arXiv subjects

Yang Guo

Publications and source records attributed to Yang Guo.

At least 91 records · Page 5Linked to original sources

arXiv2021

Multi-Stage Graph Peeling Algorithm for Probabilistic Core Decomposition

Mining dense subgraphs where vertices connect closely with each other is a common task when analyzing graphs. A very popular notion in subgraph analysis is core decomposition. Recently, Esfahani et al. presented a probabilistic core decomposition algorithm based on graph peeling and Central Limit Theorem (CLT) that is capable of handling very large graphs. Their proposed peeling algorithm (PA) starts from the lowest degree vertices and recursively deletes these vertices, assigning core numbers, and updating the degree of neighbour vertices until it reached the maximum core. However, in many applications, particularly in biology, more valuable information can be obtained from dense sub-communities and we are not interested in small cores where vertices do not interact much with others. To make the previous PA focus more on dense subgraphs, we propose a multi-stage graph peeling algorithm (M-PA) that has a two-stage data screening procedure added before the previous PA. After removing vertices from the graph based on the user-defined thresholds, we can reduce the graph complexity largely and without affecting the vertices in subgraphs that we are interested in. We show that M-PA is more efficient than the previous PA and with the properly set filtering threshold, can produce very similar if not identical dense subgraphs to the previous PA (in terms of graph density and clustering coefficient).

stat.ML↗

arXiv2021

Formation and Characteristics of Filament Threads in Double-Dipped Magnetic Flux Tubes

As one of the main formation mechanisms of solar filament formation, the chromospheric evaporation-coronal condensation model has been confirmed by numerical simulations to explain the formation of filament threads very well in flux tubes with single dips. However, coronal magnetic extrapolations indicated that some magnetic field lines might possess more than one dip. It is expected that the formation process would be significantly different in this case compared to a single-dipped magnetic flux tube. In this paper, based on the evaporation-condensation model, we study filament thread formation in double-dipped magnetic flux tubes by numerical simulations. We find that only with particular combinations of magnetic configuration and heating, e.g., concentrated localized heating and a long magnetic flux tube with deep dips, can two threads form and persist in a double-dipped magnetic flux tube. Comparing our parametric survey with observations, we conclude that such magnetically connected threads due to multiple dips are more likely to exist in quiescent filaments than in active-region filaments. Moreover, we find that these threads are usually shorter than independently trapped threads, which might be one of the reasons why quiescent filaments have short threads. These characteristics of magnetically connected threads could also explain barbs and vertical threads in quiescent filaments.

astro-ph.SR↗

arXiv2021

Magnetic Twists of Solar Filaments

Solar filaments are cold and dense materials situated in magnetic dips, which show distinct radiation characteristics compared to the surrounding coronal plasma. They are associated with coronal sheared and twisted magnetic field lines. However, the exact magnetic configuration supporting a filament material is not easy to be ascertained because of the absence of routine observations of the magnetic field inside filaments. Since many filaments lie above weak-field regions, it is nearly impossible to extrapolate their coronal magnetic structures by applying the traditional methods to noisy photospheric magnetograms, in particular the horizontal components. In this paper, we construct magnetic structures for some filaments with the regularized Biot--Savart laws and calculate their magnetic twists. Moreover, we make a parameter survey for the flux ropes of the Titov-Demoulin-modified model to explore the factors affecting the twist of a force-free magnetic flux rope. It is found that the twist of a force-free flux rope is proportional to its axial length to minor radius ratio, and is basically independent of the overlying background magnetic field strength. Thus, we infer that long quiescent filaments are likely to be supported by more twisted flux ropes than short active-region filaments, which is consistent with observations.

astro-ph.SR↗

arXiv2021

Data-constrained Magnetohydrodynamic Simulation of a Long Duration Eruptive Flare

