In search of patterns of land-use in Mexico City using logistic regression at the plot level

The study of big cities’ tendency to decentralisation is in the current agenda to understand the structure of Latin American cities. In general, centres and subcentres are related to specific functions. According to the theories of the movement economy and centrality as a process, the urban grid shapes land use distribution through movement and therefore is the main determinant of the location of ‘live centres’, a key component of centres. Activities related to ‘live centres’ include retail, catering and other movement dependent uses. However, the distribution of this kind of activity in cities like Mexico is not as spatially clear as it is in organically grown cities. In this paper we show that, nonetheless complex, there is a relationship between the location of ‘live centre’ uses and spatial configuration. We use multiple logistic regression to evaluate exactly how much influence each variable has on the outcome ‘shop’ given the presence of all the others. The results also suggest different spatial influences for different types of retail on different scales of centres

The atomistic structure and energy of nascent dislocation loops

An harmonic lattice theory is used, in conjunction with Mura's theory of eigendistorsions, to study the structure and energetics of nascent dislocation loops in face-centred-cubic (FCC) crystals. An analytical expression for the activation energies of such loops is derived. The results obtained herein indicate that thermal activation of small dislocation loops is possible at high stress levels such as those found in the vicinity of a crack tip. The implications of these results in understanding phenomena such as the brittle-ductile transition are discussed

Dislocation subgrain structures and modeling the plastic hardening of metallic single crystals

A single crystal plasticity theory for insertion into finite element simulation is formulated using sequential laminates to model subgrain dislocation structures. It is known that local models do not adequately account for latent hardening, as latent hardening is not only a material property, but a nonlocal property (e.g. grain size and shape). The addition of the nonlocal energy from the formation of subgrain structure dislocation walls and the boundary layer misfits provide both latent and self-hardening of a crystal slip. Latent hardening occurs as the formation of new dislocation walls limits motion of new mobile dislocations, thus hardening future slip systems. Self-hardening is accomplished by an evolution of the subgrain structure length scale. The substructure length scale is computed by minimizing the nonlocal energy. The minimization of the nonlocal energy is a competition between the dislocation wall energy and the boundary layer energies. The nonlocal terms are also directly minimized within the subgrain model as they affect deformation response. The geometrical relationship between the dislocation walls and slip planes affecting the dislocation mean free path is taken into account, giving a first-order approximation to shape effects. A coplanar slip model is developed due to requirements while modeling the subgrain structure. This subgrain structure plasticity model is noteworthy as all material parameters are experimentally determined rather than fit. The model also has an inherit path dependence due to the formation of the subgrain structures. Validation is accomplished by comparison with single crystal tension test results

Quasicontinuum Models of Interfacial Structure and Deformation

Microscopic models of the interaction between grain boundaries (GBs) and both dislocations and cracks are of importance in understanding the role of microstructure in altering the mechanical properties of a material. A recently developed mixed atomistic and continuum method is extended to examine the interaction between GBs, dislocations and cracks. These calculations elucidate plausible microscopic mechanisms for these defect interactions and allow for the quantitative evaluation of critical parameters such as the stress to nucleate a dislocation at a step on a GB and the force needed to induce GB migration.Comment: RevTex, 4 pages, 4 figure

Legal Ontologies for the spanish e-Government

The Electronic Government is a new field of applications for the semantic web where ontologies are becoming an important research technology. The e-Government faces considerable challenges to achieve interoperability given the semantic differences of interpretation, complexity and width of scope. In this paper we present the results obtained in an ongoing project commissioned by the Spanish government that seeks strategies for the e-Government to reduce the problems encountered when delivering services to citizens. We also introduce an e-Government ontology model; within this model a set of legal ontologies are devoted to representing the Real-estate transaction domain used to illustrate this paper

Quasicontinuum simulation of fracture at the atomic scale

We study the problem of atomic scale fracture using the recently developed quasicontinuum method in which there is a systematic thinning of the atomic-level degrees of freedom in regions where they are not needed. Fracture is considered in two distinct settings. First, a study is made of cracks in single crystals, and second, we consider a crack advancing towards a grain boundary (GB) in its path. In the investigation of single crystal fracture, we evaluate the competition between simple cleavage and crack-tip dislocation emission. In addition, we examine the ability of analytic models to correctly predict fracture behaviour, and find that the existing analytical treatments are too restrictive in their treatment of nonlinearity near the crack tip. In the study of GB-crack interactions, we have found a number of interesting deformation mechanisms which attend the advance of the crack. These include the migration of the GB, the emission of dislocations from the GB, and deflection of the crack front along the GB itself. In each case, these mechanisms are rationalized on the basis of continuum mechanics arguments

On the acquisition and representation of procedural knowledge

Historically knowledge acquisition has proven to be one of the greatest barriers to the development of intelligent systems. Current practice generally requires lengthy interactions between the expert whose knowledge is to be captured and the knowledge engineer whose responsibility is to acquire and represent knowledge in a useful form. Although much research has been devoted to the development of methodologies and computer software to aid in the capture and representation of some of some types of knowledge, little attention has been devoted to procedural knowledge. NASA personnel frequently perform tasks that are primarily procedural in nature. Previous work is reviewed in the field of knowledge acquisition and then focus on knowledge acquisition for procedural tasks with special attention devoted to the Navy's VISTA tool. The design and development is described of a system for the acquisition and representation of procedural knowledge-TARGET (Task Analysis and Rule Generation Tool). TARGET is intended as a tool that permits experts to visually describe procedural tasks and as a common medium for knowledge refinement by the expert and knowledge engineer. The system is designed to represent the acquired knowledge in the form of production rules. Systems such as TARGET have the potential to profoundly reduce the time, difficulties, and costs of developing knowledge-based systems for the performance of procedural tasks

Correlation energy, pair-distribution functions and static structure factors of jellium

We discuss and clarify a simple and accurate interpolation scheme for the spin-resolved electron static structure factor (and corresponding pair correlation function) of the 3D unpolarized homogeneous electron gas which, along with some analytic properties of the spin-resolved pair-correlation functions, we have just published. We compare our results with the very recent spin-resolved scheme by Schmidt et al., and focus our attention on the spin-resolved correlation energies and the high-density limit of the correlation functions.Comment: 8 pages, 3 figures. Proceedings of the conference on Statistical Mechanics and Strongly Correlated Systems (Bachelet, Parisi & Vulpiani Eds.) to appear as a special issue of Physica A (Elsevier, Amsterdam 2000