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Shu Wang

Shu Wang appears in the imported research catalog. Authorship, coauthor and topic links are available while profile ownership is still unclaimed.

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astro-ph.GAastro-ph.SRmath.APmath.OCComputer VisionApplicationscond-mat.supr-conCryptography and SecurityArtificial Intelligenceastro-ph.COastro-ph.HEastro-ph.IMBiological PhysicsComputation and Languagecond-mat.mtrl-sciDiscrete Mathematicseess.ASHuman-Computer InteractionMachine Learningmath.CONeural and Evolutionary Computingphysics.app-phphysics.flu-dynphysics.med-phRoboticsSoftware EngineeringSoundstat.OT

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Published work

50 published item(s)

preprint2026arXiv

SkillRAE: Agent Skill-Based Context Compilation for Retrieval-Augmented Execution

Large Language Model (LLM)-based agents (e.g., OpenClaw) increasingly rely on reusable skill libraries to solve artifact-rich tasks such as document-centric workflows and data-intensive analysis. As these libraries grow, a few works have attempted to study the Retrieval-Augmented Execution (RAE), which often first retrieves some external skills and other knowledge, then compiles the context using retrieved skills, and finally executes the task. Existing works mainly focus on optimizing skill retrieval and task execution, and they pay little attention to how to effectively organize the selected skill evidence in a form that is compact, grounded, and immediately usable for the downstream executors to complete tasks. To fill this gap, we propose SkillRAE, a two-stage RAE approach focusing on skill-based context compilation, which consists of the offline and online stages. Specifically, in the offline indexing stage, it builds a multi-level skill graph over skill communities, skills, and reusable subunits, for capturing their relationships. In the online retrieval stage, it first performs skill-ranked retrieval with selected-subunit evidence export in the graph, and then applies rescue-aware compact compilation to recover the key evidence. Together, these components compile a coarse-ranked skill set into a task-specific context that is compact, grounded, and immediately usable. Experiments on two public benchmarks show that SkillRAE achieves a significant improvement over baselines for RAE. For example, on SkillsBench, it achieves an improvement of 11.7% over the SOTA method. Ablation studies further show that our context compilation is crucial, instead of a mere prompt addition.

0 citations0 reviewsNo code linkedOpen access
preprint2025arXiv

Variation of the 2175 Å extinction feature in Andromeda galaxy

Extinction curves contain key information on interstellar dust composition and size distribution, with the 2175 Å bump being the most prominent feature. We analyze 20 sightlines toward M31 using HST/STIS UV spectroscopy combined with multi-band photometry to characterize this feature. The extinction curves show substantial diversity, from MW-like shapes to flatter profiles with $R_V$ reaching up to $\sim5.8$. The strength of the 2175 Å feature varies widely, including two sightlines where the bump is essentially absent. The bump central wavelength spans a broader range than previously reported, while its width remains consistent with earlier studies. A moderate positive correlation is found between bump strength ($c_3$) and width ($γ$). We derive an average UV extinction curve toward M31 with $R_V \approx 3.53$. These results provide new constraints on dust properties and their spatial variations in this galaxy.

0 citations0 reviewsNo code linkedOpen access
preprint2023arXiv

PatchRNN: A Deep Learning-Based System for Security Patch Identification

With the increasing usage of open-source software (OSS) components, vulnerabilities embedded within them are propagated to a huge number of underlying applications. In practice, the timely application of security patches in downstream software is challenging. The main reason is that such patches do not explicitly indicate their security impacts in the documentation, which would be difficult to recognize for software maintainers and users. However, attackers can still identify these "secret" security patches by analyzing the source code and generate corresponding exploits to compromise not only unpatched versions of the current software, but also other similar software packages that may contain the same vulnerability due to code cloning or similar design/implementation logic. Therefore, it is critical to identify these secret security patches to enable timely fixes. To this end, we propose a deep learning-based defense system called PatchRNN to automatically identify secret security patches in OSS. Besides considering descriptive keywords in the commit message (i.e., at the text level), we leverage both syntactic and semantic features at the source-code level. To evaluate the performance of our system, we apply it on a large-scale real-world patch dataset and conduct a case study on a popular open-source web server software - NGINX. Experimental results show that the PatchRNN can successfully detect secret security patches with a low false positive rate.

0 citations0 reviewsNo code linkedOpen access
preprint2022arXiv

A Simulation Study Evaluating Phase I Clinical Trial Designs for Combinational Agents

Nowadays, more and more clinical trials choose combinational agents as the intervention to achieve better therapeutic responses. However, dose-finding for combinational agents is much more complicated than single agent as the full order of combination dose toxicity is unknown. Therefore, regular phase I designs are not able to identify the maximum tolerated dose (MTD) of combinational agents. Motivated by such needs, plenty of novel phase I clinical trial designs for combinational agents were proposed. With so many available designs, research that compare their performances, explore parameters' impacts, and provide recommendations is very limited. Therefore, we conducted a simulation study to evaluate multiple phase I designs that proposed to identify single MTD for combinational agents under various scenarios. We also explored influences of different design parameters. In the end, we summarized the pros and cons of each design, and provided a general guideline in design selection.

0 citations0 reviewsNo code linkedOpen access
preprint2022arXiv

Dependence of pulsation mode of Cepheids on metallicity

The Cepheid variables in SMC, LMC, the Milky Way, M33 and M31 are used to examine the dependence of pulsation mode on metallicity which was previously found in red supergiants. The initial samples of Cepheids are collected from the Cepheid catalogs identified from the OGLE, PS1, DIRECT, WISE and ZTF surveys. The contaminants are removed with the help of the Gaia/EDR3 astrometric information for extra galaxies or by comparing the geometric distance and the distance from the P-L relation for the Milky Way. The division of fundamental and first-overtone mode is refined according to the gap between the two modes in the P-L diagram of the objects in each galaxy. The ratio of FU/(FU+1O) is found to be 0.59, 0.60, 0.69, 0.83 and 0.85 for SMC, LMC, the Milky Way, M33 and M31 respectively in order of metallicity, which confirms that the pulsation mode depends on metallicity in the way that the ratio of FU/(FU+1O) increases with metallicity. This dependence is not changed if the incompleteness of the samples is taken into account.

0 citations0 reviewsNo code linkedOpen access
preprint2022arXiv

Multi Receptive Field Network for Semantic Segmentation

Semantic segmentation is one of the key tasks in computer vision, which is to assign a category label to each pixel in an image. Despite significant progress achieved recently, most existing methods still suffer from two challenging issues: 1) the size of objects and stuff in an image can be very diverse, demanding for incorporating multi-scale features into the fully convolutional networks (FCNs); 2) the pixels close to or at the boundaries of object/stuff are hard to classify due to the intrinsic weakness of convolutional networks. To address the first issue, we propose a new Multi-Receptive Field Module (MRFM), explicitly taking multi-scale features into account. For the second issue, we design an edge-aware loss which is effective in distinguishing the boundaries of object/stuff. With these two designs, our Multi Receptive Field Network achieves new state-of-the-art results on two widely-used semantic segmentation benchmark datasets. Specifically, we achieve a mean IoU of 83.0 on the Cityscapes dataset and 88.4 mean IoU on the Pascal VOC2012 dataset.

