Now showing items 21-40 of 60016

    • Low-voltage ultrafast nonvolatile memory via direct charge injection through a threshold resistive-switching layer

      Li, Yuan; Zhang, Zhi Cheng; Li, Jiaqiang; Chen, Xu-Dong; Kong, Ya; Wang, Fu-Dong; Zhang, Guo-Xin; Lu, Tong-Bu; Zhang, Jin (Nature communications, Springer Science and Business Media LLC, 2022-08-06) [Article]
      The explosion in demand for massive data processing and storage requires revolutionary memory technologies featuring ultrahigh speed, ultralong retention, ultrahigh capacity and ultralow energy consumption. Although a breakthrough in ultrafast floating-gate memory has been achieved very recently, it still suffers a high operation voltage (tens of volts) due to the Fowler-Nordheim tunnelling mechanism. It is still a great challenge to realize ultrafast nonvolatile storage with low operation voltage. Here we propose a floating-gate memory with a structure of MoS2/hBN/MoS2/graphdiyne oxide/WSe2, in which a threshold switching layer, graphdiyne oxide, instead of a dielectric blocking layer in conventional floating-gate memories, is used to connect the floating gate and control gate. The volatile threshold switching characteristic of graphdiyne oxide allows the direct charge injection from control gate to floating gate by applying a nanosecond voltage pulse (20 ns) with low magnitude (2 V), and restricts the injected charges in floating gate for a long-term retention (10 years) after the pulse. The high operation speed and low voltage endow the device with an ultralow energy consumption of 10 fJ. These results demonstrate a new strategy to develop next-generation high-speed low-energy nonvolatile memory.
    • Investigating calcification-related candidates in a non-symbiotic scleractinian coral, Tubastraea spp.

      Capasso, Laura; Aranda, Manuel; Cui, Guoxin; Pousse, Melanie; Tambutté, Sylvie; Zoccola, Didier (Scientific Reports, Springer Science and Business Media LLC, 2022-08-06) [Article]
      In hermatypic scleractinian corals, photosynthetic fixation of CO2 and the production of CaCO3 are intimately linked due to their symbiotic relationship with dinoflagellates of the Symbiodiniaceae family. This makes it difficult to study ion transport mechanisms involved in the different pathways. In contrast, most ahermatypic scleractinian corals do not share this symbiotic relationship and thus offer an advantage when studying the ion transport mechanisms involved in the calcification process. Despite this advantage, non-symbiotic scleractinian corals have been systematically neglected in calcification studies, resulting in a lack of data especially at the molecular level. Here, we combined a tissue micro-dissection technique and RNA-sequencing to identify calcification-related ion transporters, and other candidates, in the ahermatypic non-symbiotic scleractinian coral Tubastraea spp. Our results show that Tubastraea spp. possesses several calcification-related candidates previously identified in symbiotic scleractinian corals (such as SLC4-γ, AMT-1like, CARP, etc.). Furthermore, we identify and describe a role in scleractinian calcification for several ion transporter candidates (such as SLC13, -16, -23, etc.) identified for the first time in this study. Taken together, our results provide not only insights about the molecular mechanisms underlying non-symbiotic scleractinian calcification, but also valuable tools for the development of biotechnological solutions to better control the extreme invasiveness of corals belonging to this particular genus.
    • Multilevel Importance Sampling for McKean-Vlasov Stochastic Differential Equation

