Frameworks, Symmetry and Rigidity

Symmetry equations are obtained for the rigidity matrix of a bar-joint framework in R^d. These form the basis for a short proof of the Fowler-Guest symmetry group generalisation of the Calladine-Maxwell counting rules. Similar symmetry equations are obtained for the Jacobian of diverse framework systems, including constrained point-line systems that appear in CAD, body-pin frameworks, hybrid systems of distance constrained objects and infinite bar-joint frameworks. This leads to generalised forms of the Fowler-Guest character formula together with counting rules in terms of counts of symmetry-fixed elements. Necessary conditions for isostaticity are obtained for asymmetric frameworks, both when symmetries are present in subframeworks and when symmetries occur in partition-derived frameworks.Comment: 5 Figures. Replaces Dec. 2008 version. To appear in IJCG

Solutions for discharge chamber sputtering and anode deposit spalling in small mercury ion thrusters

Proposed solutions to the problems of sputter erosion and sputtered material spalling in the discharge chamber of small mercury ion thrusters are presented. The accelerated life test evaluated three such proposed solutions: (1) the use of tantalum as a single low sputter yield material for the exposed surfaces of the discharge chamber components subject to sputtering, (2) the use of a severely roughened anode surface to improve the adhesion of the sputter-deposited coating, and (3) the use of a wire cloth anode surface in order to limit the size of any coating flakes which might spall from it. Because of the promising results obtained in the accelerated life test with anode surfaces roughened by grit-blasting, experiments were carried out to optimize the grit-blasting procedure. The experimental results and an optimal grit-blasting procedure are presented

On Term Structure Models of Commodity Futures Prices and the Kaldor-Working Hypothesis

Both prices and the volatility of storable agricultural commodity futures contracts have been rising since 2005 and particularly since 2007. This paper aims to answer two principal questions: (i) How has the behavior of these futures prices over time and across maturities changed with the rise of biofuels and their demand-side pres- sure on corn and related crops?, and (ii) Is there now stronger or weaker evidence of the Kaldor-Working convenience yield-storage hypothesis, whereby futures price backwardation can be explained by the high value of remaining inventory stocks when these are near stockouts? The empirical application is to Chicago Board of Trade corn, wheat and soybeans futures. To make use of all available futures data rather than only the nearby, this paper adopts a recently developed affine term structure model approach and conducts estimation in state-space form using the Kalman filter. A novel aspect of the research is that it allows an arbitrary number N of state vari- ables, where more variables provide further precision and curvature but at a higher computational cost. It is found that a three-state variable model containing both ran- dom walk and mean reversion components provides the most parsimonious fit during 1988-2004, but that a simple one-state variable model is optimal for the period 2005- 2007. The main implication is that futures prices since 2005 behave much more like a \random walk" than before. Also, the model allows us to estimate the term struc- ture of volatility and it is found that distant maturity futures should be expected to be much more volatile than historically normal. Two practical but only tentative implications are: (a) hedgers should use significantly lower hedge ratios than before, and (b) for traders, the classic Black-Scholes option pricing solution should perform better now than it has historically. Lastly, the paper finds partial empirical support for the convenience yield relationship with relative inventory stocks, especially for soybeans and wheat.Agricultural Finance, C52, C53, G12, G13, Q13, Q14,

Preliminary investigation of high power microwave plasmas for electrothermal thruster use

Results are reported from preliminary tests to evaluate the high power microwave electrothermal thruster (MET) concept, which employs a free-floating plasma discharge maintained by applied CW microwave power to heat a propellant gas flow. Stable plasmas have been created and maintained in helium (He), nitrogen (N2), and hydrogen (H2) as propellants in both the TM(sub 011) and TM(sub 012) modes at discharge pressures from 10 Pa to 69 kPa. Reproducible starting conditions of pressure and power have been documented for all the plasmas. Vortical inflow of the propellant gas was observed to cause the formation of on-axis 'spike' plasmas. The formation and unformation conditions of these plasmas were studied. Operation in the spike plasma condition enables maximum power absorption with minimum wall heating and offers maximum efficiency in heating the propellant gas. In the spike condition, plasmas of the three propellant gases were investigated in an open channel configuration to a maximum applied power level of 11.2 kW (in N2). Microwave power coupling efficiencies of over 90 percent were routinely obtained at absorbed power levels up to 2 kW. Magnetic nozzle effects were investigated with a superconducting solenoid Al magnet applying a high magnetic field to the plasmas in and exiting from the discharge tube

Cotton Futures Dynamics: Structural Change, Index Traders and the Returns to Storage

The commodity bull cycle of 2006-2008 and subsequent dramatic price decline have been a source of hardship for traditional commodity market participants such as producers and merchant/shippers. The usefulness of futures markets has been called into question, especially given that some market movements did not appear to be justified by economic fundamentals. An emerging research literature examines the possible influence of futures traders, and particularly the non-traditional Index Traders, on the well-functioning of futures markets and underlying commodity markets. Cotton is a relatively under-studied commodity that is of particular importance for producers in the South and Southwest. To this end, this paper asks the following questions: (1) What role have (primarily long-only) Index Traders played, if we simultaneously account for important ongoing changes in cotton economic fundamentals? (2) Have seasonal and long-run patterns of convenience yield and price volatility changed during or since the commodity bull cycle? (3) How well do the data support a theory of storage model using the concept of convenience yield, and has the relationship changed with the commodity bull cycle? The results presented in this paper suggest that traditional, well-established economic relationships for cotton futures markets clearly have been disrupted during the period 2006- 2009. However, we find no direct evidence to support the claim that Index Traders are responsible for changes in prices or volatility.Cotton, futures markets, theory of storage, convenience yield, Index Traders, Agribusiness, Agricultural Finance, Crop Production/Industries, Demand and Price Analysis, Farm Management, Financial Economics, Marketing, Research Methods/ Statistical Methods, Risk and Uncertainty,

Predicting the Corn Basis in the Texas Triangle Area

This study develops a new and straightforward economic model of basis forecasting that outperforms the simple three-year average method suggested in much of the literature. We use monthly data of the corn basis in the Texas Triangle Area from February 1997 to July 2008. The results and the graphs indicate that the new model based on economic fundamentals performs better than basis estimates using a three-year moving average.Hedging, basis, corn, Agribusiness, Agricultural Finance, Financial Economics, Marketing, Risk and Uncertainty,

Dynamics and Asymptotics of Correlations in a Many-Body Localized System

We examine the dynamics of nearest-neighbor bipartite concurrence and total correlations in the spin-1/2 $XXZ$ model with random fields. We show, starting from factorized random initial states, that the concurrence can suffer entanglement sudden death in the long time limit and therefore may not be a useful indicator of the properties of the system. In contrast, we show that the total correlations capture the dynamics more succinctly, and further reveal a fundamental difference in the dynamics governed by the ergodic versus many-body localized phases, with the latter exhibiting dynamical oscillations. Finally, we consider an initial state composed of several singlet pairs and show that by fixing the correlation properties, while the dynamics do not reveal noticeable differences between the phases, the long-time values of the correlation measures appear to indicate the critical region.Comment: 5 pages, 5 figures. Close to published versio