Twisted equivariant K-theory for proper actions of discrete groups

We give a construction for twisted equivariant K-theory in the case of a proper action of a discrete group using twisted bundles. Our construction uses results of Lueck and Oliver to extend a construction of Adem and Ruan. We also show the existence of a Chern character to twisted Bredon cohomology. This gives a partial answer to the question of when you can construct twisted equivariant K-theory out of finite rank twisted bundles.Comment: 13 page

Improved Optimal and Approximate Power Graph Compression for Clearer Visualisation of Dense Graphs

Drawings of highly connected (dense) graphs can be very difficult to read. Power Graph Analysis offers an alternate way to draw a graph in which sets of nodes with common neighbours are shown grouped into modules. An edge connected to the module then implies a connection to each member of the module. Thus, the entire graph may be represented with much less clutter and without loss of detail. A recent experimental study has shown that such lossless compression of dense graphs makes it easier to follow paths. However, computing optimal power graphs is difficult. In this paper, we show that computing the optimal power-graph with only one module is NP-hard and therefore likely NP-hard in the general case. We give an ILP model for power graph computation and discuss why ILP and CP techniques are poorly suited to the problem. Instead, we are able to find optimal solutions much more quickly using a custom search method. We also show how to restrict this type of search to allow only limited back-tracking to provide a heuristic that has better speed and better results than previously known heuristics.Comment: Extended technical report accompanying the PacificVis 2013 paper of the same nam

A Re-examination of the Determinants of Australia’s Imports

The trend rise in import penetration in Australia has renewed interest in the determinants of import volumes. In this paper, an attempt is made to explain the growth in Australia’s imports in terms of the increased openness of the economy. Openness is proxied by the effective rate of protection. It is found that whilst reductions in protection do not help explain the growth in aggregate imports, they do explain a substantial share of the growth in imports of consumption and intermediate goods. This is said to reflect the impact of reductions in protection on both the demand for imports and the domestic supply of import substitutes.

Reconciling pastoral agriculture and nature conservation: developing a co-management approach in the English uplands

The article assesses the influence of the Common Agricultural Policy (CAP) on the pastoral farming systems in a National Park within the south west of England and more recent attempts to use innovative and participatory techniques to reconcile pastoral farming systems with wildlife management. The paper confirms evidence that the economic sustainability of farm businesses in the UK involved in pastoral farming is reducing, and that wildlife-orientated schemes are changing traditional farming systems in a way that might not be in the long-term interest of wildlife. The data gathered raise questions about the cost-effectiveness of the current environmental approaches, which are government-run with centrally determined prescriptions relating to the natural resource and wildlife. Drawing on the concept of co-management, an alternative approach is identified and explored with both farmers and nature conservation regulators. This approach places more emphasis upon adjusting the traditional farming system of this marginal type of farming to align with the sustainable management of a fragile ecosystem

Exchange Rate Pass-through: The Different Responses of Importers and Exporters

This paper examines exchange rate pass-through for the prices of both imports and manufactured exports. It is found that, in the long run, exchange rate pass-through over the docks is complete for both classes of good. However, in the short run, responses to currency movements differ significantly. Differences occur with respect to the speed of pass-through and its pattern over time. Pass-through to import prices is found to be more rapid than that to manufactured export prices. However, evidence is presented of a recent and substantial increase in pass-through to manufactured export prices, in keeping with increased international integration. Conversely, existing patterns of exchange rate pass-through to import prices are found to accord with historical experience. The implications of this are discussed with respect to the balance of payments and inflation.

Spatial estimation of herbaceous biomass using remote sensing in Southern African savannas