      Rached, Nadhir Ben; Haji-Ali, Abdul-Lateef; Mohan, Shyam; Tempone, Raul (arXiv, 2022-08-05) [Preprint]
      This work combines multilevel Monte Carlo methods with importance sampling (IS) to estimate rare event quantities that can be expressed as the expectation of a Lipschitz observable of the solution to the McKean-Vlasov stochastic differential equation. We first extend the double loop Monte Carlo (DLMC) estimator, introduced in this context in our previous work [Ben Rached et al. 2022], to the multilevel setting. We formulate a novel multilevel DLMC (MLDLMC) estimator, and perform a comprehensive work-error analysis yielding new and improved complexity results. Crucially, we also devise an antithetic sampler to estimate level differences that guarantees reduced work complexity for the MLDLMC estimator compared with the single level DLMC estimator. To tackle rare events, we apply the same single level IS scheme, obtained via stochastic optimal control in [Ben Rached et al. 2022], over all levels of the MLDLMC estimator. Combining IS and MLDLMC not only reduces computational complexity by one order, but also drastically reduces the associated constant, ensuring feasible estimates for rare event quantities. We illustrate effectiveness of proposed MLDLMC estimator on the Kuramoto model from statistical physics with Lipschitz observables, confirming reduced complexity from O(TOL−4r) for the single level DLMC estimator to O(TOL−3r) while providing feasible estimation for rare event quantities up to the prescribed relative error tolerance TOLr.
    • A Fractional Image Inpainting Model Using a Variant of Mumford-Shah Model

      Halim, Abdul; Rohim, Abdur; Kumar, B. V. Rathish; Saha, Ripan (arXiv, 2022-08-05) [Preprint]
      In this paper, we propose a fourth order PDE model for image inpainting based on a variant of the famous Mumford-Shah (MS) image segmentation model. Convexity splitting is used to discrtised the time and we replace the Laplacian by its fractional counterpart in the time discretised scheme. Fourier spectral method is used for space discretization. Consistency, stability and convergence of the time discretised model has been carried out. The model is tested on some standard test images and compared them with the result of some models existed in the literature.
    • Cellular automata imbedded memristor-based recirculated logic in-memory computing

      Liu, Yanming; Tian, He; Wu, Fan; Liu, Anhan; Li, Yihao; Sun, Hao; Lanza, Mario; Ren, Tian-Ling (Research Square Platform LLC, 2022-08-05) [Preprint]
      Cellular automata is an important tool to study the emergent properties of complex systems based on its well-known parallel, bio-inspired, computational characteristics. However, running cellular automata on conventional chips suffer from low parallelism, and high hardware cost. Establish dedicated hardware for cellular automata remains elusive. Here, we propose a recirculate logic operations scheme (RLOS) based on memristive hardware combined with 2D transistors to realize cellular automata evolution. The scheme utilizes the storage and calculation characteristics of memristive devices, which greatly reduces hardware complexity. The versatility of the RLOS scheme allows implementing multiple different cellular automata algorithms on the same circuitry. The entire rule (rule 1-254) of elementary cellular automata and more complicated 1D CA model majority classification algorithm have been verified to be applicable to this circuitry. Further, the edge detection algorithm based on 2D cellular automata has been authenticated through RLOS. The experimental and evaluation results reveal that the scheme reduces the hardware cost up to 79 times comparing to the Field Programmable Gate Array (FPGA) approach. To our best knowledge, RLOS has the lowest hardware cost (6 components/per cell) among state-of-art hardware implementations. This work can pave the road towards high-efficiency and low-cost cellular automata hardware realization, and also facilitates the exploration of memristive applications.
    • Topochemical Synthesis of Ca3CrN3H Involving a Rotational Structural Transformation for Catalytic Ammonia Synthesis

      Cao, Yu; Kirsanova, Maria; Ochi, Masayuki; Almaksoud, Walid; Zhu, Tong; Rai, Rohit Kumar; Gao, Shenghan; Tsumori, Tatsuya; Kobayashi, Shintaro; Kawaguchi, Shogo; Abou-Hamad, Edy; Kuroki, Kazuhiko; Tassel, Cédric; Abakumov, Artem; Kobayashi, Yoji; Kageyama, Hiroshi (Angewandte Chemie (International ed. in English), Wiley, 2022-08-05) [Article]
      Topochemical reactions have led to great progress in the discovery of new metastable compounds with novel chemical and physical properties. With these reactions, the overall crystal structure of the host material is generally maintained. Here we report a topochemical synthesis of a hexagonal nitride hydride, h-Ca3CrN3H, by heating an orthorhombic nitride, o-Ca3CrN3, under hydrogen at 673 K, accompanied by a rotational structural transformation. The hydrogen intercalation modifies the Ca-N rock-salt-like atomic packing in o-Ca3CrN3 to a face-sharing octahedral chain in h-Ca3CrN3H, mimicking a 'hinged tessellation' movement. In addition, the h-Ca3CrN3H exhibited stable ammonia synthesis activity when used as a catalyst.
    • Various Wavefront Sensing and Control Developments on the Santa Cruz Extreme AO Laboratory (SEAL) Testbed

