12,557 research outputs found
Well-Conditioned Fractional Collocation Methods Using Fractional Birkhoff Interpolation Basis
The purpose of this paper is twofold. Firstly, we provide explicit and
compact formulas for computing both Caputo and (modified) Riemann-Liouville
(RL) fractional pseudospectral differentiation matrices (F-PSDMs) of any order
at general Jacobi-Gauss-Lobatto (JGL) points. We show that in the Caputo case,
it suffices to compute F-PSDM of order to compute that of any
order with integer while in the modified RL case, it is only
necessary to evaluate a fractional integral matrix of order
Secondly, we introduce suitable fractional JGL Birkhoff interpolation problems
leading to new interpolation polynomial basis functions with remarkable
properties: (i) the matrix generated from the new basis yields the exact
inverse of F-PSDM at "interior" JGL points; (ii) the matrix of the highest
fractional derivative in a collocation scheme under the new basis is diagonal;
and (iii) the resulted linear system is well-conditioned in the Caputo case,
while in the modified RL case, the eigenvalues of the coefficient matrix are
highly concentrated. In both cases, the linear systems of the collocation
schemes using the new basis can solved by an iterative solver within a few
iterations. Notably, the inverse can be computed in a very stable manner, so
this offers optimal preconditioners for usual fractional collocation methods
for fractional differential equations (FDEs). It is also noteworthy that the
choice of certain special JGL points with parameters related to the order of
the equations can ease the implementation. We highlight that the use of the
Bateman's fractional integral formulas and fast transforms between Jacobi
polynomials with different parameters, are essential for our algorithm
development.Comment: 30 pages, 10 figures and 1 tabl
Robust Decoding from 1-Bit Compressive Sampling with Least Squares
In 1-bit compressive sensing (1-bit CS) where target signal is coded into a
binary measurement, one goal is to recover the signal from noisy and quantized
samples. Mathematically, the 1-bit CS model reads: , where ,
, and is the random error before
quantization and is a random vector modeling the sign
flips. Due to the presence of nonlinearity, noise and sign flips, it is quite
challenging to decode from the 1-bit CS. In this paper, we consider least
squares approach under the over-determined and under-determined settings. For
, we show that, up to a constant , with high probability, the least
squares solution approximates with precision
as long as . For , we
prove that, up to a constant , with high probability, the
-regularized least-squares solution lies in the ball with
center and radius provided that and . We introduce a Newton type method, the
so-called primal and dual active set (PDAS) algorithm, to solve the nonsmooth
optimization problem. The PDAS possesses the property of one-step convergence.
It only requires to solve a small least squares problem on the active set.
Therefore, the PDAS is extremely efficient for recovering sparse signals
through continuation. We propose a novel regularization parameter selection
rule which does not introduce any extra computational overhead. Extensive
numerical experiments are presented to illustrate the robustness of our
proposed model and the efficiency of our algorithm
Suppression of the emittance growth induced by coherent synchrotron radiation in triple-bend achromats
The coherent synchrotron radiation (CSR) effect in a bending path plays an
important role in transverse emittance dilution in high-brightness light
sources and linear colliders, where the electron beams are of short bunch
length and high peak current. Suppression of the emittance growth induced by
CSR is critical to preserve the beam quality and help improve the machine
performance. It has been shown that the CSR effect in a double-bend achromat
(DBA) can be analyzed with the two-dimensional point-kick analysis method. In
this paper, this method is applied to analyze the CSR effect in a triple-bend
achromat (TBA) with symmetric layout, which is commonly used in the optics
designs of energy recovery linacs (ERLs). A condition of cancelling the CSR
linear effect in such a TBA is obtained, and is verified through numerical
simulations. It is demonstrated that emittance preservation can be achieved
with this condition, and to a large extent, has a high tolerance to the
fluctuation of the initial transverse phase space distribution of the beam.Comment: 9 pages, 4 figure
Approximation Algorithm for Fault-Tolerant Virtual Backbone in Wireless Sensor Networks
To save energy and alleviate interferences in a wireless sensor network, the
usage of virtual backbone was proposed. Because of accidental damages or energy
depletion, it is desirable to construct a fault tolerant virtual backbone,
which can be modeled as a -connected -fold dominating set (abbreviated as
-CDS) in a graph. A node set is a -CDS of graph
if every node in is adjacent with at least nodes in
and the subgraph of induced by is -connected. In this paper, we
present an approximation algorithm for the minimum -CDS problem with
. The performance ratio is at most , where
for and
for , and is the performance ratio for the minimum
-CDS problem. Using currently best known value of , the
performance ratio is , where is the maximum
degree of the graph, which is asymptotically best possible in view of the
non-approximability of the problem. This is the first performance-guaranteed
algorithm for the minimum -CDS problem on a general graph. Furthermore,
applying our algorithm on a unit disk graph which models a homogeneous wireless
sensor network, the performance ratio is less than 27, improving previous ratio
62.3 by a large amount for the -CDS problem on a unit disk graph.Comment: IEEE/ACM Transactions on Networking, 201
SNAP: A semismooth Newton algorithm for pathwise optimization with optimal local convergence rate and oracle properties
We propose a semismooth Newton algorithm for pathwise optimization (SNAP) for
the LASSO and Enet in sparse, high-dimensional linear regression. SNAP is
derived from a suitable formulation of the KKT conditions based on Newton
derivatives. It solves the semismooth KKT equations efficiently by actively and
continuously seeking the support of the regression coefficients along the
solution path with warm start. At each knot in the path, SNAP converges locally
superlinearly for the Enet criterion and achieves an optimal local convergence
rate for the LASSO criterion, i.e., SNAP converges in one step at the cost of
two matrix-vector multiplication per iteration. Under certain regularity
conditions on the design matrix and the minimum magnitude of the nonzero
