Computer simulations of planetary accretion dynamics: Sensitivity to initial conditions

The implications and limitations of program ACRETE were tested. The program is a scheme based on Newtonian physics and accretion with unit sticking efficiency, devised to simulate the origin of the planets. The dependence of the results on a variety of radial and vertical density distribution laws, the ratio of gas to dust in the solar nebula, the total nebular mass, and the orbital eccentricity of the accreting grains was explored. Only for a small subset of conceivable cases are planetary systems closely like our own generated. Many models have tendencies towards one of two preferred configurations: multiple star systems, or planetary systems in which Jovian planets either have substantially smaller masses than in our system or are absent altogether. But for a wide range of cases recognizable planetary systems are generated - ranging from multiple star systems with accompanying planets, to systems with Jovian planets at several hundred AU, to single stars surrounded only by asteroids

Molecular Analysis of Target Specific Peripheral Nerve Regeneration

Axons of the peripheral nervous system have retained the remarkable ability to regenerate. However, after peripheral nerve injury, patients often suffer from the inability to properly localize sensation and/or the loss of fine motor control, suggesting that regenerating peripheral axons reinnervate ectopic targets. Despite decades of research in myriad model systems, whether PNS axons regenerate randomly or navigate selectively to their original targets remains controversial. Moreover, while some studies suggest that regenerating axons can choose specific paths, the cell and molecular mechanisms underlying target-selective regeneration have remained elusive. Using live-cell imaging in larval zebrafish we find that after complete nerve transection regenerating motor axons exhibit a strong preference for their original muscle territory, and that axons probe both correct and incorrect trajectories extensively before selecting their original path. We demonstrate that the glycosyltransferase lysyl hydroxylase 3 is required to modify extracellular matrix collagens to provide growth and guidance to regenerating axons. Using transgenic rescue experiments, we determine that post-injury expression of lh3 and lh3 expression in Schwann cells is sufficient to restore target-selective regeneration. Moreover, we show that Schwann cells neighboring the transection site upregulate the lh3 substrate collagen4a5 and that during regeneration collagen4a5 destabilizes axons probing inappropriate trajectories to ensure target-selective regeneration. These results demonstrate that selective ECM components match subpopulations of regenerating axons with their original targets. It has long been hypothesized that regenerating axons might reuse developmental guidance cues to reestablish synaptic connections after peripheral nerve injury. Intriguingly, Collagen4 can bind the canonical axon repellants Slit and Netrin, and we find that slit1a is upregulated with collagen4a5 in the same population of Schwann cells after injury. Additionally, we demonstrate that the slit receptors robo2 and robo3 are both required for target-selective regeneration. Together these results demonstrate that regenerating peripheral nerves reemploy developmental guidance molecules and reveal a possible mechanistic framework by which the ECM constituent collagen4a5 binds and presents the repellant slit1a to convey synaptic target selection through robo2 and robo3 repulsion of regenerating axons in vivo

Is tax legislation effectively discouraging employee share ownership?

Thesis (M.Com. (Taxation))--University of the Witwatersrand, Faculty of Commerce, Law and Management, School of Accountancy, 2017Share incentive schemes have been used for many years as a mechanism to compensate, retain and attract talent by offering employees a stake in the business. Share incentives, however, usually contribute an increasingly larger portion of executive pay in comparison with general employees. The motive for larger share incentive based compensation is on the foundation that management must have a skin in the game in order for their interest to be appropriately aligned with shareholders. The Treasury and the South African Revenue Service (‘SARS’) have historically viewed share incentive schemes with suspicion. Treasury and SARS consider these schemes as salary conversion plans designed avoid tax. This has led to a litany of tax legislation that has sought to combat this so called avoidance. As things stand it appears the legislation is far too reaching and no longer reflects the commercial and economic reality of an increasingly entrepreneurial world. The aim of this research report is to ascertain whether the current tax policy is effectively discouraging employee share ownership. This paper will consider the impact of the current tax provisions on share incentive schemes for both the employees and their companies’. The United Kingdom offer tax advantages for employee share ownership plans thus the report will also include a comparison with the tax legislation governing share option schemes in the UK. The comparison will aid in recommending a more sensible and equitable way forward with regards to the taxation of share incentive schemes in South Africa. Key words: Share incentive plans, Section 8B, Section 8C, executive remuneration, equity based compensationGR201

