour over the
commonly used one. We illustrate its behaviour in a number of numerical
results. We also present a new formulation of the dynamics via Catastrophe
Theory. We find that the change from one era to the next corresponds to a fold
catastrophe, during the Kasner shifts the potential is an Implicit Function
Form whereas, as the anisotropy dissipates, the Mixmaster potential must become
a Morse 0--saddle. We compare and contrast our results to many known works on
the Mixmaster problem and indicate how extensions could be achieved. Further
exploitation of this formulation may lead to a clearer understanding of the
global Mixmaster dynamics.Comment: 24 pages, LaTeX, 5 figures (which can be obtained by sending a
message to the first author), submitted to Phys.Rev.
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