      Gerard, Benjamin L.; Perez-Soto, Javier; Chambouleyron, Vincent; Kooten, Maaike A. M. van; Dillon, Daren; Cetre, Sylvain; Jensen-Clem, Rebecca; Fu, Qiang; Amata, Hadi; Heidrich, Wolfgang (arXiv, 2022-08-05) [Preprint]
      Ground-based high contrast imaging (HCI) and extreme adaptive optics (AO) technologies have advanced to the point of enabling direct detections of gas-giant exoplanets orbiting beyond the snow lines around nearby young star systems. However, leftover wavefront errors using current HCI and AO technologies, realized as "speckles" in the coronagraphic science image, still limit HCI instrument sensitivities to detecting and characterizing lower-mass, closer-in, and/or older/colder exoplanetary systems. Improving the performance of AO wavefront sensors (WFSs) and control techniques is critical to improving such HCI instrument sensitivity. Here we present three different ongoing wavefront sensing and control project developments on the Santa cruz Extreme AO Laboratory (SEAL) testbed: (1) "multi-WFS single congugate AO (SCAO)" using the Fast Atmospheric Self-coherent camera (SCC) Technique (FAST) and a Shack Hartmann WFS, (2) pupil chopping for focal plane wavefront sensing, first with an external amplitude modulator and then with the DM as a phase-only modulator, and (3) a laboratory demonstration of enhanced linearity with the non-modulated bright Pyramid WFS (PWFS) compared to the regular PWFS. All three topics share a common theme of multi-WFS SCAO and/or second stage AO, presenting opportunities and applications to further investigate these techniques in the future.
    • Study of the Seawater Desalination Performance by Electrodialysis

      Shi, Jihong; Gong, Liang; Zhang, Tao; Sun, Shuyu (Membranes, MDPI AG, 2022-08-05) [Article]
      The global scarcity of freshwater resources has greatly contributed to the development of desalination technologies, in which electrodialysis desalination is one of the most widely used and highly regarded methods. In this work, the first step was to design and assemble a experiment module for electrodialysis desalination. The ion removal efficiency and single membrane mass transfer flux of electrodialysis desalination were investigated. The results show that the desalination performance of the module is improved by increasing the voltage gradient, increasing the concentration of seawater and electrolyte and decreasing the membrane surface flux and that the optimum operating conditions for the module at 24 V operating voltage are feedstock concentration of 35 g/L, electrolyte concentration of 1.42 g/L which and system flow rate of 15 L/h. The results of the study will help to better investigate electrodialysis desalination technology.
    • Spectrochemistry of Firefly Bioluminescence

      Al-Handawi, Marieh B.; Polavaram, Srujana; Kurlevskaya, Anastasiya; Commins, Patrick; Schramm, Stefan; Carrasco-López, César; Lui, Nathan M.; Solntsev, Kyril M.; Laptenok, Siarhei; Navizet, Isabelle; Naumov, Panče (Chemical Reviews, American Chemical Society (ACS), 2022-08-04) [Article]
      The chemical reactions underlying the emission of light in fireflies and other bioluminescent beetles are some of the most thoroughly studied processes by scientists worldwide. Despite these remarkable efforts, fierce academic arguments continue around even some of the most fundamental aspects of the reaction mechanism behind the beetle bioluminescence. In an attempt to reach a consensus, we made an exhaustive search of the available literature and compiled the key discoveries on the fluorescence and chemiluminescence spectrochemistry of the emitting molecule, the firefly oxyluciferin, and its chemical analogues reported over the past 50+ years. The factors that affect the light emission, including intermolecular interactions, solvent polarity, and electronic effects, were analyzed in the context of both the reaction mechanism and the different colors of light emitted by different luciferases. The collective data points toward a combined emission of multiple coexistent forms of oxyluciferin as the most probable explanation for the variation in color of the emitted light. We also highlight realistic research directions to eventually address some of the remaining questions related to firefly bioluminescence. It is our hope that this extensive compilation of data and detailed analysis will not only consolidate the existing body of knowledge on this important phenomenon but will also aid in reaching a wider consensus on some of the mechanistic details of firefly bioluminescence.
    • Origin of Interfacial Charges of Al2o3/Si and Al2o3/Gan Heterogeneous Heterostructures