We perform a zero-$β$ magnetohydrodynamic simulation for the C7.7 class flare initiated at 01:18 UT on 2011 June 21 using the Message Passing Interface Adaptive Mesh Refinement Versatile Advection Code (MPI-AMRVAC). The initial condition for the simulation involves a flux rope which we realize through the regularized Biot-Savart laws, whose parameters are constrained by observations from the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO) and the Extreme Ultraviolet Imager (EUVI) on the twin Solar Terrestrial Relations Observatory (STEREO). This data-constrained initial state is then relaxed to a force-free state by the magneto-frictional module in MPI-AMRVAC. The further time-evolving simulation results reproduce the eruption characteristics obtained by SDO/AIA 94 A, 304 A, and STEREO/EUVI 304 A observations fairly well. The simulated flux rope possesses similar eruption direction, height range, and velocities to the observations. Especially, the two phases of slow evolution and fast eruption are reproduced by varying the density distribution in light of the filament material draining process. Our data-constrained simulations also show other advantages, such as a large field of view (about 0.76 solar radii). We study the twist of the magnetic flux rope and the decay index of the overlying field, and find that in this event, both the magnetic strapping force and the magnetic tension force are sufficiently weaker than the magnetic hoop force, thus allowing the successful eruption of the flux rope. We also find that the anomalous resistivity is necessary in keeping the correct morphology of the erupting flux rope.

astro-ph.SR↗

arXiv2021

iCISCF: An Iterative Configuration Interaction-based Multiconfigurational Self-consistent Field Theory for Large Active Spaces

An iterative configuration interaction (iCI)-based multiconfigurational self-consistent field (SCF) theory, iCISCF, is proposed to handle systems that require large complete active spaces (CAS). The success of iCISCF stems from three ingredients: (1) efficient selection of individual configuration state functions spanning the CAS, meanwhile maintaining full spin symmetry; (2) the use of Jacobi rotation for the optimization of active orbitals, in conjunction with a quasi-Newton algorithm for the core/active-virtual and core-active orbital rotations; (3) a second-order perturbative treatment of the residual space left over by the selection procedure (i.e., iCISCF(2)). Just like selected iCI being a very accurate approximation to CASCI, iCISCF(2) is a very accurate approximation to CASSCF. Several examples that go beyond the capability of CASSCF are taken as showcases to reveal the performances of iCISCF and iCISCF(2).

physics.chem-ph↗

arXiv2021

Towards Adversarial Robustness via Transductive Learning

There has been emerging interest to use transductive learning for adversarial robustness (Goldwasser et al., NeurIPS 2020; Wu et al., ICML 2020). Compared to traditional "test-time" defenses, these defense mechanisms "dynamically retrain" the model based on test time input via transductive learning; and theoretically, attacking these defenses boils down to bilevel optimization, which seems to raise the difficulty for adaptive attacks. In this paper, we first formalize and analyze modeling aspects of transductive robustness. Then, we propose the principle of attacking model space for solving bilevel attack objectives, and present an instantiation of the principle which breaks previous transductive defenses. These attacks thus point to significant difficulties in the use of transductive learning to improve adversarial robustness. To this end, we present new theoretical and empirical evidence in support of the utility of transductive learning.

cs.LG↗

arXiv2021

dFDA-VeD: A Dynamic Future Demand Aware Vehicle Dispatching System

With the rising demand of smart mobility, ride-hailing service is getting popular in the urban regions. These services maintain a system for serving the incoming trip requests by dispatching available vehicles to the pickup points. As the process should be socially and economically profitable, the task of vehicle dispatching is highly challenging, specially due to the time-varying travel demands and traffic conditions. Due to the uneven distribution of travel demands, many idle vehicles could be generated during the operation in different subareas. Most of the existing works on vehicle dispatching system, designed static relocation centers to relocate idle vehicles. However, as traffic conditions and demand distribution dynamically change over time, the static solution can not fit the evolving situations. In this paper, we propose a dynamic future demand aware vehicle dispatching system. It can dynamically search the relocation centers considering both travel demand and traffic conditions. We evaluate the system on real-world dataset, and compare with the existing state-of-the-art methods in our experiments in terms of several standard evaluation metrics and operation time. Through our experiments, we demonstrate that the proposed system significantly improves the serving ratio and with a very small increase in operation cost.

math.OC↗

arXiv2021

The role of non-axisymmetry of magnetic flux rope in constraining solar eruptions