0 citations0 reviewsNo code linkedOpen access
preprint2022arXiv

Physical Properties of 29 sdB+dM Eclipsing Binaries in Zwicky Transient Facility

The development of large-scale time-domain surveys provides an opportunity to study the physical properties as well as the evolutionary scenario of B-type subdwarfs (sdB) and M-type dwarfs (dM). Here, we obtained 33 sdB+dM eclipsing binaries based on the Zwicky Transient Facility (ZTF) light curves and $Gaia$ early data release 3 (EDR3) parallaxes. By using the PHOEBE code for light curve analysis, we obtain probability distributions for parameters of 29 sdB+dM. $R_1$, $R_2$, and $i$ are well determined, and the average uncertainty of mass ratio $q$ is 0.08. Our parameters are in good agreement with previous works if a typical mass of sdB is assumed. Based on parameters of 29 sdB+dM, we find that both the mass ratio $q$ and the companion's radius $R_2$ decrease with the shortening of the orbital period. For the three sdB+dMs with orbital periods less than 0.075 days, their companions are all brown dwarfs. The masses and radii of the companions satisfy the mass--radius relation for low-mass stars and brown dwarfs. Companions with radii between $0.12R_\odot$ and $0.15R_\odot$ seem to be missing in the observations. As more short-period sdB+dM eclipsing binaries are discovered and classified in the future with ZTF and $Gaia$, we will have more information to constrain the evolutionary ending of sdB+dM.

0 citations0 reviewsNo code linkedOpen access
preprint2022arXiv

Possible Origin of Preformed Hole Pairs and Superconductivity in Cuprates

This paper addresses the long standing and controversial issue of the origin of superconductivity in cuprates. Their superconductivity can be attributed to amphoteric defects associated with vacancy sites in copper oxide planes. A local defect lattice relaxation results in a negative-$U$ energy binding two holes on amphoteric defects in the donor configuration that act as preformed boson pair. Thermodynamic equilibrium between defects in the donor and acceptor configurations stabilizes Fermi energy at the amphoteric defect charge transition state assuring resonant coupling between free holes and the localized hole pairs. Model calculations show that the critical temperature is primarily determined by the density of the amphoteric defects in the donor configuration, explaining the ubiquity of dome-like dependence of the critical temperature on the doping as well as its universal dependence on the superfluid density. Intentional doping with chemical acceptors or donors is neither necessary nor sufficient condition for superconductivity that is fully determined by the amphoteric defects whose concentration can be controlled by crystal nonstoichiometry. The only role of chemical doping is changing the balance between concentrations of amphoteric defects in the donor and acceptor configurations resulting in an increase of the superfluid density and thus also the critical temperature for acceptor and a decrease for donor doping. This accounts for the experimentally observed distinct asymmetry between the dome structures for the chemical doping with acceptors and donors. The unusual sensitivity of the critical temperature to external perturbations is explained by the resonant nature of the coupling between free holes and preformed hole pairs. The work has broader implications as it could be applicable to other superconductors with dome-like dependence of the critical temperature on doping.

0 citations0 reviewsNo code linkedOpen access
preprint2022arXiv

T-NGA: Temporal Network Grafting Algorithm for Learning to Process Spiking Audio Sensor Events

Spiking silicon cochlea sensors encode sound as an asynchronous stream of spikes from different frequency channels. The lack of labeled training datasets for spiking cochleas makes it difficult to train deep neural networks on the outputs of these sensors. This work proposes a self-supervised method called Temporal Network Grafting Algorithm (T-NGA), which grafts a recurrent network pretrained on spectrogram features so that the network works with the cochlea event features. T-NGA training requires only temporally aligned audio spectrograms and event features. Our experiments show that the accuracy of the grafted network was similar to the accuracy of a supervised network trained from scratch on a speech recognition task using events from a software spiking cochlea model. Despite the circuit non-idealities of the spiking silicon cochlea, the grafted network accuracy on the silicon cochlea spike recordings was only about 5% lower than the supervised network accuracy using the N-TIDIGITS18 dataset. T-NGA can train networks to process spiking audio sensor events in the absence of large labeled spike datasets.

0 citations0 reviewsNo code linkedOpen access
preprint2021arXiv

"A more probable explanation" is still impossible to explain GN-z11-flash: in response to Steinhardt et al. (arXiv:2101.12738)

In Jiang et al. (2020), we reported a possible bright flash (hereafter GN-z11-flash) from a galaxy GN-z11 at z ~ 11. Recently, Steinhardt et al. (2021; arXiv:2101.12738) found 27 images with transient signals in Keck MOSFIRE archival data and claimed that GN-z11-flash was more likely from a moving object in our Solar system. We show that the Steinhardt et al.'s definition of the chance probability and their methodology of finding GN-z11-flash-like transients are problematic in several aspects. In particular, none of their transients is analogous to GN-z11-flash, and none of them is positionally coincident with a known object in their imaging data. In Jiang et al., we performed a comprehensive analysis of the origin of GN-z11-flash and ruled out, to the best of our knowledge, the possibility of known man-made objects or moving objects in the Solar system, based on all available information and our current understanding of these objects. Steinhardt et al. did not use such information and did not analyse the GN-z11-flash event itself. The majority of their transients are apparently low-Earth orbit satellites or aircrafts. Therefore, their analysis can neither prove nor disprove our results. Finally, we present a method to estimate the chance probability of finding GN-z11-flash-like transients in archival data. Based on this method and the archival data used by Steinhardt et al., we obtain a loose upper limit of the probability that actually support the original results of Jiang et al. (2020).

0 citations0 reviewsNo code linkedOpen access
preprint2021arXiv

3D Parameter Maps of Red Clump Stars in the Milky Way -- Absolute Magnitudes and Intrinsic Colors

Red clump stars (RCs) are useful tracers of distances, extinction, chemical abundances, and Galactic structures and kinematics. Accurate estimation of the RC parameters -- absolute magnitude and intrinsic color -- is the basis for obtaining high-precision RC distances. By combining astrometric data from Gaia, spectroscopic data from APOGEE and LAMOST, and multi-band photometric data from Gaia, APASS, Pan-STARRS1, 2MASS, and WISE surveys, we use the Gaussian process regression to train machine learners to derive the multi-band absolute magnitudes $M_λ$ and intrinsic colors $(λ_1-λ_2)_0$ for each spectral RC. The dependence of $M_λ$ on metallicity decreases from optical to infrared bands, while the dependence of $M_λ$ on age is relatively similar in each band. $(λ_1-λ_2)_0$ are more affected by metallicity than age. The RC parameters are not suitable to be represented by simple constants but are related to the Galactic stellar population structure. By analyzing the variation of $M_λ$ and $(λ_1-λ_2)_0$ in the spatial distribution, we construct $(R, z)$ dependent maps of mean absolute magnitudes and mean intrinsic colors of the Galactic RCs. Through external and internal validation, we find that using three-dimensional (3D) parameter maps to determine RC parameters avoids systematic bias and reduces dispersion by about 20% compared to using constant parameters. Based on Gaia's EDR3 parallax, our 3D parameter maps, and extinction-distance profile selection, we obtain a photometric RC sample containing 11 million stars with distance and extinction measurements.

0 citations0 reviewsNo code linkedOpen access
preprint2021arXiv

Metallicity in Quasar Broad Line Regions at Redshift $\sim$ 6

Broad line regions (BLRs) in high-redshift quasars provide crucial information of chemical enrichment in the early universe. Here we present a study of BLR metallicities in 33 quasars at redshift $5.7<z<6.4$. Using the near-IR spectra of the quasars obtained from the Gemini telescope, we measure their rest-frame UV emission line flux and calculate flux ratios. We then estimate BLR metallicities with empirical calibrations based on photoionization models. The inferred median metallicity of our sample is a few times the solar value, indicating that the BLR gas had been highly metal-enriched at $z\sim6$. We compare our sample with a low-redshift quasar sample with similar luminosities and find no evidence of redshift evolution in quasar BLR metallicities. This is consistent with previous studies. The Fe II$/$Mg II flux ratio, a proxy for the Fe$/α$ element abundance ratio, shows no redshift evolution as well, further supporting rapid nuclear star formation at $z\sim6$. We also find that the black hole mass-BLR metallicity relation at $z\sim6$ is consistent with the relation measured at $2<z<5$, suggesting that our results are not biased by a selection effect due to this relation.

