452 research outputs found
Optimal control models of the goal-oriented human locomotion
In recent papers it has been suggested that human locomotion may be modeled
as an inverse optimal control problem. In this paradigm, the trajectories are
assumed to be solutions of an optimal control problem that has to be
determined. We discuss the modeling of both the dynamical system and the cost
to be minimized, and we analyze the corresponding optimal synthesis. The main
results describe the asymptotic behavior of the optimal trajectories as the
target point goes to infinity
Gravitational collapse with tachyon field and barotropic fluid
A particular class of space-time, with a tachyon field, \phi, and a
barotropic fluid constituting the matter content, is considered herein as a
model for gravitational collapse. For simplicity, the tachyon potential is
assumed to be of inverse square form i.e., V(\phi) \sim \phi^{-2}. Our purpose,
by making use of the specific kinematical features of the tachyon, which are
rather different from a standard scalar field, is to establish the several
types of asymptotic behavior that our matter content induces. Employing a
dynamical system analysis, complemented by a thorough numerical study, we find
classical solutions corresponding to a naked singularity or a black hole
formation. In particular, there is a subset where the fluid and tachyon
participate in an interesting tracking behaviour, depending sensitively on the
initial conditions for the energy densities of the tachyon field and barotropic
fluid. Two other classes of solutions are present, corresponding respectively,
to either a tachyon or a barotropic fluid regime. Which of these emerges as
dominant, will depend on the choice of the barotropic parameter, \gamma.
Furthermore, these collapsing scenarios both have as final state the formation
of a black hole.Comment: 18 pages, 7 figures. v3: minor changes. Final version to appear in
GR
Phosphorus rates on yield and quality of lettuce seeds.
Because of lack of information about phosphorus fertilization in lettuce from the standpoint of seed production, this study was undertaken. The work was carried out in Botucatu, São Paulo state, Brazil, from September 25, 2003 to February 19, 2004, in order to study the influence of crescents phosphorus rates on yield and quality of lettuce seeds, cultivar Verônica. The experimental design was randomized blocks with five treatments (0; 200; 400; 600 and 800 kg ha-1 of P2O5) and five replications. The following characteristics were evaluated: seed production per plant (g plant-1), number of seeds per plant and quality (percentage of germination and seed vigor). A linear increasing was observed on production and number of seeds per plant of lettuce with phosphorus rates, but these rates did not affect the seed physiological quality evaluated on weight of 1,000 seeds (average = 0.91 g), first count of germination (98%), standard germination test (99%), seed germination speed index (68.5), seedling emergence speed index (97.0) and emerged seedlings (49.3%)
Accelerated expansion from ghost-free bigravity: a statistical analysis with improved generality
We study the background cosmology of the ghost-free, bimetric theory of
gravity. We perform an extensive statistical analysis of the model using both
frequentist and Bayesian frameworks and employ the constraints on the expansion
history of the Universe from the observations of supernovae, the cosmic
microwave background and the large scale structure to estimate the model's
parameters and test the goodness of the fits. We explore the parameter space of
the model with nested sampling to find the best-fit chi-square, obtain the
Bayesian evidence, and compute the marginalized posteriors and mean
likelihoods. We mainly focus on a class of sub-models with no explicit
cosmological constant (or vacuum energy) term to assess the ability of the
theory to dynamically cause a late-time accelerated expansion. The model
behaves as standard gravity without a cosmological constant at early times,
with an emergent extra contribution to the energy density that converges to a
cosmological constant in the far future. The model can in most cases yield very
good fits and is in perfect agreement with the data. This is because many
points in the parameter space of the model exist that give rise to
time-evolution equations that are effectively very similar to those of the
CDM. This similarity makes the model compatible with observations as
in the CDM case, at least at the background level. Even though our
results indicate a slightly better fit for the CDM concordance model
in terms of the -value and evidence, none of the models is statistically
preferred to the other. However, the parameters of the bigravity model are in
general degenerate. A similar but perturbative analysis of the model as well as
more data will be required to break the degeneracies and constrain the
parameters, in case the model will still be viable compared to the
CDM.Comment: 42 pages, 9 figures; typos corrected in equations (2.12), (2.13),
(3.7), (3.8) and (3.9); more discussions added (footnotes 5, 8, 10 and 13)
and abstract, sections 4.2, 4.3 and 5 (conclusions) modified in response to
referee's comments; references added; acknowledgements modified; all results
completely unchanged; matches version accepted for publication in JHE
The Cosmological Constant
This is a review of the physics and cosmology of the cosmological constant.