      Wang, Chuanju; AlQatari, Feras S.; Khandelwal, Vishal; Lin, Rongyu; Li, Xiaohang (Elsevier BV, 2022-08-04) [Preprint]
      Al2O3 is a broadly employed dielectric and significant interfacial charges occur at Al2O3/semiconductor interfaces. However, the charge origin is often unclear that severely impacts device engineering and design. Al2O3/Si and Al2O3/GaN are two of the most common heterogeneous heterostructures (H2s) for many crucial devices including GaN transistors and Si solar cells. While negative charges are extensively observed in Al2O3/Si, positive charges exist in Al2O3/GaN, both of which are not well understood. In this study, we performed in-depth interfacial studies of the Al2O3/Si and Al2O3/GaN H2s to clarify the origin of the interfacial charges. Stoichiometry deviations were found at the interfaces of the two H2s where Al surpasses O for Al2O3/GaN, whereas O dominates at the Al2O3/Si interface. Therefore, we propose that the different interfacial charges are caused by nonstoichiometry atomic ratios of Al2O3 at the interface. The study indicates the important role of the semiconductor surface on the device performance, provide a deep understanding on the origin of interfacial charges at the insulator-semiconductor interfaces.
    • Visually Evaluating Generative Adversarial Networks Using Itself under Multivariate Time Series

      Pan, Qilong (arXiv, 2022-08-04) [Preprint]
      Visually evaluating the goodness of generated Multivariate Time Series (MTS) are difficult to implement, especially in the case that the generative model is Generative Adversarial Networks (GANs). We present a general framework named Gaussian GANs to visually evaluate GANs using itself under the MTS generation task. Firstly, we attempt to find the transformation function in the multivariate Kolmogorov Smirnov (MKS) test by explicitly reconstructing the architecture of GANs. Secondly, we conduct the normality test of transformed MST where the Gaussian GANs serves as the transformation function in the MKS test. In order to simplify the normality test, an efficient visualization is proposed using the chi square distribution. In the experiment, we use the UniMiB dataset and provide empirical evidence showing that the normality test using Gaussian GANs and chi sqaure visualization is effective and credible.
    • High Performance Micro Resonators-based Sensors using Multi-Mode Excitation

      Zhao, Wen; Khan, Fahimullah; Alcheikh, Nouha; Younis, Mohammad I. (IEEE Electron Device Letters, Institute of Electrical and Electronics Engineers (IEEE), 2022-08-04) [Article]
      We present a multi-mode excitation technique to significantly amplify the amplitude signal of micro-resonators-based sensors operating at their higher-order modes. We show that the multi-mode excitation can significantly reduce the noise effects, elevate the dynamic response level, and amplify the total amplitude response. We demonstrate the efficiency of the multi-mode excitation to enhance the performance of a gas sensor using electrothermally heated doubly-clamped buckled beams. The results indicate clear amplification for the response at the 2nd mode while detecting Helium compared to a single-source excitation. A 24-times amplitude magnification is achieved. The demonstrated approach provides a promising path to efficiently exploit the higher-order modes of resonant gas sensors leading to improved accuracy and resolution.
    • Single-step post-production treatment of lead acetate precursor-based perovskite using alkylamine salts for reduced grain-boundary related film defects

