A recyclable chiral Ru catalyst for enantioselective olefin metathesis. Efficient catalytic asymmetric ring-opening/cross metathesis in air

The synthesis and structure of a new chiral bidentate imidazolinylidene ligand and a derived chiral Ru-based carbene are disclosed. The Ru complex is stereogenic at the metal center; it can be prepared in >98% diastereoselectivity and purified by silica gel chromatography with undistilled solvents. The air-stable Ru complex efficiently catalyzes ring-closing and ring-opening metathesis and is recyclable. The chiral complex is highly effective (0.5-10 mol % loading) in promoting enantioselective ring-opening/cross metathesis reactions (up to >98% ee). These enantioselective transformations can be effected in air, with unpurified solvent and with substrates that would only polymerize with Mo-based catalysts.     

Mesoscopic spin-dependent reflection experiments on InSb- and InAs-based heterostructures

Spin–orbit interaction in two-dimensional electron systems can lead to a spin-dependent reflection of carriers off a lithographic barrier. Scattering of a spin-unpolarized beam from the barrier leads to the creation of two fully spin-polarized side beams in addition to an unpolarized specularly reflected beam. We experimentally demonstrate a method to create spin-polarized beams of ballistic electrons in mesoscopic samples fabricated on InSb/InAlSb and InAs/AlGaSb heterostructures. We describe two geometries, one open and one closed, in which the spin-dependent reflection and spin-dependent semiclassical trajectories were observed.     

What is the complexity of a distributed computing system?

Distributed computing systems are becoming bigger and more complex. Although the complexity of large‐scale distributed systems has been acknowledged to be an important challenge, there has not been much work in defining or measuring system complexity. Thus, today, it is difficult to compare the complexities of different systems, or to state that one system is easier to program, to manage, or to use than another. In this article, we try to understand the factors that cause computing systems to appear very complex to people. We define different aspects of system complexity and propose metrics for measuring these aspects. We also show how these aspects affect different kinds of people—viz. developers, administrators, and end‐users. On the basis of the aspects and metrics of complexity that we identify, we propose general guidelines that can help reduce the complexity of systems. © 2007 Wiley Periodicals, Inc. Complexity 12: 37–45, 2007     

Aligning simulation models: A case study and results

This paper develops the concepts and methods of a process we will call “alignment of computational models” or “docking” for short. Alignment is needed to determine whether two models can produce the same results, which in turn is the basis for critical experiments and for tests of whether one model can subsume another. We illustrate our concepts and methods using as a target a model of cultural transmission built by Axelrod. For comparison we use the Sugarscape model developed by Epstein and Axtell. The two models differ in many ways and, to date, have been employed with quite different aims. The Axelrod model has been used principally for intensive experimentation with parameter variation, and includes only one mechanism. In contrast, the Sugarscape model has been used primarily to generate rich “artificial histories”, scenarios that display stylized facts of interest, such as cultural differentiation driven by many different mechansims including resource availability, migration, trade, and combat. The Sugarscape model was modified so as to reproduce the results of the Axelrod cultural model. Among the questions we address are: what does it mean for two models to be equivalent, how can different standards of equivalence be statistically evaluated, and how do subtle differences in model design affect the results? After attaining a “docking” of the two models, the richer set of mechanisms of the Sugarscape model is used to provide two experiments in sensitivity analysis for the cultural rule of Axelrod's model. Our generally positive experience in this enterprise has suggested that it could be beneficial if alignment and equivalence testing were more widely practiced among computational modelers.     

Derivation of O(3) Electrodynamics from the Einstein–Sachs Theory of General Relativity

General relativity is reduced to O(3) electrodynamics by consideration of the irreducible representations of the Einstein group and through a particular choice of basis. The photon is shown always to possess a scalar curvature R, and so the origin of quantization is found in general relativity.     

The Role of Discrete Event Simulation Techniques in Finite Capacity Scheduling

Discrete event simulation is becoming an established tool in finite capacity scheduling. However, recent work on scheduling systems has concentrated on the development of different modelling approaches and tools, without proper examination of how they can be used to meet business objectives. This paper examines the role of scheduling in improving business performance and suggests possible considerations in the development and implementation of a finite capacity scheduler. It reviews the advantages and limitations of discrete event simulation as a modelling technique for scheduling, and argues the need for further research to develop more versatile tools. A simulation-based scheduling system incorporating bottleneck planning principles to maintain throughput with low inventory is described, as are the experiences gained during its implementation in a medium-sized press shop.     

Investigation of top mass measurements with the ATLAS detector at LHC

The European Physical Journal C - Several methods for the determination of the mass of the top quark with the ATLAS detector at the LHC are presented. All dominant decay channels of the top quark...