Focusing on recent developments, I present a pedagogical overview of cosmology
in the presence of a cosmological constant, observational constraints on its
magnitude, and the physics of a small (and potentially nonzero) vacuum energy.Comment: 50 pages. Submitted to Living Reviews in Relativity
(http://www.livingreviews.org/), December 199
Brane-World Gravity
The observable universe could be a 1+3-surface (the "brane") embedded in a
1+3+\textit{d}-dimensional spacetime (the "bulk"), with Standard Model
particles and fields trapped on the brane while gravity is free to access the
bulk. At least one of the \textit{d} extra spatial dimensions could be very
large relative to the Planck scale, which lowers the fundamental gravity scale,
possibly even down to the electroweak ( TeV) level. This revolutionary
picture arises in the framework of recent developments in M theory. The
1+10-dimensional M theory encompasses the known 1+9-dimensional superstring
theories, and is widely considered to be a promising potential route to quantum
gravity. At low energies, gravity is localized at the brane and general
relativity is recovered, but at high energies gravity "leaks" into the bulk,
behaving in a truly higher-dimensional way. This introduces significant changes
to gravitational dynamics and perturbations, with interesting and potentially
testable implications for high-energy astrophysics, black holes, and cosmology.
Brane-world models offer a phenomenological way to test some of the novel
predictions and corrections to general relativity that are implied by M theory.
This review analyzes the geometry, dynamics and perturbations of simple
brane-world models for cosmology and astrophysics, mainly focusing on warped
5-dimensional brane-worlds based on the Randall--Sundrum models. We also cover
the simplest brane-world models in which 4-dimensional gravity on the brane is
modified at \emph{low} energies -- the 5-dimensional Dvali--Gabadadze--Porrati
models. Then we discuss co-dimension two branes in 6-dimensional models.Comment: A major update of Living Reviews in Relativity 7:7 (2004)
"Brane-World Gravity", 119 pages, 28 figures, the update contains new
material on RS perturbations, including full numerical solutions of
gravitational waves and scalar perturbations, on DGP models, and also on 6D
models. A published version in Living Reviews in Relativit
Correlates of Untreated Hypercholesterolemia in Older Adults: A Community-Based Household Survey in China.
Hypercholesterolemia is common in older adults and less treated, but little is known about correlates of untreated hypercholesterolemia. Using a standard interview method we examined a random sample of 7,572 participants aged 60 years in a community-based household survey across 7 provinces of China during 2007–2012, and documented 328 cases of hypercholesterolemia from self-reported doctor diagnosis. Compared to participants with normal cholesterol, older adults with hypercholesterolemia had higher socioeconomic position and larger body mass index. In patients with hypercholesterolemia, 209 were not treated using lipid-lowering medications (63.7%, 95% confidence interval (CI) 58.5%– 68.9%). Untreated hypercholesterolemia was significantly associated with female sex (adjusted odds ratio 2.13, 95%CI 1.17–3.89), current smoking (3.48, 1.44–8.44), heavy alcohol drinking (3.13,1.11–8.84), chronic bronchitis (2.37,1.14–4.90) and high level of meat consumptions (2.85,1.22–6.65). Although having coronary heart disease exposed participants for treatment, half of participants with coronary heart disease did not receive lipid-lowering medications. Among hypercholesterolemia participants with stroke, hypertension or diabetes, more than half of them did not receive lipid-lowering medications. The high proportion of untreated hypercholesterolemia in older, high-risk Chinese adults needs to be mitigated through multi-faceted primary and secondary prevention strategies to increase population opportunities of treating hypercholesterolemia. PLOS ONEThe BUPA Foundation (Grants Nos. 45NOV06, and TBF-M09-05), and Alzheimer's Research, UK (Grant Nos. ART/PPG2007B/2
Impact of housing on the survival of persons with AIDS