      Gebremichael, Zekarias Teklu; Alam, Shahidul; Stumpf, Steffi; Diegel, Marco; Schubert, Ulrich S.; Hoppe, Harald (Nano Select, Wiley, 2022-08-04) [Article]
      Powered by the worldwide efforts of research groups experienced in dye-sensitized, and thin-film solar cells, perovskite solar cells (PSCs) reached a power conversion efficiency of 25.7% within 10 years. However, the presence of defects and trap density within the active layer's grain boundaries commonly operates as non-radiative recombination centers. Hence, intensive efforts have been reported to passivate the inevitable bulk and interface defects of the active layer using additives or post-treatment processing to enhance the efficiency and stability of PSCs. Herein, a facile post-treatment strategy based on wet processing methylammonium lead triiodide, MAPbI3 (prepared from lead acetate and methylammonium iodide precursors) films with organic amine salts (FABr and FAI) is demonstrated. As a result, high-quality films of mixed perovskites (FAxMA1-xPbI3-xBrx and FAxMA1-xPbI3) were obtained. The surface treatment has efficiently passivate the defects in the host film, suppressing the non-radiative carrier recombination. Compared to the control device, the increased open-circuit voltage (from 0.5 V to 1 V) and fill factor (FF) values of the optimized device based on FAxMA1-xPbI3 showed a PCE of 16.13%. And our findings revealed that post-treatment is possible on wet perovskite film aged for a few minutes prior to its post-treatment, which saved the energy used for pre-annealing.
    • Prevalence of anti-SARS-CoV-2 antibody in hemodialysis facilities: a cross-sectional multicenter study from Madinah

      Housawi, Abdulrahman A.; Qazi, Shazada Junaid S.; Jan, Abdulhalem A.; Osman, Rashid A.; Alshamrani, Mashil M.; AlFaadhel, Talal A.; AlHejaili, Fayez F.; Al-Tawfiq, Jaffar A.; Wafa, Ahmed A.; Hamza, Abdulmageed E.; Hassan, Moustafa A.; Alharbi, Suliman A.; Albasheer, Hamza; Almohmmdi, Majed M.; Alsisi, Salem A.; Mankowski, Michal; van de Klundert, Joris; Alhelal, Amal M.; Sala, Fatima H.; Kheyami, Ali; Alhomayeed, Bader A. (Annals of Saudi Medicine, King Faisal Specialist Hospital and Research Centre, 2022-08-04) [Article]
      BACKGROUND: Since the occurrence of coronavirus disease in 2019 (COVID-19), the global community has witnessed its exponential spread with devastating outcomes within the general population and specifically within hemodialysis patients. OBJECTIVES: Compare the state of immunity to SARS-CoV-2 among hemodialysis patients and staff. DESIGN: Cross-sectional study with a prospective follow-up period. SETTING: Hemodialysis centers in Madinah region. PATIENTS AND METHODS: We prospectively tested for SARS-CoV-2 antibodies in dialysis patients using dialysis centers staff as controls. The participants were tested on four occasions when feasible for the presence of anti-SARS-CoV-2 antibodies. We also analyzed factors that might be associated with seropositivity. MAIN OUTCOME MEASURES: SARS-CoV-2 positivity using immunoglobulin G (IgG) levels SAMPLE SIZE: 830 participants, 677 patients and 153 dialysis centers staff as controls. RESULTS: Of the total participants, 325 (257 patients and 68 staff) were positive for SARS-CoV-2 IgG antibodies, for a prevalence of 38.0% and 44.4% among patients and staff, respectively (P=.1379). Participants with a history of COVID-19 or related symptoms were more likely to have positive IgG (P<.0001). Surprisingly, positivity was also center-dependent. In a multivariable logistic regression, a history of infection and related symptoms contributed significantly to developing immunity. CONCLUSION: The high prevalence of SARS-CoV-2 antibody among hemodialysis patients and previously asymptomatic staff suggested past asymptomatic infection. Some centers showed more immunity effects than others. LIMITATIONS: Unable to collect four samples for each participant; limited to one urban center. CONFLICT OF INTEREST: None.
    • High-resolution soil moisture data reveal complex multi-scale spatial variability across the United States