Whether a solar eruption is successful or failed depends on the competition between different components of the Lorentz force exerting on the flux rope that drives the eruption. The present models only consider the strapping force generated by the background magnetic field perpendicular to the flux rope and the tension force generated by the field along the flux rope. Using the observed magnetic field on the photosphere as a time-matching bottom boundary, we perform a data-driven magnetohydrodynamic simulation for the 30 January 2015 confined eruption and successfully reproduce the observed solar flare without a coronal mass ejection. Here we show a Lorentz force component, resulting from the radial magnetic field or the non-axisymmetry of the flux rope, which can essentially constrain the eruption. Our finding contributes to the solar eruption model and presents the necessity of considering the topological structure of a flux rope when studying its eruption behaviour.

astro-ph.SR↗

arXiv2021

Low-Rank Tucker Approximation of a Tensor From Streaming Data

This paper describes a new algorithm for computing a low-Tucker-rank approximation of a tensor. The method applies a randomized linear map to the tensor to obtain a sketch that captures the important directions within each mode, as well as the interactions among the modes. The sketch can be extracted from streaming or distributed data or with a single pass over the tensor, and it uses storage proportional to the degrees of freedom in the output Tucker approximation. The algorithm does not require a second pass over the tensor, although it can exploit another view to compute a superior approximation. The paper provides a rigorous theoretical guarantee on the approximation error. Extensive numerical experiments show that that the algorithm produces useful results that improve on the state of the art for streaming Tucker decomposition.

math.NA↗

arXiv2021

Tensor Random Projection for Low Memory Dimension Reduction

Random projections reduce the dimension of a set of vectors while preserving structural information, such as distances between vectors in the set. This paper proposes a novel use of row-product random matrices in random projection, where we call it Tensor Random Projection (TRP). It requires substantially less memory than existing dimension reduction maps. The TRP map is formed as the Khatri-Rao product of several smaller random projections, and is compatible with any base random projection including sparse maps, which enable dimension reduction with very low query cost and no floating point operations. We also develop a reduced variance extension. We provide a theoretical analysis of the bias and variance of the TRP, and a non-asymptotic error analysis for a TRP composed of two smaller maps. Experiments on both synthetic and MNIST data show that our method performs as well as conventional methods with substantially less storage.

math.NA↗

arXiv2021

COSINE: A Web Server for Clonal and Subclonal Structure Inference and Evolution in Cancer Genomics

Cancers evolve from mutation of a single cell with sequential clonal and subclonal expansion of somatic mutation acquisition. Inferring clonal and subclonal structures from bulk or single cell tumor genomic sequencing data has a huge impact on cancer evolution studies. Clonal state and mutational order can provide detailed insight into tumor origin and its future development. In the past decade, a variety of methods have been developed for subclonal reconstruction using bulk tumor sequencing data. As these methods have been developed in different programming languages and using different input data formats, their use and comparison can be problematic. Therefore, we established a web server for clonal and subclonal structure inference and evolution of cancer genomic data (COSINE), which included 12 popular subclonal reconstruction methods. We decomposed each method via a detailed workflow of single processing steps with a user-friendly interface. To the best of our knowledge, this is the first web server providing online subclonal inference, including the most popular subclonal reconstruction methods. COSINE is freely accessible at www.clab-cosine.net or http://bio.rj.run:48996/cun-web.

q-bio.GN↗

arXiv2020

Null geodesics, quasinormal modes and the correspondence with shadows in high-dimensional Einstein-Yang-Mills spacetimes

Null geodesics, quasinormal modes of a massless scalar field perturbation and the correspondence with shadow radii are investigated in the background spacetime of high-dimensional Einstein-Yang-Mills black holes. Based on the properties of null geodesics, we obtain the connection between the radius of a photon sphere and the radius of a horizon in the five- and six-dimensional Einstein-Yang-Mills spacetimes. Especially in the five-dimensional case, there exist two branches for the radius of a photon sphere, but only the branch outside the event horizon satisfies the condition of circular null geodesics. Moreover, we find no reflecting points of shadow radii and no spiral-like shapes on the complex plane of quasinormal frequencies and verify the correspondence between the quasinormal modes in the eikonal limit and shadow radii in high-dimensional Einstein-Yang-Mills spacetimes.

