Noncommutative curves and noncommutative surfaces

In this survey article we describe some geometric results in the theory of noncommutative rings and, more generally, in the theory of abelian categories. Roughly speaking and by analogy with the commutative situation, the category of graded modules modulo torsion over a noncommutative graded ring of quadratic, respectively cubic growth should be thought of as the noncommutative analogue of a projective curve, respectively surface. This intuition has lead to a remarkable number of nontrivial insights and results in noncommutative algebra. Indeed, the problem of classifying noncommutative curves (and noncommutative graded rings of quadratic growth) can be regarded as settled. Despite the fact that no classification of noncommutative surfaces is in sight, a rich body of nontrivial examples and techniques, including blowing up and down, has been developed.Comment: Suggestions by many people (in particular Haynes Miller and Dennis Keeler) have been incorporated. The formulation of some results has been improve

Acceptance checkout equipment - Spacecraft Monthly progress report, 15 Jan. - 15 Feb. 1966

Acceptance checkout equipment and spacecraft testin

Application of Single-Station Sigma and Site-Response Characterization in a Probabilistic Seismic-Hazard Analysis for a New Nuclear Site

Aleatory variability in ground-motion prediction, represented by the standard deviation (sigma) of a ground-motion prediction equation, exerts a very strong influence on the results of probabilistic seismic-hazard analysis (PSHA). This is especially so at the low annual exceedance frequencies considered for nuclear facilities; in these cases, even small reductions in sigma can have a marked effect on the hazard estimates. Proper separation and quantification of aleatory variability and epistemic uncertainty can lead to defensible reductions in sigma. One such approach is the single-station sigma concept, which removes that part of sigma corresponding to repeatable site-specific effects. However, the site-to-site component must then be constrained by site-specific measurements or else modeled as epistemic uncertainty and incorporated into the modeling of site effects. The practical application of the single-station sigma concept, including the characterization of the dynamic properties of the site and the incorporation of site-response effects into the hazard calculations, is illustrated for a PSHA conducted at a rock site under consideration for the potential construction of a nuclear power plant.Civil, Architectural, and Environmental Engineerin

Building health: an epidemiological study of "sick building syndrome" in the Whitehall II study

Objectives: Sick building syndrome (SBS) is described as a group of symptoms attributed to the physical environment of specific buildings. Isolating particular environmental features responsible for the symptoms has proved difficult. This study explores the role and significance of the physical and psychosocial work environment in explaining SBS. Methods: Cross sectional data on the physical environment of a selection of buildings were added to individual data from the Whitehall II study—an ongoing health survey of office based civil servants. A self-report questionnaire was used to capture 10 symptoms of the SBS and psychosocial work stress. In total, 4052 participants aged 42–62 years working in 44 buildings were included in this study. Results: No significant relation was found between most aspects of the physical work environment and symptom prevalence, adjusted for age, sex, and employment grade. Positive (non-significant) relations were found only with airborne bacteria, inhalable dust, dry bulb temperature, relative humidity, and having some control over the local physical environment. Greater effects were found with features of the psychosocial work environment including high job demands and low support. Only psychosocial work characteristics and control over the physical environment were independently associated with symptoms in the multivariate analysis. Conclusions: The physical environment of office buildings appears to be less important than features of the psychosocial work environment in explaining differences in the prevalence of symptoms

Stability of Metal Nanowires at Ultrahigh Current Densities

We develop a generalized grand canonical potential for the ballistic nonequilibrium electron distribution in a metal nanowire with a finite applied bias voltage. Coulomb interactions are treated in the self-consistent Hartree approximation, in order to ensure gauge invariance. Using this formalism, we investigate the stability and cohesive properties of metallic nanocylinders at ultrahigh current densities. A linear stability analysis shows that metal nanowires with certain {\em magic conductance values} can support current densities up to 10^11 A/cm^2, which would vaporize a macroscopic piece of metal. This finding is consistent with experimental studies of gold nanowires. Interestingly, our analysis also reveals the existence of reentrant stability zones--geometries that are stable only under an applied bias.Comment: 12 pages, 6 figures, version published in PR