1,911 research outputs found
Reweighting for Nonequilibrium Markov Processes Using Sequential Importance Sampling Methods
We present a generic reweighting method for nonequilibrium Markov processes.
With nonequilibrium Monte Carlo simulations at a single temperature, one
calculates the time evolution of physical quantities at different temperatures,
which greatly saves the computational time. Using the dynamical finite-size
scaling analysis for the nonequilibrium relaxation, one can study the dynamical
properties of phase transitions together with the equilibrium ones. We
demonstrate the procedure for the Ising model with the Metropolis algorithm,
but the present formalism is general and can be applied to a variety of systems
as well as with different Monte Carlo update schemes.Comment: accepted for publication in Phys. Rev. E (Rapid Communications
A population Monte Carlo scheme with transformed weights and its application to stochastic kinetic models
This paper addresses the problem of Monte Carlo approximation of posterior
probability distributions. In particular, we have considered a recently
proposed technique known as population Monte Carlo (PMC), which is based on an
iterative importance sampling approach. An important drawback of this
methodology is the degeneracy of the importance weights when the dimension of
either the observations or the variables of interest is high. To alleviate this
difficulty, we propose a novel method that performs a nonlinear transformation
on the importance weights. This operation reduces the weight variation, hence
it avoids their degeneracy and increases the efficiency of the importance
sampling scheme, specially when drawing from a proposal functions which are
poorly adapted to the true posterior.
For the sake of illustration, we have applied the proposed algorithm to the
estimation of the parameters of a Gaussian mixture model. This is a very simple
problem that enables us to clearly show and discuss the main features of the
proposed technique. As a practical application, we have also considered the
popular (and challenging) problem of estimating the rate parameters of
stochastic kinetic models (SKM). SKMs are highly multivariate systems that
model molecular interactions in biological and chemical problems. We introduce
a particularization of the proposed algorithm to SKMs and present numerical
results.Comment: 35 pages, 8 figure
Multilevel ensemble transform particle filtering
This paper extends the multilevel Monte Carlo variance reduction technique to nonlinear filtering. In particular, multilevel Monte Carlo is applied to a certain variant of the particle filter, the ensemble transform particle filter (EPTF). A key aspect is the use of optimal transport methods to re-establish correlation between coarse and fine ensembles after resampling; this controls the variance of the estimator. Numerical examples present a proof of concept of the effectiveness of the proposed method, demonstrating significant computational cost reductions (relative to the single-level ETPF counterpart) in the propagation of ensembles
Nonequilibrium Reweighting on the Driven Diffusive Lattice Gas
The nonequilibrium reweighting technique, which was recently developed by the
present authors, is used for the study of the nonequilibrium steady states. The
renewed formulation of the nonequlibrium reweighting enables us to use the very
efficient multi-spin coding. We apply the nonequilibrium reweighting to the
driven diffusive lattice gas model. Combining with the dynamical finite-size
scaling theory, we estimate the critical temperature Tc and the dynamical
exponent z. We also argue that this technique has an interesting feature that
enables explicit calculation of derivatives of thermodynamic quantities without
resorting to numerical differences.Comment: Accepted for publication in J. Phys. A (Lett.
On Reasoning on Time and Location on the Web
Reasoning on time and location is receiving increasing attention on the Web due to emerging fields like Web adaptation, mobile computing, and the Semantic Web. Web applications in these fields often refer to rather complex temporal, calendric, and location information. Unfortunately, today’s Web languages and formalisms have merely primitive temporal and location data types and temporal and location reasoning capabilities – if any. This article reports on work in progress aiming at integrating temporal and locational reasoning into XML query and transformation operations. We analyze the problem and propose a concrete architecture. A prototype of the temporal reasoner, the WebCal system has already been realized
CI observations in the CQ Tau proto-planetary disk: evidence for a very low gas-to-dust ratio ?
Gas and dust dissipation processes of proto-planetary disks are hardly known.