0 citations0 reviewsNo code linkedOpen access
preprint2020arXiv

Distances to the Supernova Remnants in the Inner Disk

Distance measurements of supernova remnants (SNRs) are essential and important. Accurate estimates of physical size, dust masses, and some other properties of SNRs depend critically on accurate distance measurements. However, the determination of SNR distances is still a tough task. Red clump stars (RCs) have a long history been used as standard candles. In this work, we take RCs as tracers to determine the distances to a large group of SNRs in the inner disk. We first select RC stars based on the near-infrared (IR) color-magnitude diagram (CMD). Then, the distance to and extinction of RC stars are calculated. To extend the measurable range of distance, we combine near-IR photometric data from the 2MASS survey with the deeper UKIDSS and VVV surveys. With the help of the Gaia parallaxes, we also remove contaminants including dwarfs and giants. Because an SN explosion compresses the surrounding interstellar medium, the SNR region would become denser and exhibit higher extinction than the surroundings. The distance of a SNR is then recognized by the position where the extinction and its gradient is higher than that of the ambient medium. A total of 63 SNRs' distances in the Galactic inner disk are determined and divided into three Levels A, B, and C with decreasing reliability. The distances to 43 SNRs are well determined with reliability A or B. The diameters and dust masses of SNRs are estimated with the obtained distance and extinction.

0 citations0 reviewsNo code linkedOpen access
preprint2020arXiv

Evolved Massive Stars at Low-metallicity II. Red Supergiant Stars in the Small Magellanic Cloud

We present the most comprehensive RSG sample for the SMC up to now, including 1,239 RSG candidates. The initial sample is derived based on a source catalog for the SMC with conservative ranking. Additional spectroscopic RSGs are retrieved from the literature, as well as RSG candidates selected from the inspection of CMDs. We estimate that there are in total $\sim$ 1,800 or more RSGs in the SMC. We purify the sample by studying the infrared CMDs and the variability of the objects, though there is still an ambiguity between AGBs and RSGs. There are much less RSGs candidates ($\sim$4\%) showing PAH emission features compared to the Milky Way and LMC ($\sim$15\%). The MIR variability of RSG sample increases with luminosity. We separate the RSG sample into two subsamples ("risky" and "safe") and identify one M5e AGB star in the "risky" subsample. Most of the targets with large variability are also the bright ones with large MLR. Some targets show excessive dust emission, which may be related to previous episodic mass loss events. We also roughly estimate the total gas and dust budget produced by entire RSG population as $\rm \sim1.9^{+2.4}_{-1.1}\times10^{-6}~M_{\odot}/yr$ in the most conservative case. Based on the MIST models, we derive a linear relation between $T_{\rm eff}$ and observed $\rm J-K_S$ color with reddening correction for the RSG sample. By using a constant bolometric correction and this relation, the Geneva evolutionary model is compared with our RSG sample, showing a good agreement and a lower initial mass limit of $\sim$7 $\rm M_\odot $ for the RSG population. Finally, we compare the RSG sample in the SMC and the LMC. Despite the incompleteness of LMC sample in the faint end, the result indicates that the LMC sample always shows redder color (except for the $\rm IRAC1-IRAC2$ and $\rm WISE1-WISE2$ colors due to CO absorption) and larger variability than the SMC sample.

0 citations0 reviewsNo code linkedOpen access
preprint2020arXiv

Evolved Massive Stars at Low-metallicity III. A Source Catalog for the Large Magellanic Cloud

We present a clean, magnitude-limited (IRAC1 or WISE1$\leq$15.0 mag) multiwavelength source catalog for the LMC. The catalog was built upon crossmatching ($1''$) and deblending ($3''$) between the SEIP source list and Gaia DR2, with strict constraints on the Gaia astrometric solution to remove the foreground contamination. The catalog contains 197,004 targets in 52 different bands including 2 ultraviolet, 21 optical, and 29 infrared bands. Additional information about radial velocities and spectral/photometric classifications were collected from the literature. The bright end of our sample is mostly comprised of blue helium-burning stars (BHeBs) and red HeBs with inevitable contamination of main sequence stars at the blue end. After applying modified magnitude and color cuts based on previous studies, we identify and rank 2,974 RSG, 508 YSG, and 4,786 BSG candidates in the LMC in six CMDs. The comparison between the CMDs of the LMC and SMC indicates that the most distinct difference appears at the bright red end of the optical and near-infrared CMDs, where the cool evolved stars (e.g., RSGs, AGB, and RGs) are located, which is likely due to the effect of metallicity and SFH. Further quantitative comparison of colors of massive star candidates in equal absolute magnitude bins suggests that, there is basically no difference for the BSG candidates, but large discrepancy for the RSG candidates as LMC targets are redder than the SMC ones, which may be due to the combined effect of metallicity on both spectral type and mass-loss rate, and also the age effect. The $T_{\rm eff}$ of massive star populations are also derived from reddening-free color of $(J-K_{\rm S})_0$. The $T_{\rm eff}$ ranges are $3500<T_{\rm eff}<5000$ K for RSG population, $5000<T_{\rm eff}<8000$ K for YSG population, and $T_{\rm eff}>8000$ K for BSG population, with larger uncertainties towards the hotter stars.

0 citations0 reviewsNo code linkedOpen access
preprint2020arXiv

Influence of Ln elements (Ln = La, Pr, Nd, Sm) on the structure and oxygen permeability of Ca-containing dual-phase membranes

Developing good performance and low-cost oxygen permeable membranes for CO2 capture based on the oxy-fuel concept is greatly desirable but challenging. Despite tremendous efforts in exploring new CO2-stable dual-phase membranes, its presence is however still far from meeting the industrial requirements. Here we report a series of new Ca-containing CO2-resistant oxygen transporting membranes with composition 60wt.%Ce0.9Ln0.1O2-40wt.%Ln0.6Ca0.4FeO3(CLnO-LnCFO; Ln = La, Pr, Nd, Sm) synthesized via a Pechini one-pot method. Our results indicate all investigated compounds are composed of perovskite and fluorite phases, while the perovskite phases in the CNO-NCFO and CSO-SCFO membranes after sintering generates Ca-rich and Ca-less two kinds of grains with different morphologies, where the Ca-less small perovskite grains block the transport of oxygen ions and eventually result in poor oxygen permeability. Among our investigated CLnO-LnCFO membranes, CPO-PCFO exhibits the highest oxygen permeability and excellent CO2 stability, which were mainly associated with the improvement in crystal symmetry, non-negligible electronic conductivity of fluorite phase and the enhancement in electronic conductivity of perovskite. Our results establish Ca-containing oxides as candidate material platforms for membrane engineering devices that combine CO2 capture and oxygen separation.

