The power of inequality semijoins

Semijoin is a relational operator used in many relational query processing algorithms. Semijoins can be used to “reduce” the database by delimitting portions of the database that contain data relevant to a given query. For some queries, there exist sequences of semijoins that delimit the exact portions of the database needed to answer the query. Such sequences are called full reducers.

This paper considers a class of queries called natural inequality queries (NI queries), and characterizes a subclass for which full reducers exist. We also present an efficient algorithm that decides whether an NI query lies within this subclass, and constructs a full reducer for the query. The NI queries are a subset of the aggregate-free, conjunctive queries of QUEL, and permit join clauses to include <, , =, , >.     

Computers and the 16th-century Chanson a pilot project at the University of Chicago

Conclusions One cardinal principle guided our evaluation of the data provided for us by the computer: never to be deluded into believing that the computer was in any way capable of analyzing a piece of music. Rather, we used the information made available by the computer as a means of substantiating or explaining the insights into a given composition that we gained by means of a direct confrontation with the music. In this way, the data supplied by the computer proved to be very useful in explaining why a particular portion of a chanson generates a sense of tension, while another section of the same piece creates a feeling of rest. Similarly, the electronically derived information was particularly helpful in setting up stylistic profiles of the various composers whose chansons were included in our sampling. In fact, the computer output was instrumental in helping to settle more than one problem of conflicting attributions, largely on the basis of these stylistic profiles. Like any other computerized process, our program elicited from the computer no analytical operation that could not have been done without the assistance of the computer. Very little precise measurement has been done in the analysis of the music of the Renaissance, however, and for good reason: the compilation of statistical data of the type we have described takes so much time that it can be considered feasible only with mechanical assistance. The ability we gained to apply to the chanson repertory a somewhat more precise system of analytical measurement was the most encouraging aspect of the pilot project. candidate for the Ph.D. in Physics     

The process of crack initiation and effective grain size for cleavage fracture in pearlitic eutectoid steel

The process of cleavage crack initiation and the character of the effective grain size which controls the fracture toughness of pearlitic eutectoid steel has been investigated using smooth tensile and precracked Charpy impact specimens. The results demonstrated that initial cracking in both specimens was largely the result of shear cracking of pearlite;i.e., localized slip bands in ferrite promoted cracking of the cementite plates, which was then followed by tearing of the adjacent ferrite laths. Such behavior initially results in a fibrous crack. In the tensile specimen, the initiation site was identified as a fibrous region which grew under the applied stress, eventually initiating an unstable cleavage crack. In precracked impact specimens, this critical crack size was much smaller due to the high state of stress near the precrack tip. Fracture mechanics analysis showed that the first one or two dimples formed by the shear cracking process can initiate a cleavage crack. Using thin foil transmission electron microscopy, a cleavage facet was found to be an orientation unit where the ferrites (and the cementites) of contiguous colonies share a common orientation. The size of this orientation unit, which is equal to the cleavage facet size, is controlled by the prior austenite grain size. The influence of austenite grain size on toughness is thus explained by the fact that the austenite grain structure can control the resultant orientation of ferrite and cementite in pearlitic structures. Y.J. PARK, formerly with Carnegie-Mellon University, Pittsburgh, PA.     

建筑物火灾烟气流动性状预测

述及国家自然科学基金资助项目《建筑物火灾烟气流动性状预测》的主要研究成果。介绍建筑物火灾烟流数学模型及其应用软件,并将数值模拟结果与实体火灾试验结果进行比较。     

基于Matlab的多层模糊聚类分析在学评教系统中的应用

基于Matlab平台实现了多层模糊聚类分析算法的编制。利用此算法对学评教系统进行了分类区划，为多层模糊聚类分析的应用提供了一种简便的快速运算方法。     

TinyOS任务调度机制与实时调度构件设计

TinyOS是一个开源的构件化操作系统，它采用构件化描述语言nesC进行开发，主要针对资源非常有限的无线传感器网络节点而设计。分析了TinyOS 2.x的任务调度机制，针对其在实时应用领域的调度缺陷，设计并实现了一种软实时任务调度构件。根据构件在TOSSIM仿真器中的验证分析，能有效增强TinyOS的实时性能。     

蚁群系统模型的比较及参数优化

蚁群算法是一种模拟进化算法,根据信息素更新策略的不同,蚁群系统模型分为蚁密系统、蚁量系统和蚁周系统。本文对三种模型的原理、特点进行研究,并通过仿真实验分析三种模型的性能及参数对其性能的影响,进而提出的参数优化规则,有利于蚁群算法在组合优化问题中的推广和应用。     

厚规格船板钢DH36的生产实践

厚规格高强度船板钢的生产实践表明，采取适宜的成分与控制轧制和控制冷却工艺设计，可确保高强度船板钢具有良好的综合力学性能；同时，分析了影响高强度船板钢组织、性能的因素，发现适当提高终冷设定温度不会影响细晶强化，且可改善表层过冷组织。     

基于X3D技术的虚拟现实电子商务网站

本文给出了一种结合虚拟现实技术与三维视频技术的电子商务应用的开发过程,通过应用X3D技术,可以充分结合XML技术与VRML技术的优势,更好地被各种主流开发技术所支持,并且使传统的网站用户界面进入了三维的现实应用效果。由于X3D技术是一种发展中的技术,本文使用了Delone & McLean的IS success模型对该网站系统进行了评估,并且分析了X3D技术在现有条件下的相关不足,为该技术的改进和发展提供了有价值的信息和数据。