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Cryo-EM structure of the mature dengue virus at 3.5-Ã… resolution.
Regulated by pH, membrane-anchored proteins E and M function during dengue virus maturation and membrane fusion. Our atomic model of the whole virion from cryo-electron microscopy at 3.5-Ã… resolution reveals that in the mature virus at neutral extracellular pH, the N-terminal 20-amino-acid segment of M (involving three pH-sensing histidines) latches and thereby prevents spring-loaded E fusion protein from prematurely exposing its fusion peptide. This M latch is fastened at an earlier stage, during maturation at acidic pH in the trans-Golgi network. At a later stage, to initiate infection in response to acidic pH in the late endosome, M releases the latch and exposes the fusion peptide. Thus, M serves as a multistep chaperone of E to control the conformational changes accompanying maturation and infection. These pH-sensitive interactions could serve as targets for drug discovery
Non-intrusive stochastic analysis with parameterized imprecise probability models: I. Performance estimation
© 2019 Elsevier Ltd Uncertainty propagation through the simulation models is critical for computational mechanics engineering to provide robust and reliable design in the presence of polymorphic uncertainty. This set of companion papers present a general framework, termed as non-intrusive imprecise stochastic simulation, for uncertainty propagation under the background of imprecise probability. This framework is composed of a set of methods developed for meeting different goals. In this paper, the performance estimation is concerned. The local extended Monte Carlo simulation (EMCS) is firstly reviewed, and then the global EMCS is devised to improve the global performance. Secondly, the cut-HDMR (High-Dimensional Model Representation) is introduced for decomposing the probabilistic response functions, and the local EMCS method is used for estimating the cut-HDMR component functions. Thirdly, the RS (Random Sampling)-HDMR is introduced to decompose the probabilistic response functions, and the global EMCS is applied for estimating the RS-HDMR component functions. The statistical errors of all estimators are derived, and the truncation errors are estimated by two global sensitivity indices, which can also be used for identifying the influential HDMR components. In the companion paper, the reliability and rare event analysis are treated. The effectiveness of the proposed methods are demonstrated by numerical and engineering examples
Non-intrusive stochastic analysis with parameterized imprecise probability models: II. Reliability and rare events analysis
© 2019 Elsevier Ltd Structural reliability analysis for rare failure events in the presence of hybrid uncertainties is a challenging task drawing increasing attentions in both academic and engineering fields. Based on the new imprecise stochastic simulation framework developed in the companion paper, this work aims at developing efficient methods to estimate the failure probability functions subjected to rare failure events with the hybrid uncertainties being characterized by imprecise probability models. The imprecise stochastic simulation methods are firstly improved by the active learning procedure so as to reduce the computational costs. For the more challenging rare failure events, two extended subset simulation based sampling methods are proposed to provide better performances in both local and global parameter spaces. The computational costs of both methods are the same with the classical subset simulation method. These two methods are also combined with the active learning procedure so as to further substantially reduce the computational costs. The estimation errors of all the methods are analyzed based on sensitivity indices and statistical properties of the developed estimators. All these new developments enrich the imprecise stochastic simulation framework. The feasibility and efficiency of the proposed methods are demonstrated with numerical and engineering test examples
Efficient Mixing at low Reynolds numbers using polymer additives
Mixing in fluids is a rapidly developing field of fluid mechanics
\cite{Sreen,Shr,War}, being an important industrial and environmental problem.
The mixing of liquids at low Reynolds numbers is usually quite weak in simple
flows, and it requires special devices to be efficient. Recently, the problem
of mixing was solved analytically for a simple case of random flow, known as
the Batchelor regime \cite{Bat,Kraich,Fal,Sig,Fouxon}. Here we demonstrate
experimentally that very viscous liquids at low Reynolds number, . Here we
show that very viscous liquids containing a small amount of high molecular
weight polymers can be mixed quite efficiently at very low Reynolds numbers,
for a simple flow in a curved channel. A polymer concentration of only 0.001%
suffices. The presence of the polymers leads to an elastic instability
\cite{LMS} and to irregular flow \cite{Ours}, with velocity spectra
corresponding to the Batchelor regime \cite{Bat,Kraich,Fal,Sig,Fouxon}. Our
detailed observations of the mixing in this regime enable us to confirm sevearl
important theoretical predictions: the probability distributions of the
concentration exhibit exponential tails \cite{Fal,Fouxon}, moments of the
distribution decay exponentially along the flow \cite{Fouxon}, and the spatial
correlation function of concentration decays logarithmically.Comment: 11 pages, 5 figure
Ages and Masses of 0.64 million Red Giant Branch stars from the LAMOST Galactic Spectroscopic Survey
We present a catalog of stellar age and mass estimates for a sample of
640\,986 red giant branch (RGB) stars of the Galactic disk from the LAMOST
Galactic Spectroscopic Survey (DR4). The RGB stars are distinguished from the
red clump stars utilizing period spacing derived from the spectra with a
machine learning method based on kernel principal component analysis (KPCA).
