Decentralized Stochastic Bilevel Optimization with Improved per-Iteration Complexity

Bilevel optimization recently has received tremendous attention due to its great success in solving important machine learning problems like meta learning, reinforcement learning, and hyperparameter optimization. Extending single-agent training on bilevel problems to the decentralized setting is a natural generalization, and there has been a flurry of work studying decentralized bilevel optimization algorithms. However, it remains unknown how to design the distributed algorithm with sample complexity and convergence rate comparable to SGD for stochastic optimization, and at the same time without directly computing the exact Hessian or Jacobian matrices. In this paper we propose such an algorithm. More specifically, we propose a novel decentralized stochastic bilevel optimization (DSBO) algorithm that only requires first order stochastic oracle, Hessian-vector product and Jacobian-vector product oracle. The sample complexity of our algorithm matches the currently best known results for DSBO, and the advantage of our algorithm is that it does not require estimating the full Hessian and Jacobian matrices, thereby having improved per-iteration complexity.Comment: ICML 202

Design of ganymede-synchronous frozen orbit around Europa

A Ganymede-synchronous frozen orbit around Europa provides a stable spatial geometry between a Europa probe and a Ganymede lander, which facilitates the observation of Ganymede and data transmission between probes. However, the third-body gravitation perturbation of Ganymede continues to accumulate and affect the long-term evolution of the Europa probe. In this paper, the relative orbit of Ganymede with respect to Europa is considered to accurately capture the perturbation potential. The orbital evolution behaviors of the Europa probe under the non-spherical gravitation of Europa and the third-body gravitation of Jupiter and Ganymede are studied based on a double-averaging framework. Then, the initial orbital conditions of the Ganymede-synchronous frozen orbit are developed. A station-keeping maneuver was performed to maintain the orbital elements to achieve the Ganymede-synchronous and frozen behaviors. A numerical simulation showed that the consumption for orbital maintenance is acceptablePeer ReviewedPostprint (published version

Photocatalytic oxidation of methane over silver decorated zinc oxide nanocatalysts

The search for active catalysts that efficiently oxidize methane under ambient conditions remains a challenging task for both C1 utilization and atmospheric cleansing. Here, we show that when the particle size of zinc oxide is reduced down to the nanoscale, it exhibits high activity for methane oxidation under simulated sunlight illumination, and nano silver decoration further enhances the photo-activity via the surface plasmon resonance. The high quantum yield of 8% at wavelengths \u3c 400 nm and over 0.1% at wavelengths ¿ 470 nm achieved on the silver decorated zinc oxide nanostructures shows great promise for atmospheric methane oxidation. Moreover, the nano-particulate composites can efficiently photo-oxidize other small molecular hydrocarbons such as ethane, propane and ethylene, and in particular, can dehydrogenize methane to generate ethane, ethylene and so on. On the basis of the experimental results, a two-step photocatalytic reaction process is suggested to account for the methane photo-oxidation

Design of Ganymede-synchronous frozen orbit around Europa

A Ganymede-synchronous frozen orbit around Europa provides a stable spatial geometry between a Europa probe and a Ganymede lander, which facilitates the observation of Ganymede and data transmission between probes. However, the third-body gravitation perturbation of Ganymede continues to accumulate and affect the long-term evolution of the Europa probe. In this paper, the relative orbit of Ganymede with respect to Europa is considered to accurately capture the perturbation potential. The orbital evolution behaviors of the Europa probe under the non-spherical gravitation of Europa and the third-body gravitation of Jupiter and Ganymede are studied based on a double-averaging framework. Then, the initial orbital conditions of the Ganymede-synchronous frozen orbit are developed. A station-keeping maneuver was performed to maintain the orbital elements to achieve the Ganymede-synchronous and frozen behaviors. A numerical simulation showed that the consumption for orbital maintenance is acceptable

Design of High-Inclination Artificial Frozen Orbits Around Europa

High-inclination near-circular orbits are valuable for Europa’s global observation. However, the eccentricity drifts due to Jovian third-body gravity and Europa’s nonspherical gravity perturbations. Previous studies have concentrated on long lifetime orbits, but seldom maintained the orbits with low-thrust propulsion. In this work, we propose methods for designing near-circle and high-inclination artificial frozen orbits around Europa. The design method is separated into two parts. First, based on an analytical method, two frozen strategies are developed for maintaining the Europa probe, which use indirect and direct neutralization of the eccentricity drift, respectively. The analysis of the evolution characteristic of the artificial frozen orbit is presented. Second, through a semi-analytical approach, fuel-optimal low-thrust strategies which satisfy the proposed frozen strategies are derived to generate the artificial frozen orbits. Numerical simulations show that these artificial orbits have the ability of achieving the frozen behavior, with acceptable required velocity increments. The low-cost control strategies are also obtained