0 citations0 reviewsNo code linkedOpen access
preprint2020arXiv

Joint Mind Modeling for Explanation Generation in Complex Human-Robot Collaborative Tasks

Human collaborators can effectively communicate with their partners to finish a common task by inferring each other's mental states (e.g., goals, beliefs, and desires). Such mind-aware communication minimizes the discrepancy among collaborators' mental states, and is crucial to the success in human ad-hoc teaming. We believe that robots collaborating with human users should demonstrate similar pedagogic behavior. Thus, in this paper, we propose a novel explainable AI (XAI) framework for achieving human-like communication in human-robot collaborations, where the robot builds a hierarchical mind model of the human user and generates explanations of its own mind as a form of communications based on its online Bayesian inference of the user's mental state. To evaluate our framework, we conduct a user study on a real-time human-robot cooking task. Experimental results show that the generated explanations of our approach significantly improves the collaboration performance and user perception of the robot. Code and video demos are available on our project website: https://xfgao.github.io/xCookingWeb/.

0 citations0 reviewsNo code linkedOpen access
preprint2020arXiv

Phase-dependent study of near-infrared disk emission lines in LB-1

The mass, origin and evolutionary stage of the binary system LB-1 has been the subject of intense debate, following the claim that it hosts an $\sim$70$M_{\odot}$ black hole, in stark contrast with the expectations for stellar remnants in the Milky Way. We conducted a high-resolution, phase-resolved spectroscopic study of the near-infrared Paschen lines in this system, using the 3.5-m telescope at Calar Alto Observatory. We find that Pa$β$ and Pa$γ$ (after proper subtraction of the stellar absorption component) are well fitted with a standard double-peaked model, typical of disk emission. We measured the velocity shifts of the red and blue peaks at 28 orbital phases: the line center has an orbital motion in perfect antiphase with the stellar motion, and the radial velocity amplitude ranges from 8 to 13 km/s for different choices of lines and profile modelling. We interpret this curve as proof that the disk is tracing the orbital motion of the primary, ruling out the circumbinary disk and the hierarchical triple scenarios. The phase-averaged peak-to-peak half-separation (proxy for the projected rotational velocity of the outer disk) is $\sim$70 km s$^{-1}$, larger than the stellar orbital velocity and also inconsistent with a circumbinary disk. From those results, we infer a primary mass 4--8 times higher than the secondary mass. Moreover, we show that the ratio of the blue and red peaks (V/R intensity ratio) has a sinusoidal behaviour in phase with the secondary star, which can be interpreted as the effect of external irradiation by the secondary star on the outer disk. Finally, we briefly discuss our findings in the context of alternative scenarios recently proposed for LB-1. Definitive tests between alternative solutions will require further astrometric data from $Gaia$.

0 citations0 reviewsNo code linkedOpen access
preprint2020arXiv

The comparison of spectral analyses and flow features in upper airways with obstructive sleep apnea (OSA) between successful and failed surgeries

Obstructive sleep apnea (OSA) is a common sleep disorder and widening the upper airway is often used in clinical practice. However, the success rate of this surgery is limited; the failed surgery would even make the situation worse, indicating the widened airway is not the unique criterion to evaluate the breathing quality. Therefore, we carried out both experimental measurement and numerical simulation on OSA upper airways and found that there existed an intrinsic dominant 3-5 Hz signal and the signal-to-noise ratio (SNR) at 3-5 Hz is inversely correlated with apnea-hypopnea index (AHI). Firstly, to validate the suitability of simulation methods, we carried out Laser Doppler measurement in 3D-printing OSA upper airway models, and found excellent agreement between the measured and calculated velocity profiles in two upper airway models for the first time. Then we carried out large eddy simulation (LES) to investigate four pairs of OSA upper airway models with 8 different AHI values for both pre- and post-surgery; among them, three surgeries were successful and one failed. The decreased the pressure drop for failed case, proving that only widening airway cannot guarantee to improve OSA. In our analysis, it is indicating that a dominant recirculation downstream of the minimum cross-section should be a main feature of a successful surgery, and the strength of 3-5 Hz signal induced by flow separation in the upper airway plays an important role in appraising breathing quality. This provides a new guideline for surgery planning. Furthermore, we found a strong correlation between AHI and the area ratio of minimum cross-section near the retro-palate to the maximum cross-section behind the tongue base, and this correlation is highly significant, r = - 0.833, p = 0.01 < 0.05.

0 citations0 reviewsNo code linkedOpen access
preprint2020arXiv

The Third Data Release of the Beijing-Arizona Sky Survey

The Beijing-Arizona Sky Survey (BASS) is a wide and deep imaging survey to cover a 5400 deg$^2$ area in the Northern Galactic Cap with the 2.3m Bok telescope using two filters ($g$ and $r$ bands). The Mosaic $z$-band Legacy Survey (MzLS) covers the same area in $z$ band with the 4m Mayall telescope. These two surveys will be used for spectroscopic targeting of the Dark Energy Spectroscopic Instrument (DESI). The BASS survey observations were completed in 2019 March. This paper describes the third data release (DR3) of BASS, which contains the photometric data from all BASS and MzLS observations between 2015 January and 2019 March. The median astrometric precision relative to {\it Gaia} positions is about 17 mas and the median photometric offset relative to the PanSTARRS1 photometry is within 5 mmag. The median $5σ$ AB magnitude depths for point sources are 24.2, 23.6, and 23.0 mag for $g$, $r$, and $z$ bands, respectively. The photometric depth within the survey area is highly homogeneous, with the difference between the 20\% and 80\% depth less than 0.3 mag. The DR3 data, including raw data, calibrated single-epoch images, single-epoch photometric catalogs, stacked images, and co-added photometric catalogs, are publicly accessible at \url{http://batc.bao.ac.cn/BASS/doku.php?id=datarelease:home}.

0 citations0 reviewsNo code linkedOpen access
preprint2020arXiv

The Zwicky Transient Facility Catalog of Periodic Variable Stars

The number of known periodic variables has grown rapidly in recent years. Thanks to its large field of view and faint limiting magnitude, the Zwicky Transient Facility (ZTF) offers a unique opportunity to detect variable stars in the northern sky. Here, we exploit ZTF Data Release 2 (DR2) to search for and classify variables down to r ~ 20.6 mag. We classify 781,602 periodic variables into 11 main types using an improved classification method. Comparison with previously published catalogs shows that 621,702 objects (79.5%) are newly discovered or newly classified, including ~700 Cepheids, ~5000 RR Lyrae stars, ~15,000 Delta Scuti variables, ~350,000 eclipsing binaries, ~100,000 long-period variables, and about 150,000 rotational variables. The typical misclassification rate and period accuracy are on the order of 2% and 99%, respectively. 74% of our variables are located at Galactic latitudes, $|b|<10^\circ$. This large sample of Cepheids, RR Lyrae, Delta Scuti stars, and contact (EW-type) eclipsing binaries is helpful to investigate the Galaxy's disk structure and evolution with an improved completeness, areal coverage, and age resolution. Specifically, the northern warp and the disk's edge at distances of 15--20 kpc are significantly better covered than previously. Among rotational variables, RS Canum Venaticorum and BY Draconis-type variables can be separated easily. Our knowledge of stellar chromospheric activity would benefit greatly from a statistical analysis of these types of variables.