elements of the target regression coefficients, we show that SNAP hits a
solution with the same signs as the regression coefficients and achieves a
sharp estimation error bound in finite steps with high probability. The
computational complexity of SNAP is shown to be the same as that of LARS and
coordinate descent algorithms per iteration. Simulation studies and real data
analysis support our theoretical results and demonstrate that SNAP is faster
and accurate than LARS and coordinate descent algorithms
Suppression of the emittance growth induced by CSR in a DBA cell
The Emittace growth induced by Coherent Synchrotron Radiation(CSR) is an
important issue when electron bunches with short bunch length and high peak
current are transported in a bending magnet. In this paper, a single kick
method is introduced which could give the same result as the R-matrix method,
and much easier to use. Then with this method, an optics design technique which
could minimize the emittance dilution within a single achromatic cell.Comment: 7 pages, 6 figure
Measurements of Outflow Velocities in On-Disk Plumes from EIS Hinode Observations
The contribution of plumes to the solar wind has been subject to hot debate
in the past decades. The EUV Imaging Spectrometer (EIS) on board Hinode
provides a unique means to deduce outflow velocities at coronal heights via
direct Doppler shift measurements of coronal emission lines. Such direct
Doppler shift measurements were not possible with previous spectrometers. We
measure the outflow velocity at coronal heights in several on-disk
long-duration plumes, which are located in coronal holes and show significant
blue shifts throughout the entire observational period. In one case, a plume is
measured 4 hours apart. The deduced outflow velocities are consistent,
suggesting that the flows are quasi-steady. Furthermore, we provide an outflow
velocity profile along the plumes, finding that the velocity corrected for the
line-of-sight effect can reach 10 km s at 1.02 , 15 km
s at 1.03 , and 25 km s at 1.05 . This
clear signature of steady acceleration, combined with the fact that there is no
significant blue shift at the base of plumes, provides an important constraint
on plume models. At the height of 1.03 , EIS also deduced a density
of 1.3 cm, resulting in a proton flux of about
4.2 cms scaled to 1AU, which is an order of magnitude
higher than the proton input to a typical solar wind if a radial expansion is
assumed. This suggests that, coronal hole plumes may be an important source of
the solar wind.Comment: accepted for publication in ApJ, 13 pages, 9 figure
Real-Time Dense Stereo Embedded in A UAV for Road Inspection
The condition assessment of road surfaces is essential to ensure their
serviceability while still providing maximum road traffic safety. This paper
presents a robust stereo vision system embedded in an unmanned aerial vehicle
(UAV). The perspective view of the target image is first transformed into the
reference view, and this not only improves the disparity accuracy, but also
reduces the algorithm's computational complexity. The cost volumes generated
from stereo matching are then filtered using a bilateral filter. The latter has
been proved to be a feasible solution for the functional minimisation problem
in a fully connected Markov random field model. Finally, the disparity maps are
transformed by minimising an energy function with respect to the roll angle and
disparity projection model. This makes the damaged road areas more
distinguishable from the road surface. The proposed system is implemented on an
NVIDIA Jetson TX2 GPU with CUDA for real-time purposes. It is demonstrated
through experiments that the damaged road areas can be easily distinguished
from the transformed disparity maps.Comment: 9 pages, 8 figures, In Proceedings of the IEEE Conference on Computer
Vision and Pattern Recognition (CVPR) Workshops, June 16-20, 2019, Long
Beach, US
A Unified Primal Dual Active Set Algorithm for Nonconvex Sparse Recovery
In this paper, we consider the problem of recovering a sparse signal based on
penalized least squares formulations. We develop a novel algorithm of
primal-dual active set type for a class of nonconvex sparsity-promoting
penalties, including , bridge, smoothly clipped absolute deviation,
capped and minimax concavity penalty. First we establish the existence
of a global minimizer for the related optimization problems. Then we derive a
novel necessary optimality condition for the global minimizer using the
associated thresholding operator. The solutions to the optimality system are
coordinate-wise minimizers, and under minor conditions, they are also local
minimizers. Upon introducing the dual variable, the active set can be
determined using the primal and dual variables together. Further, this relation
lends itself to an iterative algorithm of active set type which at each step
involves first updating the primal variable only on the active set and then
updating the dual variable explicitly. When combined with a continuation
strategy on the regularization parameter, the primal dual active set method is
shown to converge globally to the underlying regression target under certain
regularity conditions. Extensive numerical experiments with both simulated and
real data demonstrate its superior performance in efficiency and accuracy
compared with the existing sparse recovery methods
A Self-Training Method for Machine Reading Comprehension with Soft Evidence Extraction
Neural models have achieved great success on machine reading comprehension
(MRC), many of which typically consist of two components: an evidence extractor
and an answer predictor. The former seeks the most relevant information from a
reference text, while the latter is to locate or generate answers from the
extracted evidence. Despite the importance of evidence labels for training the
evidence extractor, they are not cheaply accessible, particularly in many
non-extractive MRC tasks such as YES/NO question answering and multi-choice
MRC.
To address this problem, we present a Self-Training method (STM), which
supervises the evidence extractor with auto-generated evidence labels in an
iterative process. At each iteration, a base MRC model is trained with golden
answers and noisy evidence labels. The trained model will predict pseudo
evidence labels as extra supervision in the next iteration. We evaluate STM on
seven datasets over three MRC tasks. Experimental results demonstrate the
improvement on existing MRC models, and we also analyze how and why such a
self-training method works in MRC. The source code can be obtained from
https://github.com/SparkJiao/Self-Training-MRCComment: 12 pages, accepted by ACL 202
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