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Effects of diffusion time and diffusion gradient strength on the mean diffusivity of water molecules in healthy human brain

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Progress in Natural Science:Materials International 2006年第7期16卷 706-711页

作者： GAO Song, WANG Xiaoying and BAO Shanglian ( Laboratory of Medical Physics & Engineering, peking university, beijing 100871, china department of Radiology, The First Affiliated Hospital of peking university, beijing 100034, china) peking Univ Lab Med Phys & Engn Beijing 100871 Peoples R China peking Univ Affiliated Hosp 1 Dept Radiol Beijing 100034 Peoples R China

Diffusion time and diffusion gradient strength are two important parameters of pulsed gradient spin echo (PGSE) that is the most commonly used method in diffusion-weighted magnetic resonance imaging. However, the effects of the diffusion time on the mean diffusivity (MD) in previous studies are controversial. In this study, a new experimental protocol was designed to evaluate the effects of the two parameters on the results of PGSE. It was found that MD decreased significantly with the increased diffusion time or increased diffusion gradient strength, which cannot be explained by the previous theories. Through theoretical analysis, we found the flaw in the pulsed gradient spin echo (PGSE) method, which is the explanation for the controversial results of diffusion time obtained by oth- ers.

关键词： diffusion time diffusion gradient strength diffusion-weighted imaging diffusion tensor imaging mean diffusivity.

A stochastic method for convective storm identification,tracking and nowcasting

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Progress in Natural Science:Materials International 2008年第12期18卷 1557-1563页

作者： Lei Han~(a,b) Shengxue Fu~a Guang Yang~c Hongqing Wang~b Yongguang Zheng~d Yingjing Lin~b a College of Information Science and Engineering.Ocean university of china,Qingdao 266101,china b department of Atmospheric Science,peking university,beijing 100871,china c department of Probability and Statistics,peking university,beijing 100871,china d National Meteorological Center,beijing 100081,china [a]College of Information Science and Engineering Ocean University of China Qingdao 266101 China [b]department of Atmospheric Science Peking University Beijing 100871 China [c]department of Probability and Statistics Peking University Beijing 100871 China [d]National Meteorological Center Beijing 100081 China

The convective storm identification,tracking and nowcasting method is one of the important nowcasting methodologies against severe convective *** severe convective cases,such as storm shape or rapid velocity changes,existing methods are apt to provide unsatisfied storm identification,tracking and nowcasting *** overcome these difficulties,this paper proposes a novel approach to identify,track and short-term forecast (nowcast) of convective storms.A mathematical morphology-based storm identification method is adopted which can identify storm cells accurately in a cluster of *** for the difficult tracking problem,sequential Monte Carlo (SMC) method is utilized to simplify the tracking *** is not only inherently suitable for handling complicated splits and mergers, but also capable of handling the case of storm-missing *** order to provide more accurate forecast of a storm position,this method takes the advantages of the cross-correlation *** qualitative and quantitative evaluations show the efficiency and robustness of the proposed approach.

关键词： Convective storm Doppler radar Nowcasting

Enantioselective Addition of Diethylzinc to Benzaldehyde by Catalysts of Chiral Heterocyclic Ligands

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Chinese Chemical Letters 1999年第9期10卷 717-718页

作者： Yan HE Yue Lan LI Ye Di GUAN(department of Chemistry, peking university, beijing 100871) peking Univ Dept Chem Beijing 100871 Peoples R China

Several kinds of chiral heterocyclic ligands without hydroxyl group have been synthesized and used as catalysts for the asymmetric addition of diethylzinc to benzaldehyde. These ligands promoted the formation of (S)-1... 详细信息

关键词： chiral heterocyclic ligands hydroxyl group asymmetric catalysis enantioselectivity

Phase relationship of complex multi-component system in chromate cleaner production

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Progress in Natural Science:Materials International 2007年第7期17卷 845-850页

