1,262 research outputs found
Concentration in flux-function limits of solutions to a deposition model
This paper is concerned with a singular flux-function limit of the Riemann
solutions to a deposition model. As a result, it is shown that the Riemann
solutions to the deposition model just converge to the corresponding Riemann
solutions to the limit system, which is one of typical models admitting
delta-shocks. Especially, the phenomenon of concentration and the formation of
delta-shocks in the limit are analyzed in detail, and the process of
concentration is numerically simulated.Comment: 18 page
Majorana Positivity and the Fermion sign problem of Quantum Monte Carlo Simulations
The sign problem is a major obstacle in quantum Monte Carlo simulations for
many-body fermion systems. We examine this problem with a new perspective based
on the Majorana reflection positivity and Majorana Kramers positivity. Two
sufficient conditions are proven for the absence of the fermion sign problem.
Our proof provides a unified description for all the interacting lattice
fermion models previously known to be free of the sign problem based on the
auxiliary field quantum Monte Carlo method. It also allows us to identify a
number of new sign-problem-free interacting fermion models including, but not
limited to, lattice fermion models with repulsive interactions but without
particle-hole symmetry and interacting topological insulators with spin-flip
terms.Comment: small corrections to Supplementary Material/Appendix
Graph-to-Tree Neural Networks for Learning Structured Input-Output Translation with Applications to Semantic Parsing and Math Word Problem
The celebrated Seq2Seq technique and its numerous variants achieve excellent
performance on many tasks such as neural machine translation, semantic parsing,
and math word problem solving. However, these models either only consider input
objects as sequences while ignoring the important structural information for
encoding, or they simply treat output objects as sequence outputs instead of
structural objects for decoding. In this paper, we present a novel
Graph-to-Tree Neural Networks, namely Graph2Tree consisting of a graph encoder
and a hierarchical tree decoder, that encodes an augmented graph-structured
input and decodes a tree-structured output. In particular, we investigated our
model for solving two problems, neural semantic parsing and math word problem.
Our extensive experiments demonstrate that our Graph2Tree model outperforms or
matches the performance of other state-of-the-art models on these tasks.Comment: Long Paper in EMNLP 2020. 12 pages including reference
Properties on n-dimensional convolution for image deconvolution
Convolution system is linear and time invariant, and can describe the optical
imaging process. Based on convolution system, many deconvolution techniques
have been developed for optical image analysis, such as boosting the space
resolution of optical images, image denoising, image enhancement and so on.
Here, we gave properties on N-dimensional convolution. By using these
properties, we proposed image deconvolution method. This method uses a series
of convolution operations to deconvolute image. We demonstrated that the method
has the similar deconvolution results to the state-of-art method. The core
calculation of the proposed method is image convolution, and thus our method
can easily be integrated into GPU mode for large-scale image deconvolution
Recurrent MVSNet for High-resolution Multi-view Stereo Depth Inference
Deep learning has recently demonstrated its excellent performance for
multi-view stereo (MVS). However, one major limitation of current learned MVS
approaches is the scalability: the memory-consuming cost volume regularization
makes the learned MVS hard to be applied to high-resolution scenes. In this
paper, we introduce a scalable multi-view stereo framework based on the
recurrent neural network. Instead of regularizing the entire 3D cost volume in
one go, the proposed Recurrent Multi-view Stereo Network (R-MVSNet)
sequentially regularizes the 2D cost maps along the depth direction via the
gated recurrent unit (GRU). This reduces dramatically the memory consumption
and makes high-resolution reconstruction feasible. We first show the
state-of-the-art performance achieved by the proposed R-MVSNet on the recent
MVS benchmarks. Then, we further demonstrate the scalability of the proposed
method on several large-scale scenarios, where previous learned approaches
often fail due to the memory constraint. Code is available at
https://github.com/YoYo000/MVSNet.Comment: Accepted by CVPR201
Metallic rugate structures for perfect absorbers in visible and near-infrared regions
Metallic rugate structures are theoretically investigated for achieving
perfect absorption in the visible and near-infrared regions. Our model builds
on nanoporous metal films whose porosity (volume fraction of voids) follows a
sine-wave along the film thickness. By setting the initial phase of porosity at
the top surface as 0, perfect absorption is obtained. The impacts of various
structural parameters on the characteristic absorption behaviors are studied.