      Vergopolan, Noemi; Sheffield, Justin; Chaney, Nathaniel W.; Pan, Ming; Beck, Hylke E.; Ferguson, Craig R.; Torres-Rojas, Laura; Eigenbrod, Felix; Crow, Wade; Wood, Eric F. (Geophysical Research Letters, American Geophysical Union (AGU), 2022-08-04) [Article]
      Soil moisture (SM) spatiotemporal variability critically influences water resources, agriculture, and climate. However, besides site-specific studies, little is known about how SM varies locally (1–100-m scale). Consequently, quantifying the SM variability and its impact on the Earth system remains a long-standing challenge in hydrology. We reveal the striking variability of local-scale SM across the United States using SMAP-HydroBlocks — a novel satellite-based surface SM dataset at 30-m resolution. Results show how the complex interplay of SM with landscape characteristics and hydroclimate is primarily driven by local variations in soil properties. This local-scale complexity yields a remarkable and unique multi-scale behavior at each location. However, very little of this complexity persists across spatial scales. Experiments reveal that on average 48% and up to 80% of the SM spatial information is lost at the 1-km resolution, with complete loss expected at the scale of current state-of-the-art SM monitoring and modeling systems (1–25 km).
    • Highly Stable Lead-Free Perovskite Single Crystals with NIR Emission Beyond 1100 nm

      Liu, Zhuang; Qin, Xian; Chen, Qihao; Chen, Qiushui; Jing, Yuhang; Zhou, Zhonghao; Zhao, Yong Sheng; Chen, Jingsheng; Liu, Xiaogang (Advanced Optical Materials, Wiley, 2022-08-04) [Article]
      Materials that emit in the near-infrared (NIR) region are at the forefront of both research and industry, mainly due to their wide applications in national security, nondestructive bioimaging, long-wave communications, and photothermal conversion for medical care. As a key member of the luminescent materials family, metal halide perovskites have been intensively demonstrated to emit light in ultraviolet and visible regions. However, NIR-emitting perovskites suffer from severe limitations, such as low photoluminescence quantum yield and poor chemical/optical stability, thereby preventing them from practical applications. Herein, the synthesis and growth of Cs2MoCl6 and Cs2WCl6 perovskite single crystals with ultrahigh chemical and optical resistance to heat, moisture, polar solvents, and high-energy radiation is reported. Upon ultraviolet or blue excitation, these lead-free single crystals emit light beyond 1100 nm, the longest wavelength ever reported for perovskite hosts. Mechanistic studies indicate that self-trapped excitons are responsible for the NIR emission. The authors fabricate optoelectronic devices using these single crystals and demonstrate their broad applications in noninvasive palm vein imaging, night vision, and nondestructive food analysis. These results may stimulate research in the development of high-efficiency NIR perovskite phosphors for fast, accurate biometric identification and food inspection.
    • A new species of Bathypathes (Cnidaria, Anthozoa, Antipatharia, Schizopathidae) from the Red Sea and its phylogenetic position

      Chimienti, Giovanni; Terraneo, Tullia Isotta; Vicario, Silvia; Marchese, Fabio; Purkis, Sam J.; Abdulla Eweida, Ameer; Rodrigue, Mattie; Benzoni, Francesca (ZooKeys, Pensoft Publishers, 2022-08-04) [Article]
      A black coral, Bathypathes thermophila Chimienti, sp. nov. is described from the Saudi Arabian coasts of the Gulf of Aqaba and north Red Sea (Neom area) using an integrated taxonomic approach. The morphological distinctiveness of the new species is confirmed by molecular analyses. The species thrives in warm and high salinity waters typical of the Red Sea at bathyal depths. It can form colony aggregations on muddy bottoms with scattered, small hard substrates. Colonies are monopodial, feather-like, and attached to a hard substrate through a thorny basal plate. Pinnules are simple, arranged biserially and alternately, and all the same length (up to approximately 20 cm) except for few, proximal ones. Spines are triangular, laterally compressed, subequal, smooth, and simple or rarely bifurcated. Polyps are elongated transversely, 1.5–2.0 mm in transverse diameter. Large colonies can have one or few branches, whose origin is discussed. The phylogenetic position of B. thermophila sp. nov. within the order Antipatharia, recovered using three mitochondrial markers, shows that it is nested within the family Schizopathidae. It is close to species in the genera Parantipathes, Lillipathes, Alternatipathes, and Umbellapathes rather than to the other available representatives of the genus Bathypathes, as currently defined based on morphology. In agreement with previous findings, our results question the evolutionary significance of morphological characters traditionally used to discriminate Antipatharia at higher taxonomic level.
    • Beamforming Design for Integrated Sensing and Communication Systems with Finite Alphabet Input

