A latent class vector model for preference ratings

A latent class formulation of the well-known vector model for preference data is presented. Assuming preference ratings as input data, the model simultaneously clusters the subjects into a small number of homogeneous groups (or latent classes) and constructs a joint geometric representation of the choice objects and the latent classes according to a vector model. The distributional assumptions on which the latent class approach is based are analogous to the distributional assumptions that are consistent with the common practice of fitting the vector model to preference data by least squares methods. An EM algorithm for fitting the latent class vector model is described as well as a procedure for selecting the appropriate number of classes and the appropriate number of dimensions. Some illustrative applications of the latent class vector model are presented and some possible extensions are discussed. Geert De Soete is supported as “Bevoegdverklaard Navorser” of the Belgian “Nationaal Fonds voor Wetenschappelijk Onderzoek.”     

A modified CANDECOMP method for fitting the extended INDSCAL model

A modified CANDECOMP algorithm is presented for fitting the metric version of the Extended INDSCAL model to three-way proximity data. The Extended INDSCAL model assumes, in addition to the common dimensions, a unique dimension for each object. The modified CANDECOMP algorithm fits the Extended INDSCAL model in a dimension-wise fashion and ensures that the subject weights for the common and the unique dimensions are nonnegative. A Monte Carlo study is reported to illustrate that the method is fairly insensitive to the choice of the initial parameter estimates. A second Monte Carlo study shows that the method is able to recover an underlying Extended INDSCAL structure if present in the data. Finally, the method is applied for illustrative purposes to some empirical data on pain relievers. In the final section, some other possible uses of the new method are discussed. Geert De Soete is supported as “Bevoegdverklaard Navorser” of the Belgian “Nationaal Fonds voor Wetenschappelijik Onderzoek”.     

柔性加工输送系统总体优化

着重讨论柔性加工输送系统中的货台布置问题．通过优化，选择合理的货台位置，可以在不追加投资的情况下缩短货箱输送时间，提高效率，减少输送车的机械损耗，提高经济效益，     

辅酶B12模型化合物的合成及性质研究——纯电子效应的轴向苄基类水溶性辅酶B12模型化合物

以电化学和光谱电化学方法探讨了合成辅酶B12模型化合物的条件。在此基础上用化学方法合成了系列轴向基团具有纯电子效应的辅酶B12模型化合物RPhCH_2-CoTMAPI(R=CH_(3-),NO_(2-),H-,Br-,KCO_(2-),COTMAP I为碘化四[N,N,N-三甲胺基]苯基钴卟啉)。以红外,紫外可见光谱对它们进行了表征。用量子化学方法讨论电子效应对Co-C键的影响。     

Single-copy T-DNAs integrated at different positions in the Arabidopsis genome display uniform and comparable β-glucuronidase accumulation levels

This study aimed at determining whether transgene expression variability is observed in single-copy T-DNA plants and whether it can be correlated with the T-DNA integration position. Among a population of 135 Arabidopsis thaliana transformants, selected on the basis of antibiotic resistance marker expression, 21 single-copy T-DNA transformants were identified and characterized. In 19 of these 21 lines, 35S-beta-glucuronidase transgene expression, measured in two subsequent generations, was similar. This observation means that the intra-transformant variability was as high as the inter-transformant variability. Integration into an intergenic or genic region, into an exon or intron, in sense or antisense orientation, did not result in differential transgene expression. Remarkably, single-copy transformants were not always the highest expressers, implying that low transgene expression is not always induced by multicopy transformants. In only 2 of the 21 single-copy plants was the transgene expression more than 20-fold lower. However, characteristics of the insertion position in one of these lines did not differ significantly when compared to high-expressing lines. In the remaining line, methylation of the transgene was clearly demonstrated. In conclusion, screening for single-copy T-DNA transformants greatly enriches for stable and high transgene expression, because the integration position is not a major determinant of transgene expression variability in Arabidopsis.     