0 citations0 reviewsNo code linkedOpen access
preprint2020arXiv

When the Differences in Frequency Domain are Compensated: Understanding and Defeating Modulated Replay Attacks on Automatic Speech Recognition

Automatic speech recognition (ASR) systems have been widely deployed in modern smart devices to provide convenient and diverse voice-controlled services. Since ASR systems are vulnerable to audio replay attacks that can spoof and mislead ASR systems, a number of defense systems have been proposed to identify replayed audio signals based on the speakers' unique acoustic features in the frequency domain. In this paper, we uncover a new type of replay attack called modulated replay attack, which can bypass the existing frequency domain based defense systems. The basic idea is to compensate for the frequency distortion of a given electronic speaker using an inverse filter that is customized to the speaker's transform characteristics. Our experiments on real smart devices confirm the modulated replay attacks can successfully escape the existing detection mechanisms that rely on identifying suspicious features in the frequency domain. To defeat modulated replay attacks, we design and implement a countermeasure named DualGuard. We discover and formally prove that no matter how the replay audio signals could be modulated, the replay attacks will either leave ringing artifacts in the time domain or cause spectrum distortion in the frequency domain. Therefore, by jointly checking suspicious features in both frequency and time domains, DualGuard can successfully detect various replay attacks including the modulated replay attacks. We implement a prototype of DualGuard on a popular voice interactive platform, ReSpeaker Core v2. The experimental results show DualGuard can achieve 98% accuracy on detecting modulated replay attacks.

0 citations0 reviewsNo code linkedOpen access
preprint2019arXiv

Hybrid Density- and Partition-based Clustering Algorithm for Data with Mixed-type Variables

Clustering is an essential technique for discovering patterns in data. The steady increase in amount and complexity of data over the years led to improvements and development of new clustering algorithms. However, algorithms that can cluster data with mixed variable types (continuous and categorical) remain limited, despite the abundance of data with mixed types particularly in the medical field. Among existing methods for mixed data, some posit unverifiable distributional assumptions or that the contributions of different variable types are not well balanced. We propose a two-step hybrid density- and partition-based algorithm (HyDaP) that can detect clusters after variables selection. The first step involves both density-based and partition-based algorithms to identify the data structure formed by continuous variables and recognize the important variables for clustering; the second step involves partition-based algorithm together with a novel dissimilarity measure we designed for mixed data to obtain clustering results. Simulations across various scenarios and data structures were conducted to examine the performance of the HyDaP algorithm compared to commonly used methods. We also applied the HyDaP algorithm on electronic health records to identify sepsis phenotypes.

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preprint2019arXiv

NbSeTe -A New Layered Transition Metal Dichalcogenide Superconductor

Transition metal dichalcogenides (TMDCs) usually exhibit layered polytypic structures due to the weak interlayer coupling. 2H-NbSe2 is one of the most widely studied in the pristine TMDC family due to its high superconducting transition temperature (Tc = 7.3K) and the occurrence of a charge-density wave (CDW) order below 33 K. The coexistence of CDW with superconductivity poses an intriguing open question about the relationship between Fermi surface nesting and Cooper pairing. Past studies of this issue have mostly been focused on doping 2H-NbSe2 by 3d transition metals without significantly changing its crystal structure. Here we replaced the Se by Te in 2H-NbSe2 in order to design a new 1T polytype layered TMDC NbSeTe, which adopts a trigonal structure with space group P-3m1. We successfully grew large size and high-quality single crystals of 1T-NbSeTe via the vapor transport method using I2 as the transport agent. Temperature-dependent resistivity and specific heat data revealed a bulk Tc at 1.3 K, which is the first observation of superconductivity in pure 1T-NbSeTe phase. This compound enlarged the family of superconducting TMDCs and provides an opportunity to study the interplay between CDW and superconductivity in the trigonal structure.

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preprint2019arXiv

Understanding Broad Mg II Variability in Quasars with Photoionization: Implications for Reverberation Mapping and Changing-Look Quasars

The broad Mg II line in quasars has distinct variability properties compared with broad Balmer lines: it is less variable, and usually does not display a "breathing" mode, the increase in the average cloud distance when luminosity increases. We demonstrate that these variability properties of Mg II can be reasonably well explained by simple Locally Optimally Emitting Cloud (LOC) photoionization models, confirming earlier photoionization results. In the fiducial LOC model, the Mg II-emitting gas is on average more distant from the ionizing source than the H$α$/H$β$ gas, and responds with a lower amplitude to continuum variations. If the broad-line region (BLR) is truncated at a physical radius of $\sim 0.3$ pc (for a $10^{8.5}M_{\odot}$ BH accreting at Eddington ratio of 0.1), most of the Mg II flux will always be emitted near this outer boundary and hence will not display breathing. These results indicate that reverberation mapping results on broad Mg II, while generally more difficult to obtain due to the lower line responsivity, can still be used to infer the Mg II BLR size and hence black hole mass. But it is possible that Mg II does not have a well defined intrinsic BLR size-luminosity relation for individual quasars, even though a global one for the general population may still exist. The dramatic changes in broad H$α$/H$β$ emission in the observationally-rare changing-look quasars are fully consistent with photoionization responses to extreme continuum variability, and the LOC model provides natural explanations for the persistence of broad Mg II in changing-look quasars defined on H$α$/H$β$, and the rare population of broad Mg II emitters in the spectra of massive inactive galaxies.

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preprint2016arXiv

A Precise Determination of the Mid-Infrared Interstellar Extinction Law Based on the APOGEE Spectroscopic Survey

A precise measure of the mid-infrared interstellar extinction law is crucial to the investigation of the properties of interstellar dust, especially of the grains in the large size end. Based on the stellar parameters derived from the SDSS-III/APOGEE spectroscopic survey, we select a large sample of G- and K-type giants as the tracers of the Galactic mid-infrared extinction. We calculate the intrinsic stellar color excesses from the stellar effective temperatures and use them to determine the mid-infrared extinction for a given line of sight. For the entire sky of the Milky Way surveyed by APOGEE, we derive the extinction (relative to the K$_{\rm S}$ band at wavelength $λ=2.16μ$m) for the four WISE bands at 3.4, 4.6, 12 and 22$μ$m, the four Spitzer/IRAC bands at 3.6, 4.5, 5.8 and 8$μ$m, the Spitzer/MIPS24 band at 23.7$μ$m and for the first time, the AKARI/S9W band at 8.23$μ$m. Our results agree with previous works in that the extinction curve is flat in the ~3--8$μ$m wavelength range and is generally consistent with the $R_V=5.5$ model curve except our determination exceeds the model prediction in the WISE/W4 band. Although some previous works found that the mid-IR extinction law appears to vary with the extinction depth $A_{\rm{K_S}}$, no noticeable variation has been found in this work. The uncertainties are analyzed in terms of the bootstrap resampling method and Monte-Carlo simulation and are found to be rather small.

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preprint2016arXiv

A survey of luminous high-redshift quasars with SDSS and WISE II. the bright end of the quasar luminosity function at z ~ 5

This is the second paper in a series on a new luminous z ~ 5 quasar survey using optical and near-infrared colors. Here we present a new determination of the bright end of the quasar luminosity function (QLF) at z ~ 5. Combined our 45 new quasars with previously known quasars that satisfy our selections, we construct the largest uniform luminous z ~ 5 quasar sample to date, with 99 quasars in the range 4.7 <= z < 5.4 and -29 < M1450 <= -26.8, within the Sloan Digital Sky Survey (SDSS) footprint. We use a modified 1/Va method including flux limit correction to derive a binned QLF, and we model the parametric QLF using maximum likelihood estimation. With the faint-end slope of the QLF fixed as alpha = -2.03 from previous deeper samples, the best fit of our QLF gives a flatter bright end slope beta = -3.58+/-0.24 and a fainter break magnitude M*1450 = -26.98+/-0.23 than previous studies at similar redshift. Combined with previous work at lower and higher redshifts, our result is consistent with a luminosity evolution and density evolution (LEDE) model. Using the best fit QLF, the contribution of quasars to the ionizing background at z ~ 5 is found to be 18% - 45% with a clumping factor C of 2 - 5. Our sample suggests an evolution of radio loud fraction with optical luminosity but no obvious evolution with redshift.