Transition disks between Class II (proto-planetary disks) and Class III (debris
disks) remain difficult to detect. We investigate the carbon chemistry of the
peculiar CQ Tau gas disk. It is likely a transition disk because it exhibits
weak CO emission with a relatively strong millimeter continuum, indicating that
the disk might be currently dissipating its gas content. We used APEX to
observe the two CI lines at 492GHz and 809 GHz in the disk orbiting CQ Tau. We
compare the observations to several chemical model predictions. We focus our
study on the influence of the stellar UV radiation shape and gas-to-dust ratio.
We did not detect the CI lines. However, our upper limits are deep enough to
exclude high-CI models. The only available models compatible with our limits
imply very low gas-to-dust ratio, of the order of a few, only. These
observations strengthen the hypothesis that CQ Tau is likely a transition disk
and suggest that gas disappears before dust.Comment: 5 pages, 5 figures, accepted for publication in A&
Estrogen Receptor Hormone Agonists Limit Trauma Hemorrhage Shock-Induced Gut and Lung Injury in Rats
Acute lung injury (ALI) and the development of the multiple organ dysfunction syndrome (MODS) is a major cause of death in trauma patients. Earlier studies in trauma hemorrhagic shock (T/HS) have documented that splanchnic ischemia leading to gut inflammation and loss of barrier function is an initial triggering event that leads to gut-induced ARDS and MODS. Since sex hormones have been shown to modulate the response to T/HS and proestrous (PE) females are more resistant to T/HS-induced gut and distant organ injury, the goal of our study was to determine the contribution of estrogen receptor (ER)alpha and ERbeta in modulating the protective response of female rats to T/HS-induced gut and lung injury.The incidence of gut and lung injury was assessed in PE and ovariectomized (OVX) female rats subjected to T/HS or trauma sham shock (T/SS) as well as OVX rats that were administered estradiol (E2) or agonists for ERalpha or ERbeta immediately prior to resuscitation. Marked gut and lung injury was observed in OVX rats subjected to T/HS as compared to PE rats or E2-treated OVX rats subjected to T/HS. Both ERalpha and ERbeta agonists were equally effective in limiting T/HS-induced morphologic villous injury and bacterial translocation, whereas the ERbeta agonist was more effective than the ERalpha agonist in limiting T/HS-induced lung injury as determined by histology, Evan's blue lung permeability, bronchoalevolar fluid/plasma protein ratio and myeloperoxidase levels. Similarly, treatment with either E2 or the ERbeta agonist attenuated the induction of the intestinal iNOS response in OVX rats subjected to T/HS whereas the ERalpha agonist was only partially protective.Our study demonstrates that estrogen attenuates T/HS-induced gut and lung injury and that its protective effects are mediated by the activation of ERalpha, ERbeta or both receptors
Pediatric training and practice of Canadian chiropractic and naturopathic doctors: A 2004-2014 comparative study
Backgound:
To assess chiropractic (DC) and naturopathic doctors' (ND) knowledge, attitudes, and behaviour with respect to the pediatric patients in their practice.
Methods:
Cross-sectional surveys were developed in collaboration with DC and ND educators. Surveys were sent to randomly selected DCs and NDs in Ontario, Canada in 2004, and a national online survey was conducted in 2014. Data were analyzed using descriptive statistics, t-tests, non-parametric tests, and linear regression.
Results:
Response rates for DCs were n=172 (34%) in 2004, n=553 (15.5%) in 2014, and for NDs, n=171 (36%) in 2004, n=162 (7%) in 2014. In 2014, 366 (78.4%) of DCs and 83 (61%) of NDs saw one or more pediatric patients per week. Pediatric training was rated as inadequate by most respondents in both 2004 and 2014, with most respondents (n=643, 89.9%) seeking post-graduate training by 2014. Respondents' comfort in treating children and youth is based on experience and post-graduate training. Both DCs and NDs that see children and youth in their practices address a broad array of pediatric health concerns, from well child care and preventative health, to mild and serious illness.
Conclusions:
Although the response rate in 2014 is low, the concerns identified a decade earlier remain. The majority of responding DCs and NDs see infants, children, and youth for a variety of health conditions and issues, but self-assess their undergraduate pediatric training as inadequate. We encourage augmented pediatric educational content be included as core curriculum for DCs and NDs and suggest collaboration with institutions/organizations with expertise in pediatric education to facilitate curriculum development, especially in areas that affect patient safety
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