Computational approaches to structural and functional analysis of plastocyanin and other blue copper proteins

Computational techniques are becoming increasingly important in structural and functional biology, in particular as tools to aid the interpretation of experimental results and the design of new systems. This review reports on recent studies employing a variety of computational approaches to unravel the microscopic details of the structure-function relationships in plastocyanin and other proteins belonging to the blue copper superfamily. Aspects covered include protein recognition, electron transfer and protein-solvent interaction properties of the blue copper protein family. The relevance of integrating diverse computational approaches to address the analysis of a complex protein system, such as a cupredoxin metalloprotein, is emphasized.Received 9 May 2003; received after revision 24 November 2003; accepted 28 November 2003     

Routing of anisotropic spatial solitons and modulational instability in liquid crystals

In certain materials, the spontaneous spreading of a laser beam (owing to diffraction) can be compensated for by the interplay of optical intensity and material nonlinearity. The resulting non-diffracting beams are called 'spatial solitons' (refs 1-3), and they have been observed in various bulk media. In nematic liquid crystals, solitons can be produced at milliwatt power levels and have been investigated for both practical applications and as a means of exploring fundamental aspects of light interactions with soft matter. Spatial solitons effectively operate as waveguides, and so can be considered as a means of channelling optical information along the self-sustaining filament. But actual steering of these solitons within the medium has proved more problematic, being limited to tilts of just a fraction of a degree. Here we report the results of an experimental and theoretical investigation of voltage-controlled 'walk-off' and steering of self-localized light in nematic liquid crystals. We find not only that the propagation direction of individual spatial solitons can be tuned by several degrees, but also that an array of direction-tunable solitons can be generated by modulation instability. Such control capabilities might find application in reconfigurable optical interconnects, optical tweezers and optical surgical techniques.     

A large population of 'Lyman-break' galaxies in a protocluster at redshift z approximately 4.1

The most massive galaxies and the richest clusters are believed to have emerged from regions with the largest enhancements of mass density relative to the surrounding space. Distant radio galaxies may pinpoint the locations of the ancestors of rich clusters, because they are massive systems associated with 'overdensities' of galaxies that are bright in the Lyman-alpha line of hydrogen. A powerful technique for detecting high-redshift galaxies is to search for the characteristic 'Lyman break' feature in the galaxy colour, at wavelengths just shortwards of Lyalpha, which is due to absorption of radiation from the galaxy by the intervening intergalactic medium. Here we report multicolour imaging of the most distant candidate protocluster, TN J1338-1942 at a redshift z approximately 4.1. We find a large number of objects with the characteristic colours of galaxies at that redshift, and we show that this excess is concentrated around the targeted dominant radio galaxy. Our data therefore indicate that TN J1338-1942 is indeed the most distant cluster progenitor of a rich local cluster, and that galaxy clusters began forming when the Universe was only ten per cent of its present age.     

A precision measurement of the mass of the top quark

The standard model of particle physics contains parameters--such as particle masses--whose origins are still unknown and which cannot be predicted, but whose values are constrained through their interactions. In particular, the masses of the top quark (M(t)) and W boson (M(W)) constrain the mass of the long-hypothesized, but thus far not observed, Higgs boson. A precise measurement of M(t) can therefore indicate where to look for the Higgs, and indeed whether the hypothesis of a standard model Higgs is consistent with experimental data. As top quarks are produced in pairs and decay in only about 10(-24) s into various final states, reconstructing their masses from their decay products is very challenging. Here we report a technique that extracts more information from each top-quark event and yields a greatly improved precision (of +/- 5.3 GeV/c2) when compared to previous measurements. When our new result is combined with our published measurement in a complementary decay mode and with the only other measurements available, the new world average for M(t) becomes 178.0 +/- 4.3 GeV/c2. As a result, the most likely Higgs mass increases from the experimentally excluded value of 96 to 117 GeV/c2, which is beyond current experimental sensitivity. The upper limit on the Higgs mass at the 95% confidence level is raised from 219 to 251 GeV/c2.     

A high-intensity highly coherent soft X-ray femtosecond laser seeded by a high harmonic beam

Synchrotrons have for decades provided invaluable sources of soft X-rays, the application of which has led to significant progress in many areas of science and technology. But future applications of soft X-rays--in structural biology, for example--anticipate the need for pulses with much shorter duration (femtoseconds) and much higher energy (millijoules) than those delivered by synchrotrons. Soft X-ray free-electron lasers should fulfil these requirements but will be limited in number; the pressure on beamtime is therefore likely to be considerable. Laser-driven soft X-ray sources offer a comparatively inexpensive and widely available alternative, but have encountered practical bottlenecks in the quest for high intensities. Here we establish and characterize a soft X-ray laser chain that shows how these bottlenecks can in principle be overcome. By combining the high optical quality available from high-harmonic laser sources (as a seed beam) with a highly energetic soft X-ray laser plasma amplifier, we produce a tabletop soft X-ray femtosecond laser operating at 10 Hz and exhibiting full saturation, high energy, high coherence and full polarization. This technique should be readily applicable on all existing laser-driven soft X-ray facilities.