Stability-based validation of clustering solutions

Data clustering describes a set of frequently employed techniques in exploratory data analysis to extract "natural" group structure in data. Such groupings need to be validated to separate the signal in the data from spurious structure. In this context, finding an appropriate number of clusters is a particularly important model selection question. We introduce a measure of cluster stability to assess the validity of a cluster model. This stability measure quantifies the reproducibility of clustering solutions on a second sample, and it can be interpreted as a classification risk with regard to class labels produced by a clustering algorithm. The preferred number of clusters is determined by minimizing this classification risk as a function of the number of clusters. Convincing results are achieved on simulated as well as gene expression data sets. Comparisons to other methods demonstrate the competitive performance of our method and its suitability as a general validation tool for clustering solutions in real-world problems.     

An inverter with a quasi-synchronous dimming function

Abstract— We have developed a high-dimming-ratio inverter, which is controlled by a digital circuit. The dimming function is synchronized with a half-cycle of a Buck-Royer inverter. A dimming ratio of 100:1 was obtained by using this technique.     

In-Situ Characterization of SiC–AlN multiphase ceramics

AlN and SiC can react and form a solid solution at temperatures above 1800 °C, a result that may be beneficial for sintering silicon carbide ceramics. The pressureless sintered AlN–SiC multiphase ceramics have reached high density at a temperature of 2100 °C for 1 hr in Ar. Analytical scanning transmission electron microscopy was then used to determine the grain boundary, fracture surface, and the local compositions. Because AlN has a higher solid vaporization pressure than SiC, the vaporization rate of the AlN solid would far exceed that of SiC at a sintering temperature. The vaporizing AlN was deposited on the surface of SiC powder; SiC grains then elongated in a random arrangement. The form of elongated rod crystals of 4H SiC is 5 to 8 m in length and 1 m in width. It resulted in the sample fracture section producing pulling-out and a strong tearing-open effect. The bending strength and the fracture toughness of the material obtained are 420 MPa and 4.40 MPa × m^1/2, respectively.     

Synthesis of TiB2 powder from a mixture of TiN and amorphous boron

TiB₂ powder was synthesized by solid state reaction using amorphous boron and TiN as a source of titanium. The TiB₂ formation did not occur at all in a nitrogen atmosphere even at 1400° C. TiB₂ formed above 1100° C in argon and hydrogen atmospheres. The only crystalline phase of TiB₂ powder was favourably synthesized at 1400° C for 360 min in an argon atmosphere from a starting powder with a composition containing excess boron (B/Ti = 2.2). The synthesized powder was well dispersed and had a particle size of 0.5 to 2 µm. The powder activity was evaluated by sintering at 4 G Pa and 1300 to 1600° C for 15 min.     

Polymerization of macromers

Summary The possibility of group transfer polymerization of methyl methacrylate-terminated polystyrene macromer (MA-PSt) was explored by using 1-methoxy-1-(trimethylsiloxy)-2-methyl-1-propene and tris(dimethylamino)sulfonium difluorotrimethylsilicate as initiator and catalyst, respectively. The results obtained at the polymerization temperature of 0°C were undesirable. However, the oligomerization at -78°C gave almost quantitatively oligo(MA-PSt) of which the degree of polymerization was in good agreement with the mole ratio of macromer to initiator. The GPC curve of oligo(MA-PSt) was sharp and symmetrical. In addition, it was shown by the copolymerization with methyl methacrylate that the polymerizability of the macromer was markedly enhanced in the presence of methyl methacrylate comonomer.     

Oligoethylenes in high pressure polyethylene

Summary Series of several alkylated cycloheptanes were detected in some commercial samples of high pressure polyethylene. The structures of the alkylated cycloheptanes were found closely related to the chain transfer agents used in the polymerization process. A probable mechanism involving radical cyclization to cycloheptane structures was presented.     

Simulation of transverse crack formation on continuously cast peritectic medium carbon steel slabs

Reliable data are limited to the critical strain for the formation of transverse cracks on the slabs, owing to experimental difficulty to simulate temperature gradient in solidified shell in continuous casting mold. The present study is to determine the critical strain, ?_c, for the formation of transverse cracks on continuously cast slabs. A convenient and simple hot tensile test using rectangular test pieces with either V-notch or semi-circle notch or oscillation marks has been developed by placing the specimen under similar temperature gradient to that in solidified shell in the mold. The ?_c has been determined at a better accuracy and reproducibility, and the ?_c at a strain rate of 5?10^?4s^?1 is found to be a high 35% for test pieces without notch. It sharply decreases, however, to 10% for those with V- and semi-circle-notches, slightly decreases with increasing notch depth, and further decreases for those with oscillation marks that accompany solute segregation. Reduction of the oscillation mark depth is shown to be important measure to prevent the occurrence of transverse cracking of continuously cast slabs.     

Preliminary Flight Test Program on Telecom and Broadcasting Using High Altitude Platform Stations

Telecommunication and broadcasting systems using radio-relay/base stations on board the high altitude platforms are expected to create the largest business market among its possible applications. They could also be integrated with satellite- and terrestrial-based systems in the future to construct the more powerful mobile and/or broadband networks. Prototype onboard equipment and ground equipment are being developed on schedule in the fixed, mobile, and broadcasting services. The performance of them will be demonstrated and evaluated in the preliminary flight tests planned in 2002 using alternative aerial vehicles: a helicopter, a jet, and a solar-powered unmanned aerial vehicle (UAV), before flight tests using the high altitude airships. This paper presents activity in Japan on the R&D of wireless access systems using high altitude platform stations and the detail of the preliminary flight test program. This revised version was published online in July 2006 with corrections to the Cover Date.     

Sharing and Compatibility Study between Fixed Service Using High Altitude Platform Stations (HAPS) and Other Services in the 31/28 GHz Bands

In the World Radiocommunication Conference 2000 (WRC-2000), use of the 31 GHzand 28 GHz bands was permitted for the fixed service (FS) by using highaltitude platform stations (HAPS) in some countries. This paper examines thesharing and compatibility of the HAPS-based FS with the other services usingthe same and adjacent frequency bands and establishes the conditions of thecoexistence with those services. Feasibility of dynamic channel assignment(DCA) scheme among the different communication systems to facilitate thecoexistence is also studied.