      Cong, Dingyan; Guo, Shuaishuai; Zhang, Haixia; Ye, Jia; Alouini, Mohamed-Slim (IEEE Wireless Communications Letters, IEEE, 2022-08-04) [Article]
      Joint beamforming for integrated sensing and communication (ISAC) is an efficient way to combine two functionalities in a system at the waveform level. This letter proposes a beamforming design for ISAC systems with finite alphabet signaling. We formulate a problem to maximize the minimum Euclidean distance (MMED) among noise-free received signal vectors under a sensing constraint and a given power constraint. To tackle the formulated optimization problem, we transform it into a semi-definite programming (SDP) and solve it by the semi-definite relaxation (SDR) method. Comprehensive comparisons with existing schemes show that our proposed beamforming offers lower symbol error rate (SER), higher mutual information, and also better sensing performance than existing ISAC beamforming designs.
    • Time-Varying Dispersion Integer-Valued GARCH Models

      Barreto-Souza, Wagner; Piancastelli, Luiza S. C.; Fokianos, Konstantinos; Ombao, Hernando (arXiv, 2022-08-04) [Preprint]
      We propose a general class of INteger-valued Generalized AutoRegressive Conditionally Heteroskedastic (INGARCH) processes by allowing time-varying mean and dispersion parameters, which we call time-varying dispersion INGARCH (tv-DINGARCH) models. More specifically, we consider mixed Poisson INGARCH models and allow for a dynamic modeling of the dispersion parameter (as well as the mean), similarly to the spirit of the ordinary GARCH models. We derive conditions to obtain first and second order stationarity, and ergodicity as well. Estimation of the parameters is addressed and their associated asymptotic properties established as well. A restricted bootstrap procedure is proposed for testing constant dispersion against time-varying dispersion. Monte Carlo simulation studies are presented for checking point estimation, standard errors, and the performance of the restricted bootstrap approach. The inclusion of covariates is also addressed and applied to the daily number of deaths due to COVID-19 in Ireland. Insightful results were obtained in the data analysis, including a superior performance of the tv-DINGARCH processes over the ordinary INGARCH models.
    • Differentially Private ℓ1-norm Linear Regression with Heavy-tailed Data

      Wang, Di; Xu, Jinhui (IEEE, 2022-08-03) [Conference Paper]
      We study the problem of Differentially Private Stochastic Convex Optimization (DP-SCO) with heavy-tailed data. Specifically, we focus on the ℓ1-norm linear regression in the ϵ-DP model. While most of the previous work focuses on the case where the loss function is Lipschitz, here we only need to assume the variates has bounded moments. Firstly, we study the case where the ℓ2 norm of data has bounded second order moment. We propose an algorithm which is based on the exponential mechanism and show that it is possible to achieve an upper bound of O~(dnε−−√) (with high probability). Next, we relax the assumption to bounded θ-th order moment with some θ ∈ (1,2) and show that it is possible to achieve an upper bound of O~((dnε−−√)θ−1θ). Our algorithms can also be extended to more relaxed cases where only each coordinate of the data has bounded moments, and we can get an upper bound of O~(dnε−−√) and O~(d(nε)θ−1θ) in the second and θ-th moment case respectively.