hep-th↗

arXiv2020

FIGARO: Improving System Performance via Fine-Grained In-DRAM Data Relocation and Caching

DRAM Main memory is a performance bottleneck for many applications due to the high access latency. In-DRAM caches work to mitigate this latency by augmenting regular-latency DRAM with small-but-fast regions of DRAM that serve as a cache for the data held in the regular-latency region of DRAM. While an effective in-DRAM cache can allow a large fraction of memory requests to be served from a fast DRAM region, the latency savings are often hindered by inefficient mechanisms for relocating copies of data into and out of the fast regions. Existing in-DRAM caches have two sources of inefficiency: (1) the data relocation granularity is an entire multi-kilobyte row of DRAM; and (2) because the relocation latency increases with the physical distance between the slow and fast regions, multiple fast regions are physically interleaved among slow regions to reduce the relocation latency, resulting in increased hardware area and manufacturing complexity. We propose a new substrate, FIGARO, that uses existing shared global buffers among subarrays within a DRAM bank to provide support for in-DRAM data relocation across subarrays at the granularity of a single cache block. FIGARO has a distance-independent latency within a DRAM bank, and avoids complex modifications to DRAM. Using FIGARO, we design a fine-grained in-DRAM cache called FIGCache. The key idea of FIGCache is to cache only small, frequently-accessed portions of different DRAM rows in a designated region of DRAM. By caching only the parts of each row that are expected to be accessed in the near future, we can pack more of the frequently-accessed data into FIGCache, and can benefit from additional row hits in DRAM. Our evaluations show that FIGCache improves the average performance of a system using DDR4 DRAM by 16.3% and reduces average DRAM energy consumption by 7.8% for 8-core workloads, over a conventional system without in-DRAM caching.

cs.AR↗

arXiv2020

Scalar quasinormal modes of black holes in Einstein-Yang-Mills gravity

Quasinormal modes of a scalar field perturbation are investigated in the background spacetime of Einstein-Yang-Mills black holes. The logarithmic term in the metric function of the five-dimensional Einstein-Yang-Mills black hole eliminates the divergence of the Hawking temperature but maintains the charge-dependent behavior of the quasinormal modes. The real and imaginary parts of quasinormal mode frequencies have the same charge-dependent behavior in different dimensions. Similar to the case of high dimensional Schwarzschild black holes, a compact approximate relation, $ω_{\rm R}\sim D/r_{+}$, also exists between the real part of the quasinormal mode frequencies $ω_{\rm R}$ and the number of dimensions $D$ in the higher than five dimensional Einstein-Yang-Mills black holes.

hep-th↗

arXiv2020

The Magnetic Topology and Eruption Mechanism of a Multiple-ribbon Flare

Multiple-ribbon flares are usually complex in their magnetic topologies and eruption mechanisms. In this paper, we investigate an X2.1 flare (SOL2015-03-11T16:22) that occurred in active region 12297 near the center of the solar disk by both potential and nonlinear force-free field models extrapolated with the data observed by the Helioseismic and Magnetic Imager (HMI) on board Solar Dynamics Observatory (SDO). We calculate the three-dimensional squashing degree distribution. The results reveal that there are two flux ropes in this active region, covered by a large scale hyperbolic flux tube (HFT), which is the intersection of quasi-separatrix layers with a null point embedded in it. When the background magnetic field diminishes due to the separation of the northwest dipole and the flux cancellation, the central flux rope rises up forming the two brightest central ribbons. It then squeezes the upper lying HFT structure to generate further brightenings. This very energetic flare with a complex shape is accompanied by a coronal mass ejection (CME). We adopt the simplified line-tied force-balance equation of the current ring model and assign the observed value of the decay index to the equation to simulate the acceleration profile of the CME in the early stage. It is found that the path with an inclination of $45^\circ$ from radial best fits the profile of the actual acceleration.