Furthermore, multiple peaks or bands with high-absorption can be achieved by
integrating several periodicities in one structure. The rugate absorbers show
perfect or near-perfect absorption for TE and TM polarizations and large
incident angles
An observational revisit of band-split solar type-II radio bursts
Band split of solar type II radio bursts, discovered several decades ago, is
a fascinating phenomenon with the type-II lanes exhibiting two almost-parallel
sub-bands with similar morphology. The underlying split mechanism remains
elusive. One popular interpretation is that the splitting bands are emitted
from the shock upstream and downstream, respectively, with their frequency
ratio ({\gamma}) determined by the shock compression ratio. This interpretation
has been taken as the physical basis for many published references. Here we
report an observational analysis of type II events with nice split selected
from the ground-based RSTN data from 2001 to 2014, in the metric-decametric
wavelength. We investigate the temporal variation and distribution of {\gamma},
and conduct correlation analyses on the deduced spectral values. It is found
that {\gamma} varies in a very narrow range with >80% of {\gamma} (one-minute
averaged data) being between 1.15 to 1.25. For some well-observed and
long-lasting events, {\gamma} does not show a systematic variation trend within
observational uncertainties, from the onset to the termination of the splits.
In addition, the parameters representing the propagation speed of the radio
source (presumably the coronal shock) show a very weak or basically no
correlation with {\gamma}. We suggest that these results do not favor the
upstreamdownstream scenario of band splits
A solar type II radio burst from CME-coronal ray interaction: simultaneous radio and EUV imaging
Simultaneous radio and extreme ultraviolet (EUV)/white-light imaging data are
examined for a solar type II radio burst occurring on 2010 March 18 to deduce
its source location. Using a bow-shock model, we reconstruct the 3-dimensional
EUV wave front (presumably the type-II emitting shock) based on the imaging
data of the two STEREO spacecraft. It is then combined with the Nan\c{c}ay
radio imaging data to infer the 3-dimensional position of the type II source.
It is found that the type II source coincides with the interface between the
CME EUV wave front and a nearby coronal ray structure, providing evidence that
the type II emission is physically related to the CME-ray interaction. This
result, consistent with those of previous studies, is based on simultaneous
radio and EUV imaging data for the first time.Comment: 5 figure
Scaling Up Bayesian Uncertainty Quantification for Inverse Problems using Deep Neural Networks
Due to the importance of uncertainty quantification (UQ), Bayesian approach
to inverse problems has recently gained popularity in applied mathematics,
physics, and engineering. However, traditional Bayesian inference methods based
on Markov Chain Monte Carlo (MCMC) tend to be computationally intensive and
inefficient for such high dimensional problems. To address this issue, several
methods based on surrogate models have been proposed to speed up the inference
process. More specifically, the calibration-emulation-sampling (CES) scheme has
been proven to be successful in large dimensional UQ problems. In this work, we
propose a novel CES approach for Bayesian inference based on deep neural
network (DNN) models for the emulation phase. The resulting algorithm is not
only computationally more efficient, but also less sensitive to the training
set. Further, by using an Autoencoder (AE) for dimension reduction, we have
been able to speed up our Bayesian inference method up to three orders of
magnitude. Overall, our method, henceforth called \emph{Dimension-Reduced
Emulative Autoencoder Monte Carlo (DREAM)} algorithm, is able to scale Bayesian
UQ up to thousands of dimensions in physics-constrained inverse problems. Using
two low-dimensional (linear and nonlinear) inverse problems we illustrate the
validity this approach. Next, we apply our method to two high-dimensional
numerical examples (elliptic and advection-diffussion) to demonstrate its
computational advantage over existing algorithms.Comment: 40 pages, 20 figure
EUV and Magnetic Activities Associated with Type-I Solar Radio Bursts
Type-I bursts (i.e. noise storms) are the earliest-known type of solar radio
emission at the metre wavelength. They are believed to be excited by
non-thermal energetic electrons accelerated in the corona. The underlying
dynamic process and exact emission mechanism still remain unresolved. Here,
with a combined analysis of extreme ultraviolet (EUV), radio and photospheric
magnetic field data of unprecedented quality recorded during a type-I storm on
30 July 2011, we identify a good correlation between the radio bursts and the
co-spatial EUV and magnetic activities. The EUV activities manifest themselves
as three major brightening stripes above a region adjacent to a compact
sunspot, while the magnetic field there presents multiple moving magnetic
features (MMFs) with persistent coalescence or cancelation and a
morphologically similar three-part distribution. We find that the type-I
intensities are correlated with those of the EUV emissions at various
wavelengths with a correlation coefficient of 0.7-0.8. In addition, in the
region between the brightening EUV stripes and the radio sources there appear
consistent dynamic motions with a series of bi-directional flows, suggesting
ongoing small-scale reconnection there. Mainly based on the induced connection
between the magnetic motion at the photosphere and the EUV and radio activities
in the corona, we suggest that the observed type-I noise storms and the EUV
brightening activities are the consequence of small-scale magnetic reconnection
driven by MMFs. This is in support of the original proposal made by Bentely et
al. (Solar Phys. 193, 227, 2000)
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