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Search Results: 1 - 10 of 128629 matches for " Xun Li "
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Continuous-time mean-variance efficiency: the 80% rule
Xun Li,Xun Yu Zhou
Mathematics , 2007, DOI: 10.1214/105051606000000349
Abstract: This paper studies a continuous-time market where an agent, having specified an investment horizon and a targeted terminal mean return, seeks to minimize the variance of the return. The optimal portfolio of such a problem is called mean-variance efficient \`{a} la Markowitz. It is shown that, when the market coefficients are deterministic functions of time, a mean-variance efficient portfolio realizes the (discounted) targeted return on or before the terminal date with a probability greater than 0.8072. This number is universal irrespective of the market parameters, the targeted return and the length of the investment horizon.
Flow Injection Determination of Tramadol Based on Its Sensitizing Effect on the Chemiluminescent Reaction of Permanganate-Sulfite  [PDF]
Xun Yao, Jingkai Zhang, Jianguo Li
American Journal of Analytical Chemistry (AJAC) , 2011, DOI: 10.4236/ajac.2011.27088
Abstract: In this paper, a novel chemiluminescence (CL) method for the determination of tramadol has been developed by combining the flow injection technique and its sensitizing effect on the weak CL reaction between sulfite and acidic KMnO4. A mechanism for the CL reaction has been proposed on the basis of fluorescent and CL spectra. Under the optimized conditions, the proposed method allows the measurement of tramadol hydrochloride over the range of 0.04 - 4 ?g/mLwith a correlation coefficient of 0.9995 (n = 8) and a detection limit of 0.01 ?g/mL (3σ), and the relative standard deviation for 2.0 ?g/mL tramadol (n = 11) is 2.1%. The utility of this method was demonstrated by determining tramadol hydrochloride in tablets and injections.
Land Administration System structured Land rent residuals and China’s urban sprawl – A Case Study of Dashi, Guangzhou
Liu Xuan,Li Xun
Urbani Izziv , 2012,
Abstract: Urban sprawl has been at the centre of current debate on urban structure. Compared to the rich literature on urban sprawl in Western cities, relatively little is known about the physical consequence and the causes of urban sprawl in China. In China, for peasant collective-owned land (COL), land use right cannot be sold, transferred or leased for non-agricultural construction’. COL could be developed until it is acquired and leased out by the government. Through the process of land acquisition, the peasants could only get the compensation for 30 years' average of annual agricultural production value in addition to compensation for attachments, crops and vegetables which sometimes could hardly support the life of peasants who lost their land. To compensation for the peasants’ loss, the government would leave 10-15% of the acquired land to peasant collectives as Collectively Owned Economic Development Land (COEDL), allowing it used for industrial or commercial purpose. However, development of COEDL manifested to have low development density, quality and output. With a property rights approach, this research evaluates the development process of COEDL in Guangzhou. It has been revealed that high cost to obtain the permit of land use change (nongzhuanyong zhibiao), and to provide infrastructure and public facilities reduce the incentives of peasants to use COEDL legally and more efficiently. Furthermore, as COEDL is forbidden to transfer or mortgage, the financing ability of peasant collectives is thus impaired severely, allowing them only low investment on development. Use of COEDL thus contributes to urban sprawl in China.
The effects of economic growth on Chinese urbanization: panel data approach

LI Xun,

地理研究 , 2005,
Abstract: The coordination between the levels of urbanization and economic growth is very important to realize rural-urban and regional development as a whole. But China's urbanization level lags behind economic growth level seriously. This paper aims to probe into the economic factors that contribute to the lag and to make quantitative analysis on its force. This topic has been discussed extensively by scholars using the general model with the panel data of cross-country in the world. The empirical growth literature has been documented that the estimate of panel data approach is more robust than that of cross-sectional approach in cross-country studies. First, we divide countries into low-income group, middle-income group and high-income group, then we select 70 countries and data between 1982 and 1997 which are offered by World Bank's World Development Report, and then we construct a world model which will affect urbanization level. By using the standard analysis tool-panel analysis in transnational economic empirical analysis, we found that in the period of low and middle-income, per capita GDP has the greatest effect on urbanization. Since the tertiary industry is the main force of urbanization, the secondary industry should be the original force of urbanization. In the period of high and middle-income, the force of tertiary industry has the greatest effect while in the period of high-income, the force of higher education and international trade becomes outstanding. Based on the world model, we construct a model which includes the factors that will affect China's urbanization level. By using the fourth and fifth census data and other official statistic data, we make a quantitative analysis on the economic factors which contribute to the lag of China's urbanization and found that the elasticities of per capita GDP, secondary industry ratio, tertiary industry ratio, international trade and higher education to urbanization are 0.420, 0.196, 0.197, 0.103 and 0.048 respectively. Finally, through the comparison between different countries, we found that the force of each economic factor in China is the same as that in the world, which means that the evaluation result of the two models is robust on the one hand, and that China's urbanization is in conformity with the general law of world's urbanization on the other hand. However, further analysis shows that the force of economic growth, secondary industry, tertiary industry and higher education are relatively laggard in China. Among them, the difference of tertiary industry's elasticity to urbanization is most evident, being only 43.8% of the world, and the next is secondary industry ratio, higher education level and international trade, which are 56.3%, 56.4% and 71.5% respectively. And the economic growth's elasticity to urbanization is 76.3% of that in the world. The main reason for this phenomenon is the close relationship with China's unique urbanization mode. The bottom-up urbanization,motivated by town
Continuous-Time Mean-Variance Portfolio Selection with Constraints on Wealth and Portfolio
Xun Li,Zuo Quan Xu
Mathematics , 2015,
Abstract: We consider continuous-time mean-variance portfolio selection with bankruptcy prohibition under convex cone portfolio constraints. This is a long-standing and difficult problem not only because of its theoretical significance, but also for its practical importance. First of all, we transform the above problem into an equivalent mean-variance problem with bankruptcy prohibition without portfolio constraints. The latter is then treated using martingale theory. Our findings indicate that we can directly present the semi-analytical expressions of the pre-committed efficient mean-variance policy without a viscosity solution technique but within a general framework of the cone portfolio constraints. The numerical simulation also sheds light on results established in this paper.