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preprint2016arXiv

A survey of luminous high-redshift quasars with SDSS and WISE. I. target selection and optical spectroscopy

High-redshift quasars are important tracers of structure and evolution in the early universe. However, they are very rare and difficult to find when using color selection because of contamination from late-type dwarfs. High-redshift quasar surveys based on only optical colors suffer from incompleteness and low identification efficiency, especially at $z\gtrsim4.5$. We have developed a new method to select $4.7\lesssim z \lesssim 5.4$ quasars with both high efficiency and completeness by combining optical and mid-IR Wide-field Infrared Survey Explorer (WISE) photometric data, and are conducting a luminous $z\sim5$ quasar survey in the whole Sloan Digital Sky Survey (SDSS) footprint. We have spectroscopically observed 99 out of 110 candidates with $z$-band magnitudes brighter than 19.5 and 64 (64.6\%) of them are quasars with redshifts of $4.4\lesssim z \lesssim 5.5$ and absolute magnitudes of $-29\lesssim M_{1450} \lesssim -26.4$. In addition, we also observed 14 fainter candidates selected with the same criteria and identified 8 (57.1\%) of them as quasars with $4.7<z<5.4$ . Among 72 newly identified quasars, 12 of them are at $5.2 < z < 5.7$, which leads to an increase of $\sim$36\% of the number of known quasars at this redshift range. More importantly, our identifications doubled the number of quasars with $M_{1450}<-27.5$ at $z>4.5$, which will set strong constraints on the bright end of the quasar luminosity function. We also expand our method to select quasars at $z\gtrsim5.7$. In this paper we report the discovery of four new luminous $z\gtrsim5.7$ quasars based on SDSS-WISE selection.

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preprint2016arXiv

Approximation of the weighted maximin dispersion problem over Lp-ball: SDP relaxation is misleading

Consider the problem of finding a point in a unit $n$-dimensional $\ell_p$-ball ($p\ge 2$) such that the minimum of the weighted Euclidean distance from given $m$ points is maximized. We show in this paper that the recent SDP-relaxation-based approximation algorithm [SIAM J. Optim. 23(4), 2264-2294, 2013] will not only provide the first theoretical approximation bound of $\frac{1-O\left(\sqrt{ \ln(m)/n}\right)}{2}$, but also perform much better in practice, if the SDP relaxation is removed and the optimal solution of the SDP relaxation is replaced by a simple scalar matrix.

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preprint2016arXiv

Multispectral Deep Neural Networks for Pedestrian Detection

Multispectral pedestrian detection is essential for around-the-clock applications, e.g., surveillance and autonomous driving. We deeply analyze Faster R-CNN for multispectral pedestrian detection task and then model it into a convolutional network (ConvNet) fusion problem. Further, we discover that ConvNet-based pedestrian detectors trained by color or thermal images separately provide complementary information in discriminating human instances. Thus there is a large potential to improve pedestrian detection by using color and thermal images in DNNs simultaneously. We carefully design four ConvNet fusion architectures that integrate two-branch ConvNets on different DNNs stages, all of which yield better performance compared with the baseline detector. Our experimental results on KAIST pedestrian benchmark show that the Halfway Fusion model that performs fusion on the middle-level convolutional features outperforms the baseline method by 11% and yields a missing rate 3.5% lower than the other proposed architectures.

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preprint2016arXiv

On the Ball-Constrained Weighted Maximin Dispersion Problem

The ball-constrained weighted maximin dispersion problem $(\rm P_{ball})$ is to find a point in an $n$-dimensional Euclidean ball such that the minimum of the weighted Euclidean distance from given $m$ points is maximized. We propose a new second-order cone programming relaxation for $(\rm P_{ball})$. Under the condition $m\le n$, $(\rm P_{ball})$ is polynomial-time solvable since the new relaxation is shown to be tight. In general, we prove that $({\rm P_{ball}})$ is NP-hard. Then, we propose a new randomized approximation algorithm for solving $({\rm P_{ball}})$, which provides a new approximation bound of $\frac{1-O(\sqrt{\ln(m)/n})}{2}$.

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preprint2016arXiv

The Final SDSS High-Redshift Quasar Sample of 52 Quasars at z>5.7

We present the discovery of nine quasars at $z\sim6$ identified in the Sloan Digital Sky Survey (SDSS) imaging data. This completes our survey of $z\sim6$ quasars in the SDSS footprint. Our final sample consists of 52 quasars at $5.7<z\le6.4$, including 29 quasars with $z_{\rm AB}\le20$ mag selected from 11,240 deg$^2$ of the SDSS single-epoch imaging survey (the main survey), 10 quasars with $20\le z_{\rm AB}\le20.5$ selected from 4223 deg$^2$ of the SDSS overlap regions (regions with two or more imaging scans), and 13 quasars down to $z_{\rm AB}\approx22$ mag from the 277 deg$^2$ in Stripe 82. They span a wide luminosity range of $-29.0\le M_{1450}\le-24.5$. This well-defined sample is used to derive the quasar luminosity function (QLF) at $z\sim6$. After combining our SDSS sample with two faint ($M_{1450}\ge-23$ mag) quasars from the literature, we obtain the parameters for a double power-law fit to the QLF. The bright-end slope $β$ of the QLF is well constrained to be $β=-2.8\pm0.2$. Due to the small number of low-luminosity quasars, the faint-end slope $α$ and the characteristic magnitude $M_{1450}^{\ast}$ are less well constrained, with $α=-1.90_{-0.44}^{+0.58}$ and $M^{\ast}=-25.2_{-3.8}^{+1.2}$ mag. The spatial density of luminous quasars, parametrized as $ρ(M_{1450}<-26,z)=ρ(z=6)\,10^{k(z-6)}$, drops rapidly from $z\sim5$ to 6, with $k=-0.72\pm0.11$. Based on our fitted QLF and assuming an IGM clumping factor of $C=3$, we find that the observed quasar population cannot provide enough photons to ionize the $z\sim6$ IGM at $\sim90$\% confidence. Quasars may still provide a significant fraction of the required photons, although much larger samples of faint quasars are needed for more stringent constraints on the quasar contribution to reionization.

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preprint2016arXiv

Visual Tracking via Reliable Memories

In this paper, we propose a novel visual tracking framework that intelligently discovers reliable patterns from a wide range of video to resist drift error for long-term tracking tasks. First, we design a Discrete Fourier Transform (DFT) based tracker which is able to exploit a large number of tracked samples while still ensures real-time performance. Second, we propose a clustering method with temporal constraints to explore and memorize consistent patterns from previous frames, named as reliable memories. By virtue of this method, our tracker can utilize uncontaminated information to alleviate drifting issues. Experimental results show that our tracker performs favorably against other state of-the-art methods on benchmark datasets. Furthermore, it is significantly competent in handling drifts and able to robustly track challenging long videos over 4000 frames, while most of others lose track at early frames.

