40 research outputs found
On adaptive stochastic heavy ball momentum for solving linear systems
The stochastic heavy ball momentum (SHBM) method has gained considerable
popularity as a scalable approach for solving large-scale optimization
problems. However, one limitation of this method is its reliance on prior
knowledge of certain problem parameters, such as singular values of a matrix.
In this paper, we propose an adaptive variant of the SHBM method for solving
stochastic problems that are reformulated from linear systems using
user-defined distributions. Our adaptive SHBM (ASHBM) method utilizes iterative
information to update the parameters, addressing an open problem in the
literature regarding the adaptive learning of momentum parameters. We prove
that our method converges linearly in expectation, with a better convergence
rate compared to the basic method. Notably, we demonstrate that the
deterministic version of our ASHBM algorithm can be reformulated as a variant
of the conjugate gradient (CG) method, inheriting many of its appealing
properties, such as finite-time convergence. Consequently, the ASHBM method can
be further generalized to develop a brand-new framework of the stochastic CG
(SCG) method for solving linear systems. Our theoretical results are supported
by numerical experiments
On the convergence analysis of the greedy randomized Kaczmarz method
In this paper, we analyze the greedy randomized Kaczmarz (GRK) method
proposed in Bai and Wu (SIAM J. Sci. Comput., 40(1):A592--A606, 2018) for
solving linear systems. We develop more precise greedy probability criteria to
effectively select the working row from the coefficient matrix. Notably, we
prove that the linear convergence of the GRK method is deterministic and
demonstrate that using a tighter threshold parameter can lead to a faster
convergence rate. Our result revises existing convergence analyses, which are
solely based on the expected error by realizing that the iterates of the GRK
method are random variables. Consequently, we obtain an improved iteration
complexity for the GRK method. Moreover, the Polyak's heavy ball momentum
technique is incorporated to improve the performance of the GRK method. We
propose a refined convergence analysis, compared with the technique used in
Loizou and Richt\'{a}rik (Comput. Optim. Appl., 77(3):653--710, 2020), of
momentum variants of randomized iterative methods, which shows that the
proposed GRK method with momentum (mGRK) also enjoys a deterministic linear
convergence. Numerical experiments show that the mGRK method is more efficient
than the GRK method
Fast stochastic dual coordinate descent algorithms for linearly constrained convex optimization
The problem of finding a solution to the linear system with certain
minimization properties arises in numerous scientific and engineering areas. In
the era of big data, the stochastic optimization algorithms become increasingly
significant due to their scalability for problems of unprecedented size. This
paper focuses on the problem of minimizing a strongly convex function subject
to linear constraints. We consider the dual formulation of this problem and
adopt the stochastic coordinate descent to solve it. The proposed algorithmic
framework, called fast stochastic dual coordinate descent, utilizes sampling
matrices sampled from user-defined distributions to extract gradient
information. Moreover, it employs Polyak's heavy ball momentum acceleration
with adaptive parameters learned through iterations, overcoming the limitation
of the heavy ball momentum method that it requires prior knowledge of certain
parameters, such as the singular values of a matrix. With these extensions, the
framework is able to recover many well-known methods in the context, including
the randomized sparse Kaczmarz method, the randomized regularized Kaczmarz
method, the linearized Bregman iteration, and a variant of the conjugate
gradient (CG) method. We prove that, with strongly admissible objective
function, the proposed method converges linearly in expectation. Numerical
experiments are provided to confirm our results.Comment: arXiv admin note: text overlap with arXiv:2305.0548
Effects of steel slag application on greenhouse gas emissions and crop yield over multiple growing seasons in a subtropical paddy field in China
Use of anticoagulants and antiplatelet agents in stable outpatients with coronary artery disease and atrial fibrillation. International CLARIFY registry
Diffusion tensor imaging with direct cytopathological validation: characterisation of decorin treatment in experimental juvenile communicating hydrocephalus
The Backward Euler Fully Discrete Finite Volume Method for the Problem of Purely Longitudinal Motion of a Homogeneous Bar
We present a linear backward Euler fully discrete finite volume method for the initial-boundary-value problem of purely longitudinal motion of a homogeneous bar and an give optimal order error estimates in L2 and H1 norms. Furthermore, we obtain the superconvergence error estimate of the generalized projection of the solution u in H1 norm. Numerical experiment illustrates the convergence and stability of this scheme
Improvement of provably secure self-certified proxy convertible authenticated encryption scheme
By integrating self-certified public-key systems and the designated verifier proxy signature with message recovery, Wu and Lin proposed the first self-certified proxy convertible authenticated encryption (SP-CAE) scheme and its variants based on discrete logarithm problem (DLP) in 2009. Though their schemes are claimed provably secure, we demonstrate that their schemes are existentially forgeable under adaptive chosen warrants, unconfidentiable and verifiable under adaptive chosen messages and designated verifiers. Then we propose a provably secure SP-CAE scheme in the random oracle model
Monitoring hourly night-time light by an unmanned aerial vehicle and its implications to satellite remote sensing
Satellite-observed night-time light in urban areas has been widely used as an indicator for socioeconomic development and light pollution. Up to present, the diurnal dynamics of city light during the night, which are important to understand the nature of human activity and the underlying variables explaining night-time brightness, have hardly been investigated by remote sensing techniques due to limitation of the revisit time and spatial resolution of available satellites. In this study, we employed a consumer-grade unmanned aerial vehicle (UAV) to monitor city light in a study area located in Wuhan City, China, from 8:08 PM, April 15, 2019 to 5:08 AM, April 16, 2019, with an hourly temporal resolution. By using three ground-based Sky Quality Meters (SQMs), we found that the UAV-recorded light brightness was consistent with the ground luminous intensity measured by the SQMs in both the spatial (R = 0.72) and temporal dimensions (R > 0.94), and that the average city light brightness was consistent with the sky brightness in the temporal dimension (R = 0.98), indicating that UAV images can reliably monitor the city's night-time brightness. The temporal analysis showed that different locations had different patterns of temporal changes in their night-time brightness, implying that inter-calibration of two kinds of satellite images with different overpass times would be a challenge. Combining an urban function map of 18 classes and the hourly UAV images, we found that urban functions differed in their temporal light dynamics. For example, the outdoor sports field lost 97.28% of its measured brightness between 8: 08 PM – 4:05 AM, while an administrative building only lost 4.56%, and the entire study area lost 61.86% of its total brightness. Within our study area, the period between 9:06 PM and 10:05 PM was the period with largest amount of light loss. The spectral analysis we conducted showed that city light colors were different in some urban functions, with the major road being the reddest region at 8:08 PM and becoming even redder at 4:05 AM. This preliminary study indicates that UAVs are a good tool to investigate city light at night, and that city light is very complex in both of the temporal and spatial dimensions, requiring comprehensive investigation using more advanced UAV techniques, and emphasizing the need for geostationary platforms for night-time light sensors