The Savanna biome covers around 60% of sub-Saharan Africa. The goods and services it provides are utilised and often depended upon by rural communities, commercial farmers and managers of conservation areas existing within it. The benefits derivable by these parties depend largely on vegetation structure and species composition which can show great variation within savannas. Fire has long been used as an effective means of manipulating savanna vegetation to maximise the provision of specific benefits, usually the provision of new herbaceous growth, and to a lesser extent to control woody cover. Information on the abundance and distribution of herbaceous biomass, which is the primary fuel source for savanna fires, has emerged as one of the most important inputs for savanna management planning. Although the most popular and reliable means of obtaining this information remains field-based sampling, estimation using remote sensing data is increasingly being incorporated into the process. Its increased popularity stems from the fact that it can greatly expand the extent of the areas for which herbaceous biomass estimations can be provided. Although there have been studies conducted on the performance of individual remote sensing based herbaceous biomass estimation methods, few have focused on the relative performance of available methods. Information on the accuracy of methods when applied in relatively densely wooded savannas, or those where a large amount of herbaceous material is retained between seasons is also limited. This presents a problem for savanna managers in South Africa where these conditions prevail. It was the aim of this study to compare the accuracy and precision of two different remote sensing based herbaceous biomass estimation techniques (the use of a regression model and cokriging) when applied under such conditions. To achieve this aim a large amount of herbaceous biomass data were required to form testing and training datasets. These were acquired from the Kruger National Park’s Veld Condition Assessment (VCA) datasets for the growth seasons between 2000 and 2006, which contains herbaceous biomass estimates based on disk pasture meter readings. It was suspected early on in the study that the VCA field data was not ideal for use as remote sensing (ground truthing) field data because of the limited size of the field plots relative to the pixels of the remotely sensed imagery used. It was decided to include an additional section of analysis to determine the possible contribution of this issue to the estimation error of the methods assessed. This involved measuring and comparing mean herbaceous biomass in co-located trial 60x60m VCA sites and trial 250x250m, The Moderate Resolution Imaging Spectroradiometer (MODIS) pixels. The main section of analysis involved (i) gathering and deriving the required variables for use in the two estimation methods assessed, (ii) producing the estimates and (iii) comparing their accuracy and precision. The first method assessed was the use of a linear regression model. Seven regression models were created in total, one for each year of the growth seasons occurring between 2000 and 2006, plus another using all of the data combined. The models included variables to account for vegetation production (based on MODIS EVI), tree cover and fire history. These variables were derived using data supplied by the CSIR and Kruger National Park Scientific Services. The second method assessed was cokriging performed with the VCA herbaceous biomass field estimates as the primary variable and the MODIS EVI data as a secondary variable. The regression models were unable to account for more than 46% of the variation in herbaceous biomass, usually accounting for between just 20 and 30% (R2 of between 0.2 and 0.3). Three potential methods were identified that could improve the model fits obtained in the future, namely: 1. Increasing the dimensions of the field sample plots 2. Improving the calibration of the disk pasture meter used to collect the field data 3. Using EVI from previous seasons in conjunction with fire scar data to account for the presence of dry material from previous seasons. Cokriging produced estimates that were on average 119 kg/ha more accurate than those of the regression models. However, the performance of cokriging was poorer than expected given the results of previous studies in the area. A possible explanation for this discrepancy is that the ArcGIS geostatistical analysis extension used in this study is limited in its capabilities. Even with the poorer than expected performance recorded in this study, the cokriged maps remain the best option for fire managers as they are the most accurate to date and require the fewest resources to produce. Neither method produced estimates with less than 1000 kg/ha of error (RMSE), the upper limit initially considered useful in this study. However this error limit could be considered unrealistic given the well documented high level of heterogeneity typical of southern African savannas

Go Vegan

La méthode de mesure, due à Vernotte, des caractéristiques thermophysiques d'une plaque d'un matériau donné par application d'un film chauffant sur une face de la plaque et enregistrement en fonction du temps de l'évolution des températures de la face opposée, et éventuellement de la face chauffée, a été souvent mise en application. On précise ici quelles conditions doivent être remplies et quelles sont les incidences de divers défauts éventuels du montage quand on applique la variante due à Krischer de cette méthode. Un dispositif expérimental très simple a permis de corroborer ces considérations théoriques

The Impulsive Nature of Lightning Initiation

We report results from imaging the initiation region of lightning via 3D interferometric beamforming on data collected by the Netherlands-based core of the Low Frequency Array of Antennas (LOFAR). LOFAR achieves 1 nanosecond timing accuracy and meter-scale spatial precision in lightning imaging on pulses observed in the 30-80 MHz band via the 38 Dutch-based stations. This project complements and enhances the previous work of the LOFAR lightning group of Groningen [Hare, B.M., et al., Nature 568, 360363 (2019)], and [Scholten, O., et al., ESSOAr 10503153] in order to improve image detail in regions with weak sources. This project incorporates beamforming techniques to improve upon previously employed methods with the result of improving both spatial and time resolution of lightning sources. In doing so, we have located and imaged the first non-impulsive sources in lightning flashes. These sources are believed to be caused by a streamer-cascade-like initiation event leading to the formation of the first leader in two separate lightning flashes. The initiation starts from essentially background and within a tens of microseconds ramps up a few orders of magnitude before the first impulsive sources connected with lightning leaders are observed. The events are likely an analog of fast breakdown in narrow bipolar events, and here we report their ramp-up rate, propagation speed, and trajectories