Exploring multiple viewshed analysis using terrain features and optimisation techniques

The calculation of viewsheds is a routine operation in geographic information systems and is used in a wide range of applications. Many of these involve the siting of features, such as radio masts, which are part of a network and yet the selection of sites is normally done separately for each feature. The selection of a series of locations which collectively maximise the visual coverage of an area is a combinatorial problem and as such cannot be directly solved except for trivial cases. In this paper, two strategies for tackling this problem are explored. The first is to restrict the search to key topographic points in the landscape such as peaks, pits and passes. The second is to use heuristics which have been applied to other maximal coverage spatial problems such as location-allocation. The results show that the use of these two strategies results in a reduction of the computing time necessary by two orders of magnitude, but at the cost of a loss of 10% in the area viewed. Three different heuristics were used, of which Simulated Annealing produced the best results. However the improvement over a much simpler fast-descent swap heuristic was very slight, but at the cost of greatly increased running times. © 2004 Elsevier Ltd. All rights reserved

Association of Cystic Medial Necrosis of the Aorta and Undiagnosed Thyroiditis [Scripta Medica]

We have recently seen two patients with cystic medial necrosis of the aorta. The first patient died of a dissecting aneurysm of the thoracic aorta. At autopsy, classical Hashimoto’s thyroiditis was discovered. The second patient died of a rupture of the ascending aorta. At autopsy, chronic thyroiditis was seen with multiple large germinal center and diffuse fibrosis. Neither patient was clinically suspected of thyroid dysfunction although the second patient had had a partial thyroidectomy in the remote past

The geometric role of symmetry breaking in gravity

In gravity, breaking symmetry from a group G to a group H plays the role of describing geometry in relation to the geometry the homogeneous space G/H. The deep reason for this is Cartan's "method of equivalence," giving, in particular, an exact correspondence between metrics and Cartan connections. I argue that broken symmetry is thus implicit in any gravity theory, for purely geometric reasons. As an application, I explain how this kind of thinking gives a new approach to Hamiltonian gravity in which an observer field spontaneously breaks Lorentz symmetry and gives a Cartan connection on space.Comment: 4 pages. Contribution written for proceedings of the conference "Loops 11" (Madrid, May 2011

Online Gaming Can Make a Better World: Jane McGonigal

With personal feelings put aside and sociological theoretical depictions brought to the forefront, it is interesting to compare some of Jane\u27s ideas with that of both Emile Durkheim and Max Weber. The theorist who stood out right away, being exemplified through Jane\u27s positive attitude claims on a much larger, macro-level scale, was Emile Durkheim. Jane\u27s ideas about transcending human\u27s as a resource through the social fabrics of gaming into something that might solve world hunger, poverty, and global warming was nothing short of functionalism at it\u27s best. Jane\u27s platform for social structure and maintaining positive social order is the online world, and online gaming is the vehicle for change

Social Security and Retirement around the World: Historical Trends in Mortality and Health, Employment, and Disability Insurance Participation and Reforms - Introduction and Summary

This is the introduction and summary to the fifth phase of an ongoing project on Social Security Programs and Retirement Around the World. The first phase described the retirement incentives inherent in plan provisions and documented the strong relationship across countries between social security incentives to retire and the proportion of older persons out of the labor force. The second phase documented the large effects that changing plan provisions would have on the labor force participation of older workers. The third phase demonstrated the consequent fiscal implications that extending labor force participation would have on net program costs—reducing government social security benefit payments and increasing government tax revenues. The fourth phase presented analyses of the relationship between the labor force participation of older persons and the labor force participation of younger persons in twelve countries. We found no evidence that increasing the employment of older persons will reduce the employment opportunities of youth and no evidence that increasing the employment of older persons will increase the unemployment of youth. This phase is intended to set the stage for and inform future more formal analysis of disability insurance programs, with this key question: Given health status, to what extent are the differences in LFP across countries determined by the provisions of disability insurance programs? Here we first consider changes in mortality over time and in particular the relationship between mortality and labor force participation, thinking of mortality as one indicator of health that is comparable across countries and over time in the same country. We then consider how mortality is related to other indicators of health status, in particular self-assessed health and then how trends in DI participation are related to changes in health. Finally we consider the effect on disability insurance participation of “natural experiments” in which the disability insurance reforms were not prompted by changes in health status or by changes in the employment circumstances of older workers. We find that these “exogenous” reforms can have a very large effect on the labor force participation of older workers.

The Immirzi Parameter as an Instanton Angle

The Barbero-Immirzi parameter is a one parameter quantization ambiguity underpinning the loop approach to quantum gravity that bears tantalizing similarities to the theta parameter of gauge theories such as Yang-Mills and QCD. Despite the apparent semblance, the Barbero-Immirzi field has resisted a direct topological interpretation along the same lines as the theta-parameter. Here we offer such an interpretation. Our approach begins from the perspective of Einstein-Cartan gravity as the symmetry broken phase of a de Sitter gauge theory. From this angle, just as in ordinary gauge theories, a theta-term emerges from the requirement that the vacuum is stable against quantum mechanical tunneling. The Immirzi parameter is then identified as a combination of Newton's constant, the cosmological constant, and the theta-parameter.Comment: 24 page

Exotic Statistics for Strings in 4d BF Theory

After a review of exotic statistics for point particles in 3d BF theory, and especially 3d quantum gravity, we show that string-like defects in 4d BF theory obey exotic statistics governed by the 'loop braid group'. This group has a set of generators that switch two strings just as one would normally switch point particles, but also a set of generators that switch two strings by passing one through the other. The first set generates a copy of the symmetric group, while the second generates a copy of the braid group. Thanks to recent work of Xiao-Song Lin, we can give a presentation of the whole loop braid group, which turns out to be isomorphic to the 'braid permutation group' of Fenn, Rimanyi and Rourke. In the context 4d BF theory this group naturally acts on the moduli space of flat G-bundles on the complement of a collection of unlinked unknotted circles in R^3. When G is unimodular, this gives a unitary representation of the loop braid group. We also discuss 'quandle field theory', in which the gauge group G is replaced by a quandle.Comment: 41 pages, many figures. New version has minor corrections and clarifications, and some added reference