2-(Benzenesulfonamido)pyridinium nitrate
Jiang-Sheng Li,Xun Li
Acta Crystallographica Section E , 2009, DOI: 10.1107/s1600536809015670
Abstract: In the title compound, C11H11N2O2S+·NO3 , the dihedral angle between the benzene and pyridinium rings is 87.59 (8)°. An intramolecular C—H...O interaction occurs in the cation. In the crystal structure, ion pairs occur, being linked by two strong N—H...O interactions, forming R22(8) loops. The packing is further stabilized by weak C—H...O interactions.
A Fair-Priority MAC design in Networked Control System
Hongjun Li,Xun Li,Hongxu Ma
International Journal of Computer Network and Information Security , 2009,
Abstract: Media Access Control (MAC) protocols in Wireless Networked Control System (WNCS) must minimize the radio energy costs in sensor nodes. Latency and throughput are also important design features for MAC protocols in the applications. But most of them cannot guarantee quality for real-time traffic. This paper studies the state of the art of current real-time MAC protocols, and then introduces a medium access control protocol and a improved protocol that provide multiple priority levels and hard real-time transmission. The channel is accessed by sensors according to their priorities. Sensors send frames in a round manner with same priority. The fairness between different priorities is provided. The channel access procedure is divided into two stages: broadcast period and transmission period. Simulation and experiment results indicate that our protocol provides high channel utilization and bounded delays for real-time communication and can be well applied in the many fields especially the dynamic wireless sensor networks.
Seamless Positioning and Navigation by Using Geo-Referenced Images and Multi-Sensor Data
Xun Li,Jinling Wang,Tao Li
Sensors , 2013, DOI: 10.3390/s130709047
Abstract: Ubiquitous positioning is considered to be a highly demanding application for today’s Location-Based Services (LBS). While satellite-based navigation has achieved great advances in the past few decades, positioning and navigation in indoor scenarios and deep urban areas has remained a challenging topic of substantial research interest. Various strategies have been adopted to fill this gap, within which vision-based methods have attracted growing attention due to the widespread use of cameras on mobile devices. However, current vision-based methods using image processing have yet to revealed their full potential for navigation applications and are insufficient in many aspects. Therefore in this paper, we present a hybrid image-based positioning system that is intended to provide seamless position solution in six degrees of freedom (6DoF) for location-based services in both outdoor and indoor environments. It mainly uses visual sensor input to match with geo-referenced images for image-based positioning resolution, and also takes advantage of multiple onboard sensors, including the built-in GPS receiver and digital compass to assist visual methods. Experiments demonstrate that such a system can greatly improve the position accuracy for areas where the GPS signal is negatively affected (such as in urban canyons), and it also provides excellent position accuracy for indoor environments.
Unified Framework of Mean-Field Formulations for Optimal Multi-period Mean-Variance Portfolio Selection
Xiangyu Cui,Xun Li,Duan Li
Mathematics , 2013,
Abstract: The classical dynamic programming-based optimal stochastic control methods fail to cope with nonseparable dynamic optimization problems as the principle of optimality no longer applies in such situations. Among these notorious nonseparable problems, the dynamic mean-variance portfolio selection formulation had posted a great challenge to our research community until recently. A few solution methods, including the embedding scheme, have been developed in the last decade to solve the dynamic mean-variance portfolio selection formulation successfully. We propose in this paper a novel mean-field framework that offers a more efficient modeling tool and a more accurate solution scheme in tackling directly the issue of nonseparability and deriving the optimal policies analytically for the multi-period mean-variance-type portfolio selection problems.
Mean-Variance Policy for Discrete-time Cone Constrained Markets: The Consistency in Efficiency and Minimum-Variance Signed Supermartingale Measure
Xiangyu Cui,Duan Li,Xun Li
Mathematics , 2014,
Abstract: The discrete-time mean-variance portfolio selection formulation, a representative of general dynamic mean-risk portfolio selection problems, does not satisfy time consistency in efficiency (TCIE) in general, i.e., a truncated pre-committed efficient policy may become inefficient when considering the corresponding truncated problem, thus stimulating investors' irrational investment behavior. We investigate analytically effects of portfolio constraints on time consistency of efficiency for convex cone constrained markets. More specifically, we derive the semi-analytical expressions for the pre-committed efficient mean-variance policy and the minimum-variance signed supermartingale measure (VSSM) and reveal their close relationship. Our analysis shows that the pre-committed discrete-time efficient mean-variance policy satisfies TCIE if and only if the conditional expectation of VSSM's density (with respect to the original probability measure) is nonnegative, or once the conditional expectation becomes negative, it remains at the same negative value until the terminal time. Our findings indicate that the property of time consistency in efficiency only depends on the basic market setting, including portfolio constraints, and this fact motivates us to establish a general solution framework in constructing TCIE dynamic portfolio selection problem formulations by introducing suitable portfolio constraints.
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