Cross-validation suggests our method is capable of distinguishing RC from RGB
stars with only 2 per cent contamination rate for stars with signal-to-noise
ratio (SNR) higher than 50. The age and mass of these RGB stars are determined
from their LAMOST spectra with KPCA method by taking the LAMOST -
giant stars having asteroseismic parameters and the LAMOST-TGAS sub-giant stars
based on isochrones as training sets. Examinations suggest that the age and
mass estimates of our RGB sample stars with SNR 30 have a median error of
30 per cent and 10 per cent, respectively. Stellar ages are found to exhibit
positive vertical and negative radial gradients across the disk, and the age
structure of the disk is strongly flared across the whole disk of
\,kpc. The data set demonstrates good correlations among stellar age,
[Fe/H] and [/Fe]. There are two separate sequences in the [Fe/H] --
[/Fe] plane: a high-- sequence with stars older than
\,8\,Gyr and a low-- sequence composed of stars with ages
covering the whole range of possible ages of stars. We also examine relations
between age and kinematic parameters derived from the Gaia DR2 parallax and
proper motions. Both the median value and dispersion of the orbital
eccentricity are found to increase with age. The vertical angular momentum is
found to fairly smoothly decrease with age from 2 to 12\,Gyr, with a rate of
about 50\,kpc\,km\,s\,Gyr. A full table of the catalog is
public available online.Comment: 16 pages, 22 figures,accepted by MNRA
The Temporal and Spectral Characteristics of "Fast Rise and Exponential Decay" Gamma-Ray Burst Pulses
In this paper we have analyzed the temporal and spectral behavior of 52 Fast
Rise and Exponential Decay (FRED) pulses in 48 long-duration gamma-ray bursts
(GRBs) observed by the CGRO/BATSE, using a pulse model with two shape
parameters and the Band model with three shape parameters, respectively. It is
found that these FRED pulses are distinguished both temporally and spectrally
from those in long-lag pulses. Different from these long-lag pulses only one
parameter pair indicates an evident correlation among the five parameters,
which suggests that at least 4 parameters are needed to model burst
temporal and spectral behavior. In addition, our studies reveal that these FRED
pulses have correlated properties: (i) long-duration pulses have harder spectra
and are less luminous than short-duration pulses; (ii) the more asymmetric the
pulses are the steeper the evolutionary curves of the peak energy () in
the spectrum within pulse decay phase are. Our statistical
results give some constrains on the current GRB models.Comment: 18 pages, 7 figures, accepted for publication in the Astrophysical
Journa
Kinematic Analysis and Dimensional Synthesis of Exechon Parallel Kinematic Machine for Large Volume Machining
A parallel kinematic machine (PKM) topology can only give its best performance when its geometrical parameters are optimized. In this paper, dimensional synthesis of a newly developed PKM is presented for the first time. An optimization method is developed with the objective to maximize both workspace volume and global dexterity of the PKM. Results show that the method can effectively identify design parameter changes under different weighted objectives. The PKM with optimized dimensions has a large workspace to footprint ratio and a large well-conditioned workspace, hence justifies its suitability for large volume machining.</jats:p
On higher analogues of Courant algebroids
In this paper, we study the algebraic properties of the higher analogues of
Courant algebroid structures on the direct sum bundle
for an -dimensional manifold. As an application, we revisit Nambu-Poisson
structures and multisymplectic structures. We prove that the graph of an
-vector field is closed under the higher-order Dorfman bracket iff
is a Nambu-Poisson structure. Consequently, there is an induced Leibniz
algebroid structure on . The graph of an -form is
closed under the higher-order Dorfman bracket iff is a
premultisymplectic structure of order , i.e. \dM\omega=0. Furthermore,
there is a Lie algebroid structure on the admissible bundle
. In particular, for a 2-plectic structure, it induces
the Lie 2-algebra structure given in \cite{baez:classicalstring}.Comment: 13 page
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