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preprint2015arXiv

Capability of Quasar Selection by Combining the SCUSS and SDSS Observations

The South Galactic Cap $u$-band Sky Survey (SCUSS) provides a deep $u$-band imaging of about 5000 deg$^2$ in south Galactic cap. It is about 1.5 mag deeper than the SDSS $u$-band. In this paper we evaluate the capability of quasar selection using both SCUSS and SDSS data, based on considerations of the deep SCUSS $u$-band imaging and two-epoch $u$-band variability. We find that the combination of the SCUSS $u$-band and the SDSS $griz$ band allows us to select more faint quasars and more quasars at redshift around 2.2 than the selection only with the SDSS $ugriz$ data. Quasars have significant $u$-band variabilities. The fraction of quasars with large two-epoch variability is much higher than that of stars. The selection by variability can select both low-redshift quasars with ultraviolet excess and mid-redshift ($2 < z <3.5$) quasars where quasar selection by optical colors is inefficient. The above two selections are complementary and make full use of the SCUSS u-band advantages.

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preprint2015arXiv

S-Lemma with Equality and Its Applications

Let $f(x)=x^TAx+2a^Tx+c$ and $h(x)=x^TBx+2b^Tx+d$ be two quadratic functions having symmetric matrices $A$ and $B$. The S-lemma with equality asks when the unsolvability of the system $f(x)<0, h(x)=0$ implies the existence of a real number $μ$ such that $f(x) + μh(x)\ge0, ~\forall x\in \mathbb{R}^n$. The problem is much harder than the inequality version which asserts that, under Slater condition, $f(x)<0, h(x)\le0$ is unsolvable if and only if $f(x) + μh(x)\ge0, ~\forall x\in \mathbb{R}^n$ for some $μ\ge0$. In this paper, we show that the S-lemma with equality does not hold only when the matrix $A$ has exactly one negative eigenvalue and $h(x)$ is a non-constant linear function ($B=0, b\not=0$). As an application, we can globally solve $\inf\{f(x)\vert h(x)=0\}$ as well as the two-sided generalized trust region subproblem $\inf\{f(x)\vert l\le h(x)\le u\}$ without any condition. Moreover, the convexity of the joint numerical range $\{(f(x), h_1(x),\ldots, h_p(x)):~x\in\Bbb R^n\}$ where $f$ is a (possibly non-convex) quadratic function and $h_1(x),\ldots,h_p(x)$ are affine functions can be characterized using the newly developed S-lemma with equality.

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preprint2015arXiv

The Interstellar Oxygen Crisis, or Where Have All the Oxygen Atoms Gone?

The interstellar medium (ISM) seems to have a significant surplus of oxygen which was dubbed as the "O crisis": independent of the adopted interstellar reference abundance, the total number of O atoms depleted from the gas phase far exceeds that tied up in solids by as much as ~160ppm of O/H. Recently, it has been hypothesized that the missing O could be hidden in micrometer-sized H2O ice grains. We examine this hypothesis by comparing the infrared (IR) extinction and far-IR emission arising from these grains with that observed in the Galactic diffuse ISM. We find that it is possible for the diffuse ISM to accommodate ~160ppm of O/H in micron-sized H2O ice grains without violating the observational constraints including the absence of the 3.1micron O-H absorption feature. More specifically, H2O ice grains of radii ~4micron and O/H = 160 ppm are capable of accounting for the observed flat extinction at ~ 3-8 micron and produce no excessive emission in the far-IR. These grains could be present in the diffuse ISM through rapid exchange of material between dense molecular clouds where they form and diffuse clouds where they are destroyed by photosputtering.

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preprint2015arXiv

Uniform Quadratic Optimization and Extensions

The uniform quadratic optimizatin problem (UQ) is a nonconvex quadratic constrained quadratic programming (QCQP) sharing the same Hessian matrix. Based on the second-order cone programming (SOCP) relaxation, we establish a new sufficient condition to guarantee strong duality for (UQ) and then extend it to (QCQP), which not only covers several well-known results in literature but also partially gives answers to a few open questions. For convex constrained nonconvex (UQ), we propose an improved approximation algorithm based on (SOCP). Our approximation bound is dimensional independent. As an application, we establish the first approximation bound for the problem of finding the Chebyshev center of the intersection of several balls.

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preprint2015arXiv

Very Large Interstellar Grains as Evidenced by the Mid-Infrared Extinction

The sizes of interstellar grains are widely distributed, ranging from a few angstroms to a few micrometers. The ultraviolet (UV) and optical extinction constrains the dust in the size range of a couple hundredth micrometers to several submicrometers. The near and mid infrared (IR) emission constrains the nanometer-sized grains and angstrom-sized very large molecules. However, the quantity and size distribution of micrometer-sized grains remain unknown as they are gray in the UV/optical extinction and they are too cold and emit too little in the IR to be detected by IRAS, Spitzer, or Herschel. In this work, we employ the ~3-8 micron mid-IR extinction which is flat in both diffuse and dense regions to constrain the quantity, size, and composition of the micron-sized grain component. We find that, together with nano- and submicron-sized silicate and graphite (as well as PAHs), micron-sized graphite grains with C/H=137 ppm and a mean size of ~1.2 micron closely fit the observed interstellar extinction of the Galactic diffuse interstellar medium from the far-UV to the mid-IR as well as the near-IR to millimeter thermal emission obtained by COBE/DIRBE, COBE/FIRAS, and Planck up to lambda < 1000 micron. The micron-sized graphite component accounts for ~14.6% of the total dust mass and ~2.5% of the total IR emission.

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preprint2014arXiv

An Improved Analysis of Semidefinite Approximation Bound for Nonconvex Nonhomogeneous Quadratic Optimization with Ellipsoid Constraints

We consider the problem of approximating nonconvex quadratic optimization with ellipsoid constraints (ECQP). We show some SDP-based approximation bounds for special cases of (ECQP) can be improved by trivially applying the extened Pataki's procedure. The main result of this paper is to give a new analysis on approximating (ECQP) by the SDP relaxation, which greatly improves Tseng's result [SIAM Journal Optimization, 14, 268-283, 2003]. As an application, we strictly improve the approximation ratio for the assignment-polytope constrained quadratic program.

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preprint2014arXiv

Indeterminate Strings, Prefix Arrays & Undirected Graphs

An integer array y = y[1..n] is said to be feasible if and only if y[1] = n and, for every i \in 2..n, i \le i+y[i] \le n+1. A string is said to be indeterminate if and only if at least one of its elements is a subset of cardinality greater than one of a given alphabet Sigma; otherwise it is said to be regular. A feasible array y is said to be regular if and only if it is the prefix array of some regular string. We show using a graph model that every feasible array of integers is a prefix array of some (indeterminate or regular) string, and for regular strings corresponding to y, we use the model to provide a lower bound on the alphabet size. We show further that there is a 1-1 correspondence between labelled simple graphs and indeterminate strings, and we show how to determine the minimum alphabet size |Sigma| of an indeterminate string x based on its associated graph Gx. Thus, in this sense, indeterminate strings are a more natural object of combinatorial interest than the strings on elements of Sigma that have traditionally been studied.

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preprint2014arXiv

Modeling the Infrared Interstellar Extinction

How dust scatters and absorbs starlight in the interstellar medium (ISM) contains important clues about the size and composition of interstellar dust. While the ultraviolet (UV) and visible interstellar extinction is well studied and can be closely fitted in terms of various dust mixtures (e.g., the silicategraphite mixture), the infrared (IR) extinction is not well understood, particularly, the mid-IR extinction in the 3-8 micron wavelength range is rather flat (or "gray") and is inconsistent with the standard Mathis, Rumpl, & Nordsieck (MRN) silicate-graphite dust model. We attempt to reproduce the flat IR extinction by exploring various dust sizes and species, including amorphous silicate, graphite, amorphous carbon and iron. We find that the flat IR extinction is best explained in terms of micrometer-sized amorphous carbon dust which consumes ~ 60 carbon atoms per million hydrogen atoms (i.e., C/H ~ 60 ppm). To account for the observed UV/visible and near-IR extinction, the silicate-graphite model requires Si/H ~ 34 ppm and C/H ~ 292 ppm. We conclude that the extinction from the UV to the mid-IR could be closely reproduced by a mixture of submicrometer-sized amorphous silicate dust, submicrometer-sized graphitic dust, and micrometer-sized amorphous carbon dust, at the expense of excess C available in the ISM (i.e., this model requires a solid-phase C abundance of C/H ~ 352 ppm, considerably exceeding what could be available in the ISM).