作者： Ge Xinlei~1 Wang Xidong~(1,3**) Zheng Shili~2 Zhang Mei~1 Zhang Yi~2 (1.department of Physical Chemistry,university of Science and Technology beijing,beijing 100083,china 2.Institute of Process Engineering,Chinese Academy of Sciences,beijing 100080,china 3.College of Engineering,peking university,beijing 100871,china) [a]department of Physical Chemistry University of Science and Technology Beijing Beijing China [b]College of Engineering Peking University Beijing China [c]Institute of Process Engineering Chinese Academy of Sciences Beijing China

The phase relationships of the complex multi-component systems in the liquid-phase oxidization and separation process related with the chromate cleaner production are investigated theoretically and *** phase diagram of main system Fe- Cr2O4-KOH-O2of oxidization process and the relationship of its standard Gibbs free energy change with temperature are calculated.A se- ries of phase diagrams of the separation process at different temperatures,such as K（Na）OH-K（Na）2CrO4-H2O,KOH-KAlO2-H2O, K（Na）OH-K（Na）2CO3-H2O,K（Na）OH-K（Na）HCO3-H2O and K（Na）HCO3-K（Na）2CrO4-H2O have also been *** ex- periments are carried out to verify the calculated *** results showed that the sub-molten salt media are suitable for the oxidization and separation processes;FeCr2O4can be oxidized across a wide range of temperature;K（Na）2CrO4can be separated from K（Na）AlO2system at appropriate temperature and proper concentration;the crystallization of carbonate is easier than that of K（Na）2CrO4.

关键词： chromite chromate system thermodynamics calculation phase diagram

Deprotection of t-Butyldimethylsiloxy (TBDMS) Protecting Group with Catalytic Copper (II) Chloride Dihydrate

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Chinese Chemical Letters 2000年第9期11卷 753-756页

作者： Zhong Ping TAN Lin WANG Jian Bo WANG department of Chemistry, peking university, beijing 100871 peking Univ Dept Chem Beijing 100871 Peoples R China

t-Butyldimethylsilyl (TBDMS) ether can be cleaved upon refluxing in acetone/H2O (95 : 5) in the presence of a catalytic amount of copper (II) chloride dihydrate (5 mmol %).

关键词： TBDMS ether deprotection copper (II) chloride dihydrate

Contributions of different land cover types in Otindag Sandy Land and Bashang area of Hebei Province to the material source of sand stormy weather in beijing

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Chinese Science Bulletin 2003年第17期48卷 1853-1856页

作者： LIU Hongyan, TIAN Yuhong & DING Deng department of Ecology, College of Environmental Sciences, peking university, beijing 100871, china department of Ecology College of Environmental Sciences Peking University Beijing 100871 China

Different land cover types in Otindag Sandy Land and Bashang area of Hebei Province are linked to a material source of sand stormy weather in beijing based on results of field vegetation and soil survey and laboratory works. Results of grain size analysis show that dust release potential in per unit area of moving sandy land is small, while lowland meadow and meadow steppe in stony hills have high potential of releasing dusts in per unit area during sand stormy weather occurrence. Further considering the effects of vegetation, it is inferred that the moving sandy land served as a material source of past dust storm and the possi-bility of dust release in per unit area is low in current time. Typical steppe in stony hills is undergoing desertification and its dust release possibility in per unit area is high. Farmland has strong potential of release dusts when they are ploughed in spring, but a large amount of therophytes grow and thus prevent dusts from release when cultivation was terminated. Potentials of dust release in per unit area in fixed sand dunes, stony mountain meadow steppe and lowland meadow are low due to high cover of perennials. Sand dune reactivation and desiccation of lakes and lowlands under estimated future climatic change will make them serve as a future material source of sand stormy weather.