<p>Abstract</p> <p>Background</p> <p>Homeless persons with HIV/AIDS have greater morbidity and mortality, more hospitalizations, less use of antiretroviral therapy, and worse medication adherence than HIV-infected persons who are stably housed. We examined the effect of homelessness on the mortality of persons with AIDS and measured the effect of supportive housing on AIDS survival.</p> <p>Methods</p> <p>The San Francisco AIDS registry was used to identify homeless and housed persons who were diagnosed with AIDS between 1996 and 2006. The registry was computer-matched with a housing database of homeless persons who received housing after their AIDS diagnosis. The Kaplan-Meier product limit method was used to compare survival between persons who were homeless at AIDS diagnosis and those who were housed. Proportional hazards models were used to estimate the independent effects of homelessness and supportive housing on survival after AIDS diagnosis.</p> <p>Results</p> <p>Of the 6,558 AIDS cases, 9.8% were homeless at diagnosis. Sixty-seven percent of the persons who were homeless survived five years compared with 81% of those who were housed (p < 0.0001). Homelessness increased the risk of death (adjusted relative hazard [RH] 1.20; 95% confidence limits [CL] 1.03, 1.41). Homeless persons with AIDS who obtained supportive housing had a lower risk of death than those who did not (adjusted RH 0.20; 95% CL 0.05, 0.81).</p> <p>Conclusion</p> <p>Supportive housing ameliorates the negative effect of homelessness on survival with AIDS.</p
f(R) theories
Over the past decade, f(R) theories have been extensively studied as one of
the simplest modifications to General Relativity. In this article we review
various applications of f(R) theories to cosmology and gravity - such as
inflation, dark energy, local gravity constraints, cosmological perturbations,
and spherically symmetric solutions in weak and strong gravitational
backgrounds. We present a number of ways to distinguish those theories from
General Relativity observationally and experimentally. We also discuss the
extension to other modified gravity theories such as Brans-Dicke theory and
Gauss-Bonnet gravity, and address models that can satisfy both cosmological and
local gravity constraints.Comment: 156 pages, 14 figures, Invited review article in Living Reviews in
Relativity, Published version, Comments are welcom
HIV Protease Inhibitors Act as Competitive Inhibitors of the Cytoplasmic Glucose Binding Site of GLUTs with Differing Affinities for GLUT1 and GLUT4
The clinical use of several first generation HIV protease inhibitors (PIs) is associated with the development of insulin resistance. Indinavir has been shown to act as a potent reversible noncompetitive inhibitor of zero-trans glucose influx via direct interaction with the insulin responsive facilitative glucose transporter GLUT4. Newer drugs within this class have differing effects on insulin sensitivity in treated patients. GLUTs are known to contain two distinct glucose-binding sites that are located on opposite sides of the lipid bilayer. To determine whether interference with the cytoplasmic glucose binding site is responsible for differential effects of PIs on glucose transport, intact intracellular membrane vesicles containing GLUT1 and GLUT4, which have an inverted transporter orientation relative to the plasma membrane, were isolated from 3T3-L1 adipocytes. The binding of biotinylated ATB-BMPA, a membrane impermeable bis-mannose containing photolabel, was determined in the presence of indinavir, ritonavir, atazanavir, tipranavir, and cytochalasin b. Zero-trans 2-deoxyglucose transport was measured in both 3T3-L1 fibroblasts and primary rat adipocytes acutely exposed to these compounds. PI inhibition of glucose transport correlated strongly with the PI inhibition of ATB-BMPA/transporter binding. At therapeutically relevant concentrations, ritonavir was not selective for GLUT4 over GLUT1. Indinavir was found to act as a competitive inhibitor of the cytoplasmic glucose binding site of GLUT4 with a KI of 8.2 µM. These data establish biotinylated ATB-BMPA as an effective probe to quantify accessibility of the endofacial glucose-binding site in GLUTs and reveal that the ability of PIs to block this site differs among drugs within this class. This provides mechanistic insight into the basis for the clinical variation in drug-related metabolic toxicity
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