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preprint2014arXiv

Universality of the Near-Infrared Extinction Law Based on the APOGEE Survey

Whether the near-infrared (NIR) extinction law is universal has been a long debated topic. Based on the APOGEE H-band spectroscopic survey as a key project of SDSS-III, the intrinsic colors of a large number of giant stars are accurately determined from the stellar effective temperature. Taking this advantage and using a sample of 5942 K-type giants, the NIR extinction law is carefully re-visited. The color excess ratio E(J-H)/E(J-Ks), representative of the NIR extinction law, shows no dependence on the color excess when E(J-Ks) changes from ~0.3 to ~4.0, which implies a universal NIR extinction law from diffuse to dense regions. The constant value of E(J-H)/E(J-Ks), 0.64, corresponds to a power law index of 1.95. The other two ratios, E(H-Ks)/E(J-Ks) and E(J-H)/E(H-Ks), are 0.36 and 1.78 respectively. The results are consistent with the MRN dust size distribution.

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preprint2013arXiv

On Finite Time Singularity and Global Regularity of an Axisymmetric Model for the 3D Euler Equations

We investigate the large time behavior of an axisymmetric model for the 3D Euler equations. In \cite{HL09}, Hou and Lei proposed a 3D model for the axisymmetric incompressible Euler and Navier-Stokes equations with swirl. This model shares many properties of the 3D incompressible Euler and Navier-Stokes equations. The main difference between the 3D model of Hou and Lei and the reformulated 3D Euler and Navier-Stokes equations is that the convection term is neglected in the 3D model. In \cite{HSW09}, the authors proved that the 3D inviscid model can develop a finite time singularity starting from smooth initial data on a rectangular domain. A global well-posedness result was also proved for a class of smooth initial data under some smallness condition. The analysis in \cite{HSW09} does not apply to the case when the domain is axisymmetric and unbounded in the radial direction. In this paper, we prove that the 3D inviscid model with an appropriate Neumann-Robin boundary condition will develop a finite time singularity starting from smooth initial data in an axisymmetric domain. Moreover, we prove that the 3D inviscid model has globally smooth solutions for a class of large smooth initial data with some appropriate boundary condition.

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preprint2013arXiv

The Mid-Infrared Extinction Law and its Variation in the Coalsack Nebula

In recent years the wavelength dependence of interstellar extinction from the ultraviolet (UV), optical, through the near- and mid-infrared (IR) has been studied extensively. Although it is well established that the UV/optical extinction law varies significantly among the different lines of sight, it is not clear how the IR extinction varies among various environments. In this work, using the color-excess method and taking red giants as the extinction tracer, we determine the interstellar extinction Alambda in the four Spitzer/IRAC bands of the Coalsack nebula, a nearby starless dark cloud, based on the data obtained from the 2MASS and Spitzer/GLIMPSE surveys. We select five individual regions across the nebula that span a wide variety of physical conditions, ranging from diffuse, translucent to dense environments, as traced by the visual extinction, the Spitzer/MIPS 24micron emission, and CO emission. We find that Alambda/AKs, the mid-IR extinction relative to AKs, decreases from diffuse to dense environments, which may be explained in terms of ineffective dust growth in dense regions. The mean extinction (relative to AKs) is calculated for the four IRAC bands as well, which exhibits a flat mid-IR extinction law, consistent with previous determinations for other regions. The extinction in the IRAC 4.5micron band is anomalously high, much higher than that of the other three IRAC bands. It cannot be explained in terms of CO and CO2 ices. The mid-IR extinction in the four IRAC bands have also been derived for four representative regions in the Coalsack Globule 2 which respectively exhibit strong ice absorption, moderate or weak ice absorption, and very weak or no ice absorption. The derived mid-IR extinction curves are all flat, with Alambda/AKs increasing with the decrease of the H2O ice absorption optical depth.

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preprint2012arXiv

On one multidimensional compressible nonlocal model of the dissipative QG equations

In this paper we study the Cauchy problem for one multidimensional compressible nonlocal model of the dissipative quasi-geostrophic equations. First, we obtain the local existence and uniqueness of the smooth non-negative solution or the strong solution in time. Secondly, for the sub-critical and critical case $1\leα\leq 2$, we obtain the global existence and uniqueness results of the nonnegative smooth solution. Then, we prove the global existence of the weak solution for $0\le α\le 2$ and $ν\ge 0$. Finally, for the sub-critical case, we establish the global $H^1$ and $L^p, p>2,$ decay rate of the smooth solution as $t\to\infty$.

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preprint2012arXiv

On Singularity Formation of a 3D Model for Incompressible Navier-Stokes Equations

We investigate the singularity formation of a 3D model that was recently proposed by Hou and Lei in [16] for axisymmetric 3D incompressible Navier-Stokes equations with swirl. The main difference between the 3D model of Hou and Lei and the reformulated 3D Navier-Stokes equations is that the convection term is neglected in the 3D model. This model shares many properties of the 3D incompressible Navier-Stokes equations. One of the main results of this paper is that we prove rigorously the finite time singularity formation of the 3D inviscid model for a class of initial boundary value problems with smooth initial data of finite energy. We also prove the global regularity of the 3D inviscid model for a class of small smooth initial data.

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preprint2011arXiv

On the Local Well-posedness of a 3D Model for Incompressible Navier-Stokes Equations with Partial Viscosity

In this short note, we study the local well-posedness of a 3D model for incompressible Navier-Stokes equations with partial viscosity. This model was originally proposed by Hou-Lei in \cite{HouLei09a}. In a recent paper, we prove that this 3D model with partial viscosity will develop a finite time singularity for a class of initial condition using a mixed Dirichlet Robin boundary condition. The local well-posedness analysis of this initial boundary value problem is more subtle than the corresponding well-posedness analysis using a standard boundary condition because the Robin boundary condition we consider is non-dissipative. We establish the local well-posedness of this initial boundary value problem by designing a Picard iteration in a Banach space and proving the convergence of the Picard iteration by studying the well-posedness property of the heat equation with the same Dirichlet Robin boundary condition.

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preprint2010arXiv

On Singularity Formation of a Nonlinear Nonlocal System

We investigate the singularity formation of a nonlinear nonlocal system. This nonlocal system is a simplified one-dimensional system of the 3D model that was recently proposed by Hou and Lei in [13] for axisymmetric 3D incompressible Navier-Stokes equations with swirl. The main difference between the 3D model of Hou and Lei and the reformulated 3D Navier-Stokes equations is that the convection term is neglected in the 3D model. In the nonlocal system we consider in this paper, we replace the Riesz operator in the 3D model by the Hilbert transform. One of the main results of this paper is that we prove rigorously the finite time singularity formation of the nonlocal system for a large class of smooth initial data with finite energy. We also prove the global regularity for a class of smooth initial data. Numerical results will be presented to demonstrate the asymptotically self-similar blow-up of the solution. The blowup rate of the self-similar singularity of the nonlocal system is similar to that of the 3D model.

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