关键词：河北北京沙暴土地覆盖砂地坝上地区材料源

On the evolution and histology of some Cambrian protoconodonts, paraconodonts and primitive euconodonts

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Science china Earth Sciences 2004年第7期47卷 577-584页

作者： DONG Xiping department of Geology, peking university, beijing 100871, china Laboratory of Palaeobiology & Stratigraphy, Nanjing Institute of Ge-ology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, china 1. department of Geology Peking University 100871 Beijing China 2. Laboratory of Palaeobiology & Stratigraphy Nanjing Institute of Geology and Palaeontology Chinese Academy of Sciences 210008 Nanjing China

Previous reports on the morphological evolution of protoconodonts and paraconodonts are reviewed. The evolutionary trends exhibited by species of the protoconodont genus Gapparodus and the paraconodont genus Westergaadodina are discussed. Based on the present study on histology, genus Paibiconus is protoconodont, while genus Yongshunella is paraconodont (fig. 1 (a)). However, the nature of Huayuanodontus has not been fully understood so far. Recently, the transitional form New Gen. Sensu Miller, 1980 of late Late Cambrian has been found in Hunan Province, South china. It is considered as the Sister-Group of Proconodontus. Moreover, another kind of transitional form of middle Late Cambrian and late Late Cambrian between Prooneotodus notundatus and Pnoconodontus, which looks like Prooneotodus notundatus in gross morphology, but has anterior and posterior costae, has also been found in Hunan, South china. This kind of transitional form is believed to be probably the real intermediate link between Prooneotodus rotundatus and Pnoconodontus, and Prooneotodus rotundatus is most probably the ancestor of Proconodontus. The two transitional forms noted above are all euconodonts by the study of their histology. Unlike the euconodonts of Post-Cambrian, the evolutionary relationships among the species of protoconodonts, paraconodonts and the primitive euconodonts could not be made clear without the histological study on them.

关键词： protoconodonts, paraconodonts, evolution, histology, Cambrian.

User-experience-based availability analysis model and its application in P2P storage systems

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Science china(Information Sciences) 2011年第8期54卷 1585-1595页

作者： WU Yu, YANG Zhi,QU Zhi, XIAO Zhen & DAI YaFei department of Computer Science,peking university,beijing 100871,china 1. department of Computer Science Peking University Beijing 100871 China

Data availability is one of the most important properties of peer-to-peer (P2P) storage *** analysis model and data placement are two key design *** in P2P storage system are both providers and *** characteristic determines that the availability analysis must be user-centric,and thereby enhance the quality of service and decrease the system *** popular approach in recent studies is simple random placement with steady-state model,which has the following drawbacks:1) It ignores the up/down patterns of nodes,whose availability is over-estimated or under-estimated at different periods of time.2) It ignores the access patterns of users,so the availability perceived by users is hard to evaluate precisely.3) It ignores the huge difference of nodes’ availability, thus leading to the absence of incentive. This paper proposes a novel user-experience-based availability model, which evaluates the availability of P2P storage system in terms of user experience,which can degenerate to traditional availability analysis model. Based on the new model,this paper proposes decentralized data placement algorithms for two typical P2P storage applications:"data sharing"and"personal backup".By the trace-driven simulation, we prove that our methods can enhance the availability perceived by users greatly,reduce the variance of the availability dramatically and eliminate the nodes with low availability in data-sharing applications;meanwhile,it can provide different-level service to encourage users according to their contributions.

关键词： P2P storage user experience availability data replacement

Representation of Integers by Ternary Quadratic Forms

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Acta Mathematica Sinica,English Series 2001年第4期17卷 715-720页

作者： De Lang LI Chun Lai ZHAO department of Mathematics, Sichuan university, Chengdu 610064, P. R. china department of Mathematics, peking university, beijing 100871, P. R. china 四川大学数学系四川成都北京大学数学系北京

In this paper we give a formula for the number of representations of some square-free integers by certain ternary quadratic forms and estimate the lower bound of the 2-power appearing in this number.

关键词： Congruent elliptic curve Tate-Shafarevich group Modular form Class number