astro-ph.SR↗

arXiv2020

The Cluster-in-Molecule Local Correlation Method with an Accurate Distant Pair Correction for Large Systems

The cluster-in-molecule (CIM) local correlation approach with an accurate distant pair correlation energy correction is presented. For large systems, the inclusion of distant pair correlation energies is essential for the accurate predictions of absolute correlation energies and relative energies. Here we propose a simple and efficient scheme for evaluating the distant pair correlation energy correction. The corrections can be readily extracted from electron correlation calculations of clusters with almost no additional effort. Benchmark calculations show that the improved CIM approach can recover more than 99.97% of the conventional correlation energy. By combining the CIM approach with the domain based local pair natural orbital (DLPNO) local correlation approach, we have provided accurate binding energies at the CIM-DLPNO-CCSD(T) level for a test set consisting of eight weakly bound complexes ranging in size from 200 to 1027 atoms. With these results as the reference data, the accuracy and applicability of other electron correlation methods and a few density functional methods for large systems have been assessed.

physics.chem-ph↗

arXiv2020

A Macro-Micro Weakly-supervised Framework for AS-OCT Tissue Segmentation

Primary angle closure glaucoma (PACG) is the leading cause of irreversible blindness among Asian people. Early detection of PACG is essential, so as to provide timely treatment and minimize the vision loss. In the clinical practice, PACG is diagnosed by analyzing the angle between the cornea and iris with anterior segment optical coherence tomography (AS-OCT). The rapid development of deep learning technologies provides the feasibility of building a computer-aided system for the fast and accurate segmentation of cornea and iris tissues. However, the application of deep learning methods in the medical imaging field is still restricted by the lack of enough fully-annotated samples. In this paper, we propose a novel framework to segment the target tissues accurately for the AS-OCT images, by using the combination of weakly-annotated images (majority) and fully-annotated images (minority). The proposed framework consists of two models which provide reliable guidance for each other. In addition, uncertainty guided strategies are adopted to increase the accuracy and stability of the guidance. Detailed experiments on the publicly available AGE dataset demonstrate that the proposed framework outperforms the state-of-the-art semi-/weakly-supervised methods and has a comparable performance as the fully-supervised method. Therefore, the proposed method is demonstrated to be effective in exploiting information contained in the weakly-annotated images and has the capability to substantively relieve the annotation workload.

cs.CV↗

arXiv2020

Analyses of residual accelerations for TianQin based on the global MHD simulation

TianQin is a proposed space-based gravitational wave observatory. It is designed to detect the gravitational wave signals in the frequency range of 0.1 mHz -- 1 Hz. At a geocentric distance of $10^5$ km, the plasma in the earth magnetosphere will contribute as the main source of environmental noises. Here, we analyze the acceleration noises that are caused by the magnetic field of space plasma for the test mass of TianQin. The real solar wind data observed by the Advanced Composition Explorer are taken as the input of the magnetohydrodynamic simulation. The Space Weather Modeling Framework is used to simulate the global magnetosphere of the earth, from which we obtain the plasma and magnetic field parameters on the detector's orbits. We calculate the time series of the residual accelerations and the corresponding amplitude spectral densities on these orbit configurations. We find that the residual acceleration produced by the interaction between the TM's magnetic moment induced by the space magnetic field and the spacecraft magnetic field ($\bm{a}_{\rm M1}$) is the dominant term, which can approach $10^{-15}$ m/s$^2$/Hz$^{1/2}$ at $f \approx$ 0.2 mHz for the nominal values of the magnetic susceptibility ($χ_{\rm m} = 10^{-5}$) and the magnetic shielding factor ($ξ_{\rm m} = 10$) of the test mass. The ratios between the amplitude spectral density of the acceleration noise caused by the space magnetic field and the preliminary goal of the inertial sensor are 0.38 and 0.08 at 1 mHz and 10 mHz, respectively. We discuss the further reduction of this acceleration noise by decreasing $χ_{\rm m}$ and/or increasing $ξ_{\rm m}$ in the future instrumentation development for TianQin.

astro-ph.IM↗