On the consistency of sequentially normalized least squares

Peer reviewe

Homalg: A meta-package for homological algebra

The central notion of this work is that of a functor between categories of finitely presented modules over so-called computable rings, i.e. rings R where one can algorithmically solve inhomogeneous linear equations with coefficients in R. The paper describes a way allowing one to realize such functors, e.g. Hom, tensor product, Ext, Tor, as a mathematical object in a computer algebra system. Once this is achieved, one can compose and derive functors and even iterate this process without the need of any specific knowledge of these functors. These ideas are realized in the ring independent package homalg. It is designed to extend any computer algebra software implementing the arithmetics of a computable ring R, as soon as the latter contains algorithms to solve inhomogeneous linear equations with coefficients in R. Beside explaining how this suffices, the paper describes the nature of the extensions provided by homalg.Comment: clarified some points, added references and more interesting example

Microstrain analysis of titanium aluminides

International audienceThe aeronautic and automotive industries have shown a renewed interest in TiAl based alloys. The main reasons for such an interest are their low density (~3,8g/cm3), a good stiffness and a high strength for temperatures up to 750°C. However, these alloys exhibit, in their polycrystalline form, a poor ductility at room temperature with widely scattered values. The aim of this study is therefore to characterise their mechanical behaviour with a multiscale methodology, coupling microstructure analysis and strain field measurements. This methodology employs orientation imaging microscopy as well as digital imaging correlation techniques with an intragranular step size of a few micrometers. Two chemical compositions (47 at. % Al and 48 at. % Al) and two processing routes (casting and powder metallurgy) are studied. Thus, four different types of final microstructures are considered, from fully lamellar Ti3Al (a2) + TiAl (g) microstructure to bimodal ones composed of two-phase (a2+g) lamellar grains and monolithic g grains. Firstly, the microstructure is characterised crystallographically and morphologically. This allows the identification of a representative volume element (RVE) inside the analysed volume. Then, uniaxial mechanical tests are performed for each microstructure, and the strain fields are analysed with a multiscale approach, which determines the spatial distribution of the strain field heterogeneity with respect to the different microstructures

Understanding social-psychological determinants and effects of collaborative consumption

This doctoral thesis aims to define collaborative consumption and advance the understanding of its social-psychological determinants and effects. In order to achieve these aims, the thesis presents three studies, each of which has been accepted at scientific conferences and developed further based on feedback from experts and reviewers. Two of the studies have been published in peer-reviewed journals. The introduction provides an overview of collaborative consumption as a comparably sustainable consumption practice. Moreover, three research deficits are identified that are the motivation for the subsequent studies. First, it is shown that the basic idea and the scope of collaborative consumption remain unclear. Second, it is found that understanding of determinants is limited to isolated variables leaving relative strengths of and interdependencies between variables untapped Finally, it is assessed that actual effects of collaborative consumption on consumers mindsets are not well understood. The first study titled Prototypical collaborative consumption behaviors and their relations: A conceptual review and empirical study examines consumer behaviors that are comprised by the term collaborative consumption and the relations between these behaviors. In order to identify prototypical collaborative consumption behaviors, original definitions of collaborative consumption in the literature are reviewed. To derive hypotheses on the relationships between the prototypical behaviors, the study draws on theoretical foundations from the field of consumer lifestyles and behavioral spillover. The second study titled Understanding collaborative consumption: An extension of the theory of planned behavior with value-based personal norms aims to understand which social-psychological variables and underlying values and beliefs determine actual collaborative consumption. The theory of planned behavior is used as the primary theoretical framework, as it is a well-established model that has been shown to explain a wide range of consumer behaviors. However, reviews and meta-analyses have found the theorys ability to account for normative motives to perform a behavior is weak and have called for further theory development. As normative motives are expected to be particularly important in the context of collaborative consumption, the theory is extended with a value-based personal norm variable. The third study titled Does collaborative consumption affect consumers values, attitudes, and norms? A panel study examines the nature of causality between collaborative consumption and behavioral factors in order to determine whether collaborative consumption affects consumers values, attitudes, and norms over time. The study primarily builds on the theory of planned behavior, value theory, and the value-belief-norm theory to determine the theoretical framework linking collaborative consumption, values, attitudes, and norms over time. The theoretical framework is tested based on a two-wave panel over a time period of nine months using survey data from 168 consumers. In conclusion, the thesis contributes to the literature in six ways. First, the thesis conceptually defines collaborative consumption, a term that was used ambiguously so far. Second, it empirically advances the understanding of social-psychological determinants of collaborative consumption. Third, it explains social-psychological effects of collaborative consumption on consumers over time, something that has not been done in the literature before. Fourth, the thesis identifies and examines the relationships between five prototypical collaborative consumption behaviors. Fifth, it argues for the extension of the theory of planned behavior by a value-based personal norm variable and provides supporting empirical evidence. Finally, it advances knowledge on the causal relationship between values, attitudes, norms, and behavior.Im Rahmen dieser Dissertation wird kollaborativer Konsum definiert und das Verständnis von dessen sozial-psychologischen Determinanten und Effekten weiterentwickelt

A High Merger Fraction in the Rich Cluster MS1054-03 at z=0.83: Direct Evidence for Hierarchical Formation of Massive Galaxies

We present a morphological study of the galaxy population of the luminous X-ray cluster MS1054-03 at z=0.83. The sample consists of 81 spectroscopically confirmed cluster members in a 3 x 2 Mpc area imaged in F606W and F814W with WFPC2. We find thirteen ongoing mergers in MS1054-03, comprising 17% of the L > L* cluster population. Most of these mergers will likely evolve into luminous (\sim 2 L*) elliptical galaxies, and some may evolve into S0 galaxies. Assuming the galaxy population in MS1054-03 is typical for its redshift it is estimated that \sim 50% of present-day cluster ellipticals experienced a major merger at z < 1. The mergers are preferentially found in the outskirts of the cluster, and probably occur in small infalling clumps. Morphologies, spectra, and colors of the mergers show that their progenitors were typically E/S0s or early-type spirals with mean stellar formation redshifts z* \gtrsim 1.7. The red colors of the merger remnants are consistent with the low scatter in the color-magnitude relation in rich clusters at lower redshift. The discovery of a high fraction of mergers in this young cluster is direct evidence against formation of ellipticals in a single ``monolithic'' collapse at high redshift, and in qualitative agreement with predictions of hierarchical models for structure formation.Comment: Added GIF version of Figure 1. At http://www.astro.rug.nl/~dokkum/preprints/merger_fig1.eps.gz the PS file is available. Accepted for publication in ApJ Letter