9,170 research outputs found
Multiplicative random walk Metropolis-Hastings on the real line
In this article we propose multiplication based random walk Metropolis
Hastings (MH) algorithm on the real line. We call it the random dive MH (RDMH)
algorithm. This algorithm, even if simple to apply, was not studied earlier in
Markov chain Monte Carlo literature. The associated kernel is shown to have
standard properties like irreducibility, aperiodicity and Harris recurrence
under some mild assumptions. These ensure basic convergence (ergodicity) of the
kernel. Further the kernel is shown to be geometric ergodic for a large class
of target densities on . This class even contains realistic target
densities for which random walk or Langevin MH are not geometrically ergodic.
Three simulation studies are given to demonstrate the mixing property and
superiority of RDMH to standard MH algorithms on real line. A share-price
return data is also analyzed and the results are compared with those available
in the literature
Quantum energy teleportation in a quantum Hall system
We propose an experimental method for a quantum protocol termed quantum
energy teleportation (QET), which allows energy transportation to a remote
location without physical carriers. Using a quantum Hall system as a realistic
model, we discuss the physical significance of QET and estimate the order of
energy gain using reasonable experimental parameters
Disrupted functional brain network organization in patients with obstructive sleep apnea.
IntroductionObstructive sleep apnea (OSA) subjects show impaired autonomic, affective, executive, sensorimotor, and cognitive functions. Brain injury in OSA subjects appears in multiple sites regulating these functions, but the integrity of functional networks within the regulatory sites remains unclear. Our aim was to examine the functional interactions and the complex network organization of these interactions across the whole brain in OSA, using regional functional connectivity (FC) and brain network topological properties.MethodsWe collected resting-state functional magnetic resonance imaging (MRI) data, using a 3.0-Tesla MRI scanner, from 69 newly diagnosed, treatment-naïve, moderate-to-severe OSA (age, 48.3 ± 9.2 years; body mass index, 31 ± 6.2 kg/m(2); apnea-hypopnea index (AHI), 35.6 ± 23.3 events/h) and 82 control subjects (47.6 ± 9.1 years; body mass index, 25.1 ± 3.5 kg/m(2)). Data were analyzed to examine FC in OSA over controls as interregional correlations and brain network topological properties.ResultsObstructive sleep apnea subjects showed significantly altered FC in the cerebellar, frontal, parietal, temporal, occipital, limbic, and basal ganglia regions (FDR, P < 0.05). Entire functional brain networks in OSA subjects showed significantly less efficient integration, and their regional topological properties of functional integration and specialization characteristics also showed declined trends in areas showing altered FC, an outcome which would interfere with brain network organization (P < 0.05; 10,000 permutations). Brain sites with abnormal topological properties in OSA showed significant relationships with AHI scores.ConclusionsOur findings suggest that the dysfunction extends to resting conditions, and the altered FC and impaired network organization may underlie the impaired responses in autonomic, cognitive, and sensorimotor functions. The outcomes likely result from the prominent structural changes in both axons and nuclear structures, which occur in the condition
Ab initio theory of helix-coil phase transition
In this paper we suggest a theoretical method based on the statistical
mechanics for treating the alpha-helix-random coil transition in alanine
polypeptides. We consider this process as a first-order phase transition and
develop a theory which is free of model parameters and is based solely on
fundamental physical principles. It describes essential thermodynamical
properties of the system such as heat capacity, the phase transition
temperature and others from the analysis of the polypeptide potential energy
surface calculated as a function of two dihedral angles, responsible for the
polypeptide twisting. The suggested theory is general and with some
modification can be applied for the description of phase transitions in other
complex molecular systems (e.g. proteins, DNA, nanotubes, atomic clusters,
fullerenes).Comment: 24 pages, 3 figure
Noisy Monte Carlo: Convergence of Markov chains with approximate transition kernels
Monte Carlo algorithms often aim to draw from a distribution by
simulating a Markov chain with transition kernel such that is
invariant under . However, there are many situations for which it is
impractical or impossible to draw from the transition kernel . For instance,
this is the case with massive datasets, where is it prohibitively expensive to
calculate the likelihood and is also the case for intractable likelihood models
arising from, for example, Gibbs random fields, such as those found in spatial
statistics and network analysis. A natural approach in these cases is to
replace by an approximation . Using theory from the stability of
Markov chains we explore a variety of situations where it is possible to
quantify how 'close' the chain given by the transition kernel is to
the chain given by . We apply these results to several examples from spatial
statistics and network analysis.Comment: This version: results extended to non-uniformly ergodic Markov chain
Imaging of brain metabolism with positron emission tomography and magnetic resonance spectroscopy in connection with the pathophysiology of focal brain diseases
Imaging of brain metabolism with positron emission tomography and magnetic resonance spectroscopy in connection with the pathophysiology of focal brain diseases
Imaging of brain metabolism with positron emission tomography and magnetic resonance spectroscopy in connection with the pathophysiology of focal brain diseases
Imaging of brain metabolism with positron emission tomography and magnetic resonance spectroscopy in connection with the pathophysiology of focal brain diseases
- …