Hybrid method for accurate multi-gravity-assist trajectory design using pseudostate theory and deep neural networks

This paper presents a novel hybrid method to design the continuous and accurate multi-gravity-assist trajectory for a high-fidelity dynamics. The gravitational perturbation of the primary body is considered during the gravity assistance. The pseudostate technique is applied to approximate the gravity-assisted trajectory, where the optimal sweepback duration is solved using a trained deep neural network. The major factors that affect the optimal sweepback duration of the approach and departure segments are investigated. The results show that the optimal sweepback duration of the approach segment only relies on the shape of the approach trajectory and is independent of the flight time. Then, a gravity-assisted trajectory patched strategy and a hybrid algorithm combining the particle swarm optimization and the sequential quadratic programming are developed to optimize the multi-gravity-assist trajectory. The proposed hybrid method is applied to the Europa orbiter mission. In comparison with the traditional patched conic method, this method demonstrates outstanding performance on accuracy and significantly reduces the computational time and complexity of the trajectory correction with the high-fidelity dynamics

Impact of U.S. class action lawsuits on cross-listed foreign companies

This study evaluates the short-term valuation impact of U.S. class action lawsuits by focusing on both sued and non-sued foreign companies listed in the United States. Using a comprehensive database that includes stock- and company-level information in both the U.S. and local home markets, I examine how private U.S. securities litigations affect the market value of both sued foreign companies and peer foreign firms not accused of wrongdoing. I find that during the event period surrounding the lawsuit-filing date, there is a significant negative stock price reaction for the sued foreign companies. Moreover, investors also tend to react negatively towards non-sued foreign issuers during this period. The logistic regression results also suggest that the determinants of lawsuit propensity are similar for foreign firms cross-listed in the U.S. and U.S. domestic companies. Finally, certain firm-, lawsuit-, and country-level characteristics can explain the degree of stock market reactions. The overall results provide evidence that private class action lawsuits in the U.S. have economically significant impact on cross-listed foreign issuers, thus playing an important role in overseeing and disciplining foreign companies

Photocatalytic oxidation of methane over silver decorated zinc oxide nanocatalysts

This work was financially supported by the National Key Project on Basic Research (Grant No. 2013CB933203), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB20000000), the Natural Science Foundation of China (Grant No. 21373224, 21577143 and 51502289), the Natural Science Foundation of Fujian Province (Grant No. 2014H0054 and 2015J05044) and the One Hundred Talents Program of the Chinese Academy of Sciences

Extension Cloud Model and Grey Clustering Evaluation of Enterprise Safety Management System: Based on COWA-CRITIC Combination Weighting

In order to address the issues of unclear risk grading control, lack of safety management, and hidden danger investigation and management processes, this paper used a mining enterprise as the backdrop for an engineering example. The “evaluation model of the overall construction level of the enterprise safety management system” is constructed from four aspects: “preliminary infrastructure”, “risk grading and control”, “hidden danger investigation and management processes”, and “Post-support work”. The safety evaluation level is divided into five levels, and the evaluation model is combined weighted by using the combined ordered weighted averaging (COWA) algorithm and the criteria importance through intercriteria correlation (CRITIC) method. In addition, the cloud model, the extension cloud model, and the grey clustering evaluation method are used for a thorough evaluation. Finally, the enterprise safety management system’s overall construction level is determined to be good. In order to effectively strengthen the enterprise safety management capability and prevent the occurrence of production safety accidents, this study provides a practical and thorough evaluation method for the evaluation of the enterprise safety management system. This method makes it easier to identify system weaknesses and provides a safety guarantee for the sustainable development of enterprises

Formation Tracking Control for Multi-Agent Systems with Collision Avoidance and Connectivity Maintenance

This paper investigates the formation tracking control of multiple agents with a double-integrator model and presents a novel distributed control framework composed of three items: a potential-based gradient term, a formation term, and a navigation term. Considering the practical situation, each agent is regarded as a rigid-body with a safe radius and a sensing region. To enable collision avoidance and connectivity maintenance among multiple agents, a new potential function with fewer parameters is established. The predetermined formation is also achieved by taking the difference between the actual displacement and the desired displacement as a consensus variable. Lastly, the virtual navigator provides trajectory signals and guides the multiple agent movement. Two instances of an equilateral triangle formation and a hexagonal formation are used in the simulation to verify the proposed method