Digitisation, history, and the making of a postcolonial archive of Southern African liberation struggles

This paper describes the history of an initiative to digitize a postcolonial archive on the struggle for freedom in Southern Africa. The authors outline the intellectual architecture of the project and the complex epistemological, political, and technical challenges that they confronted in their endeavor to construct a digital archive that might help reorient scholarly debates on the struggle for liberation

Optimizing the Access to Healthcare Services in Dense Refugee Hosting Urban Areas: A Case for Istanbul

With over 3.5 million refugees, Turkey continues to host the world's largest refugee population. This introduced several challenges in many areas including access to healthcare system. Refugees have legal rights to free healthcare services in Turkey's public hospitals. With the aim of increasing healthcare access for refugees, we looked at where the lack of infrastructure is felt the most. Our study attempts to address these problems by assessing whether Migrant Health Centers' locations are optimal. The aim of this study is to improve refugees' access to healthcare services in Istanbul by improving the locations of health facilities available to them. We used call data records provided by Turk Telekom.Comment: version to submit for D4R competitio

The 1997 event in the Crab pulsar revisited

A complex event observed in the radio pulses from the Crab pulsar in 1997 included echoes, a dispersive delay, and large changes in intensity. It is shown that these phenomena were due to refraction at the edge of a plasma cloud in the outer region of the Crab Nebula. Several similar events have been observed, although in less detail. It is suggested that the plasma cloud is in the form of filaments with diameter around 3 x 10^11m and electron density of order 10^4 cm-3Comment: 5 pages 4 figs Accepted by MNRA

An IDL-based analysis package for COBE and other skycube-formatted astronomical data

UIMAGE is a data analysis package written in IDL for the Cosmic Background Explorer (COBE) project. COBE has extraordinarily stringent accuracy requirements: 1 percent mid-infrared absolute photometry, 0.01 percent submillimeter absolute spectrometry, and 0.0001 percent submillimeter relative photometry. Thus, many of the transformations and image enhancements common to analysis of large data sets must be done with special care. UIMAGE is unusual in this sense in that it performs as many of its operations as possible on the data in its native format and projection, which in the case of COBE is the quadrilateralized sphereical cube ('skycube'). That is, after reprojecting the data, e.g., onto an Aitoff map, the user who performs an operation such as taking a crosscut or extracting data from a pixel is transparently acting upon the skycube data from which the projection was made, thereby preserving the accuracy of the result. Current plans call for formatting external data bases such as CO maps into the skycube format with a high-accuracy transformation, thereby allowing Guest Investigators to use UIMAGE for direct comparison of the COBE maps with those at other wavelengths from other instruments. It is completely menu-driven so that its use requires no knowledge of IDL. Its functionality includes I/O from the COBE archives, FITS files, and IDL save sets as well as standard analysis operations such as smoothing, reprojection, zooming, statistics of areas, spectral analysis, etc. One of UIMAGE's more advanced and attractive features is its terminal independence. Most of the operations (e.g., menu-item selection or pixel selection) that are driven by the mouse on an X-windows terminal are also available using arrow keys and keyboard entry (e.g., pixel coordinates) on VT200 and Tektronix-class terminals. Even limited grey scales of images are available this way. Obviously, image processing is very limited on this type of terminal, but it is nonetheless surprising how much analysis can be done on that medium. Such flexibility has the virtue of expanding the user community to those who must work remotely on non-image terminals, e.g., via modem