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Mining association rules in relational databases is a significant computational task with lots of applications. A fundamental ingredient of this task is the discovery of sets of attributes (itemsets) whose frequency in the data exceeds some threshold value. In this paper we describe two algorithms for completing the calculation of frequent sets using a tree structure for storing partial supports, called interim‐support (IS) tree. The first of our algorithms (T‐Tree‐First (TTF)) uses a novel tree pruning technique, based on the notion of (fixed‐prefix) potential inclusion, which is specially designed for trees that are implemented using only two pointers per node. This allows to implement the IS tree in a space‐efficient manner. The second algorithm (P‐Tree‐First (PTF)) explores the idea of storing the frequent itemsets in a second tree structure, called the total support tree (T‐tree); the main innovation lies in the use of multiple pointers per node, which provides rapid access to the nodes of the T‐tree and makes it possible to design a new, usually faster, method for updating them. Experimental comparison shows that these techniques result in considerable speedup for both algorithms compared with earlier approaches that also use IS trees (Principles of Data Mining and Knowledge Discovery, Proceedings of the 5th European Conference, PKDD, 2001, Freiburg, September 2001 (Lecture Notes in Artificial Intelligence, vol. 2168). Springer: Berlin, Heidelberg, 54–66; Journal of Knowledge‐Based Syst. 2000; 13 :141–149). Further comparison between the two new algorithms, shows that the PTF is generally faster on instances with a large number of frequent itemsets, provided that they are relatively short, whereas TTF is more appropriate whenever there exist few or quite long frequent itemsets; in addition, TTF behaves well on instances in which the densities of the items of the database have a high variance. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
43.
近年来,基于簇的水声传感器网络得到广泛应用。由于水声传感器多用于军事行业,使得提高水声传感器网络的安全性变得十分重要。针对提高水声传感器网络安全性这一问题,提出了结合ECC Diffie-Hellman密钥交换、一元高阶多项式、双线性映射和哈希函数的安全认证协议。经过分析,该协议满足了认证、访问控制、数据保密性、数据完整性和不可否认性等安全需求。结合BAN逻辑进行形式化分析,证明该协议是安全的;同时,将协议的两个阶段分别与同类型认证协议进行对比,实验结果表明,所提协议的性能在时间或者空间上有所提升。 相似文献
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Mg-6%Al-5%Pb (mass fraction) anodes with different contents of zinc were prepared by melting and casting. The electrochemical
discharge behavior of these anodes in 3.5% NaCl solutions was investigated by galvanostatic test and electrochemical impedance
spectroscopy (EIS). The microstructures and the corroded surfaces of these anodes were studied by scanning electron microscopy
(SEM) and emission spectrum analysis (ESA). The phase structures and the corrosion products of the anodes were analyzed by
X-ray diffraction (XRD). The results show that zinc promotes the grain refinement of Mg-6%Al-5%Pb anode and makes the average
discharge potential of Mg-6%Al-5%Pb anode more negative during galvanostatic test. Mg-6%Al-5%Pb anode with the addition of
1% (mass fraction) zinc has the best electrochemical performance. The activation mechanism of zinc to Mg-6%Al-5%Pb anode is
as follows: The hydrolyzation of dissolved Zn2+ ions reduces the pH value of the solution near the surface of the anode and accelerates the dissolution of Mg(OH)2 film; The precipitated Zn(OH)2 with similar structure as Mg(OH)2 combines with Mg(OH)2 film easily and makes it break down. 相似文献
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Wenjian Leng Chuanren Yang Hong Ji Jihua Zhang Jinlong Tang Hongwei Chen 《Journal of Materials Science: Materials in Electronics》2006,17(12):1041-1045
Ferroelectric (Pb0.92La0.08)(Zr0.65Ti0.35)O3 (PLZT) films have been prepared on Pt/Ti/SiO2/Si and fused quartz substrates using radio-frequency (rf) magnetron sputtering at a deposition temperature of 650°C. X-ray diffraction analysis shows that the PLZT thin films on platinized silicon are polycrystalline with (100)-preferential orientation. A Al/PLZT/Pt capacitor has been fabricated and it shows that the films have excellent ferroelectric character, with saturation polarization (P
s), remanent polarization (P
r) and coercive field (E
c) of 32.8μC/cm2, 24.3μC/cm2 and 142 kV/cm, respectively. The PLZT thin films exhibit good insulating property and the leakage current density of the films on platinized silicon is only about 0.86 × 10−7 A/cm2 at 200 kV/cm. By the optical transmission spectra measurements, the energy gap (E
g) of the PLZT films on fused quartz is found to be about 3.54 eV. The optical constants (n and k) of the films in the wavelength range of 250–900 nm are obtained by a Filmetrics F20 reflectance spectrometer. 相似文献