Orbit Transfer Vehicle (OTV) engine study, phase "A" extension 2

The operation of the Advanced Expander Cycle OTV engine was evaluated at high mixture ratios, and intermediate and low thrust levels to establish the impact upon the engine design and costs, and/or operating characteristics. The engine operation was analyzed at tank head and pumped idle conditions with the nozzle extension retracted. The engine performance, weight, and envelope parametrics data were expanded to include mixture ratios of 7.5 and 8.0

Effects on herbicides on hedgerow biodiversity

Low dosages of herbicides may reduce the number of flowers on non-target plants. Effect of herbicides on flowering and on pollen and nectar production are relevant end-points for effect assessment on non-target species. Herbicides have the potential to reduce the number of flowers in hedgerow vegetation and thereby also decrease the availability of pollen, nectar and seeds that are important food for many insects and birds. The biodiversity of hedgerow bottom vegetation is lower at conventional than at organic farms, presumably as a consequence of the herbicide use

The devil is in the detail: hints for practical optimisation

Finding the minimum of an objective function, such as a least squares or negative log-likelihood function, with respect to the unknown model parameters is a problem often encountered in econometrics. Consequently, students of econometrics and applied econometricians are usually well-grounded in the broad differences between the numerical procedures employed to solve these problems. Often, however, relatively little time is given to understanding the practical subtleties of implementing these schemes when faced with illbehaved problems. This paper addresses some of the details involved in practical optimisation, such as dealing with constraints on the parameters, specifying starting values, termination criteria and analytical gradients, and illustrates some of the general ideas with several instructive examples

I give at the office: A review of workplace giving research, theory, and practice

Workplace giving is a widely used philanthropic tool. Although it may have great unmet potential, it is also facing a number of challenges, including competition from informal crowdfunding campaigns. In the face of such challenges, we take stock of the extant research to better understand the value and future of workplace giving, emphasizing employee actions and preferences in our review. Workplace giving studies can also augment knowledge about contextual giving or bounded settings for exploring basic philanthropic questions (e.g., donor control or gift elasticity). We use a three‐part conceptual framework to synthesize and discuss research on individual workplace giving in the context of broader giving behaviors. We address what researchers know, do not know, and need to know on the topic of workplace giving

Examining the Efficiency of the U.S. Courts of Appeals: Pathologies and Prescriptions

Until recently (e.g., Lindquist 2007), few studies have examined the factors that might affect aspects of judicial efficiency, including the time it takes a court to decide a case. In our analysis of a sample o f U.S. Courts of Appeals decisions from 1971-1996, we examine a variety of potential causes of inefficiency, or pathologies, before suggesting a series of prescriptions. 1 Both authors equally contributed to this manuscript. The authors would like to thank Reese Manceaux for his assistance in merging a variety of seemingly incompatible databases, as well as Nicole Arnold for her assistance in collecting data.judicial efficiency, Courts of Appeals, litigation

Transitions in non-conserving models of Self-Organized Criticality

We investigate a random--neighbours version of the two dimensional non-conserving earthquake model of Olami, Feder and Christensen [Phys. Rev. Lett. {\bf 68}, 1244 (1992)]. We show both analytically and numerically that criticality can be expected even in the presence of dissipation. As the critical level of conservation, $\alpha_c$, is approached, the cut--off of the avalanche size distribution scales as $\xi\sim(\alpha_c-\alpha)^{-3/2}$. The transition from non-SOC to SOC behaviour is controlled by the average branching ratio $\sigma$ of an avalanche, which can thus be regarded as an order parameter of the system. The relevance of the results are discussed in connection to the nearest-neighbours OFC model (in particular we analyse the relevance of synchronization in the latter).Comment: 8 pages in latex format; 5 figures available upon reques

Scaling laws in spherical shell dynamos with free-slip boundaries

Numerical simulations of convection driven rotating spherical shell dynamos have often been performed with rigid boundary conditions, as is appropriate for the metallic cores of terrestrial planets. Free-slip boundaries are more appropriate for dynamos in other astrophysical objects, such as gas-giants or stars. Using a set of 57 direct numerical simulations, we investigate the effect of free-slip boundary conditions on the scaling properties of heat flow, flow velocity and magnetic field strength and compare it with earlier results for rigid boundaries. We find that the nature of the mechanical boundary condition has only a minor influence on the scaling laws. We also find that although dipolar and multipolar dynamos exhibit approximately the same scaling exponents, there is an offset in the scaling pre-factors for velocity and magnetic field strength. We argue that the offset can be attributed to the differences in the zonal flow contribution between dipolar and multipolar dynamos.Comment: 10 pages, 9 figures, 1 table. To appear in ICARU

Groups which do not admit ghosts

A ghost in the stable module category of a group G is a map between representations of G that is invisible to Tate cohomology. We show that the only non-trivial finite p-groups whose stable module categories have no non-trivial ghosts are the cyclic groups of order 2 and 3. We compare this to the situation in the derived category of a commutative ring. We also determine for which groups G the second power of the Jacobson radical of kG is stably isomorphic to a suspension of k.Comment: 9 pages, improved exposition and fixed several typos, to appear in the Proceedings of the AM